
simdutf: Unicode validation and transcoding at billions of characters per second
===============================================

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# Table of contents

- [Table of contents](#table-of-contents)
- [simdutf: Unicode validation and transcoding at billions of characters per second](#simdutf-unicode-validation-and-transcoding-at-billions-of-characters-per-second)
  - [Real-World Usage](#real-world-usage)
  - [How fast is it?](#how-fast-is-it)
  - [Requirements](#requirements)
  - [Usage (Usage)](#usage-usage)
  - [Usage (CMake)](#usage-cmake)
  - [Single-header version](#single-header-version)
  - [Single-header version with limited features](#single-header-version-with-limited-features)
  - [Packages](#packages)
  - [Example](#example)
  - [API](#api)
  - [Base64](#base64)
  - [Find](#find)
  - [C++20 and std::span usage in simdutf](#c20-and-stdspan-usage-in-simdutf)
  - [The sutf command-line tool](#the-sutf-command-line-tool)
  - [Manual implementation selection](#manual-implementation-selection)
  - [Thread safety](#thread-safety)
  - [References](#references)
  - [License](#license)


Most modern software relies on the [Unicode standard](https://en.wikipedia.org/wiki/Unicode).
In memory, Unicode strings are represented using either
UTF-8 or UTF-16. The UTF-8 format is the de facto standard on the web (JSON, HTML, etc.) and it has been adopted as the default in many popular
programming languages (Go, Zig, Rust, Swift, etc.). The UTF-16 format is standard in Java, C# and in many Windows technologies.

Not all sequences of bytes are valid Unicode strings. It is unsafe to use Unicode strings in UTF-8 and UTF-16LE without first validating them. Furthermore, we often need to convert strings from one encoding to another, by a process called [transcoding](https://en.wikipedia.org/wiki/Transcoding). For security purposes, such transcoding should be validating: it should refuse to transcode incorrect strings.

This library provide fast Unicode functions such as

- ASCII, UTF-8, UTF-16LE/BE and UTF-32 validation, with and without error identification,
- Latin1 to UTF-8 transcoding,
- Latin1 to UTF-16LE/BE transcoding
- Latin1 to UTF-32 transcoding
- UTF-8 to Latin1 transcoding, with or without validation, with and without error identification,
- UTF-8 to UTF-16LE/BE transcoding, with or without validation, with and without error identification,
- UTF-8 to UTF-32 transcoding, with or without validation, with and without error identification,
- UTF-16LE/BE to Latin1 transcoding, with or without validation, with and without error identification,
- UTF-16LE/BE to UTF-8 transcoding, with or without validation, with and without error identification,
- UTF-32 to Latin1 transcoding, with or without validation, with and without error identification,
- UTF-32 to UTF-8 transcoding, with or without validation, with and without error identification,
- UTF-32 to UTF-16LE/BE transcoding, with or without validation, with and without error identification,
- UTF-16LE/BE to UTF-32 transcoding, with or without validation, with and without error identification,
- From an UTF-8 string, compute the size of the Latin1 equivalent string,
- From an UTF-8 string, compute the size of the UTF-16 equivalent string,
- From an UTF-8 string, compute the size of the UTF-32 equivalent string (equivalent to UTF-8 character counting),
- From an UTF-16LE/BE string, compute the size of the Latin1 equivalent string,
- From an UTF-16LE/BE string, compute the size of the UTF-8 equivalent string,
- From an UTF-32 string, compute the size of the UTF-8 or UTF-16LE equivalent string,
- From an UTF-16LE/BE string, compute the size of the UTF-32 equivalent string (equivalent to UTF-16 character counting),
- UTF-8 and UTF-16LE/BE character counting,
- UTF-16 endianness change (UTF16-LE/BE to UTF-16-BE/LE),
- [WHATWG forgiving-base64](https://infra.spec.whatwg.org/#forgiving-base64-decode) (with or without URL encoding) to binary,
- Binary to base64 (with or without URL encoding).

The functions are accelerated using SIMD instructions (e.g., ARM NEON, SSE, AVX, AVX-512, RISC-V Vector Extension, LoongSon, POWER, etc.). When your strings contain hundreds of characters, we can often transcode them at speeds exceeding a billion characters per second. You should expect high speeds not only with English strings (ASCII) but also Chinese, Japanese, Arabic, and so forth. We handle the full character range (including, for example, emojis).

The library compiles down to a small library of a few hundred kilobytes. Our functions are exception-free and non allocating. We have extensive tests and extensive benchmarks.

We have exhaustive tests, including an elaborate fuzzing setup. The library has been used in production systems for years.


Real-World Usage
-----

The simdutf library is used by:
- [Node.js](https://nodejs.org/en/) (19.4.0 or better, 20.0 or better, 18.15 or better), a standard JavaScript runtime environment,
- [Bun](https://bun.sh), a fast JavaScript runtime environment,
- [WebKit](https://github.com/WebKit/WebKit/pull/9990), the Web engine behind the Safari browser (iOS, macOS),
- [Chromium](https://chromium-review.googlesource.com/c/chromium/src/+/6054817), the Web engine behind the Google Chrome, Microsoft Edge and Brave,
- [StarRocks](https://www.starrocks.io), an Open-Source, High-Performance Analytical Database,
- [Oracle GraalVM JavaScript](https://github.com/oracle/graaljs), a JavaScript implementation by Oracle,
- [Couchbase](https://www.couchbase.com), a popular database system,
- [Ladybird](https://ladybird.org), an independent Web browser,
- [StarRocks](https://www.starrocks.io), a High-Performance Analytical Database,
- [Cloudflare workerd](https://github.com/cloudflare/workerd), a JavaScript/Wasm Runtime,
- [haskell/text](https://github.com/haskell/text), a library for fast operations over Unicode text,
- [klogg](https://github.com/variar/klogg), a Really fast log explorer,
- [Pixie](https://github.com/pixie-io/pixie), observability tool for Kubernetes applications.



How fast is it?
-----------------

The adoption of the simdutf library by the popular Node.js JavaScript runtime lead to a significant
performance gain:

> Decoding and Encoding becomes considerably faster than in Node.js 18. With the addition of simdutf for UTF-8 parsing the observed benchmark, results improved by 364% (an extremely impressive leap) when decoding in comparison to Node.js 16. ([State of Node.js Performance 2023](https://blog.rafaelgss.dev/state-of-nodejs-performance-2023))


<img src="doc/node2023.png" width="70%" />


Over a wide range of realistic data sources, the simdutf library transcodes a billion characters per second or more. Our approach can be 3 to 10 times faster than the popular ICU library on difficult (non-ASCII) strings. We can be 20x faster than ICU when processing easy strings (ASCII). Our good results apply to both recent x64 and ARM processors.


To illustrate, we present a benchmark result with values are in billions of characters processed by second. Consider the following figures.



<img src="doc/utf8utf16.png" width="70%" />

<img src="doc/utf16utf8.png" width="70%" />

If your system supports AVX-512, the simdutf library can provide very high performance. We get the following speed results on an Ice Lake Intel processor (both AVX2 and AVX-512) are simdutf kernels:

<img src="doc/avx512.png" width="70%" />


Datasets: https://github.com/lemire/unicode_lipsum

Please refer to our benchmarking tool for a proper interpretation of the numbers. Our results are reproducible.




Requirements
-------

- C++11 compatible compiler. We support LLVM clang, GCC, Visual Studio. (Our tests and benchmark tools requires C++17.) Be aware that GCC under Windows is buggy and thus unsupported.
- For high speed, you should have a recent 64-bit system (e.g., ARM, x64, RISC-V with vector extensions, Loongson, POWER).
- If you rely on CMake, you should use a recent CMake (at least 3.15); otherwise you may use the [single header version](#single-header-version). The library is also available from [Microsoft's vcpkg](https://github.com/simdutf/simdutf-vcpkg), from [conan](https://conan.io/center/recipes/simdutf), from [FreeBSD's port](https://cgit.freebsd.org/ports/tree/converters/simdutf), from [brew](https://formulae.brew.sh/formula/simdutf), and many other systems.
- AVX-512 support require a processor with AVX512-VBMI2 (Ice Lake or better, AMD Zen 4 or better) and a recent compiler (GCC 8 or better, Visual Studio 2022 or better, LLVM clang 6 or better). You need a correspondingly recent assembler such as gas (2.30+) or nasm (2.14+): recent compilers usually come with recent assemblers. If you mix a recent compiler with an incompatible/old assembler (e.g., when using a recent compiler with an old Linux distribution), you may get errors at build time because the compiler produces instructions that the assembler does not recognize: you should update your assembler to match your compiler (e.g., upgrade binutils to version 2.30 or better under Linux) or use an older compiler matching the capabilities of your assembler.
- To benefit from RISC-V Vector Extensions on RISC-V systems, you should compile specifically for the desired architecture. E.g., add `-march=rv64gcv` as a compiler flag when using a version of GCC or LLVM which supports these extensions (such as GCC 14 or better). The command `CXXFLAGS=-march=rv64gcv cmake -B build` may suffice.
- We recommend that Visual Studio users compile with LLVM (ClangCL). Using LLVM as a front-end inside Visual Studio provides faster release builds and better runtime performance.

Usage (Usage)
-------


We made a video to help you get started with the library.

[![the simdutf library](http://img.youtube.com/vi/H9NZtb7ykYs/0.jpg)](https://www.youtube.com/watch?v=H9NZtb7ykYs)<br />


Quick Start
-----------

Linux or macOS users might follow the following instructions if they have a recent C++ compiler installed and the standard utilities (`wget`, `unzip`, etc.)


1. Pull the library in a directory
   ```
   wget https://github.com/simdutf/simdutf/releases/download/v7.4.0/singleheader.zip
   unzip singleheader.zip
   ```
   You can replace `wget` by `curl -OL https://...` if you prefer.
2. Compile
   ```
   c++ -std=c++17 -o amalgamation_demo amalgamation_demo.cpp
   ```
3. `./amalgamation_demo`

   ```
   valid UTF-8
   wrote 4 UTF-16LE words.
   valid UTF-16LE
   wrote 4 UTF-8 words.
   1234
   perfect round trip
   ```

Usage (CMake)
-------

```
cmake -B build
cmake --build build
cd build
ctest .
```

Visual Studio users must specify whether they want to build the Release or Debug version.

To run transcoding benchmarks, execute the `benchmark` command. You can get help on its
usage by first building it and then calling it with the `--help` flag.
E.g., under Linux you may do the following:

```
cmake -B build -D SIMDUTF_BENCHMARKS=ON
cmake --build build
./build/benchmarks/benchmark --help
./build/benchmarks/base64/base64_benchmark --help
```

E.g., to run base64 decoding benchmarks on DNS data (short inputs), do

```
./build/benchmarks/base64/benchmark_base64 -d pathto/base64data/dns/*.txt
```

where pathto/base64data should contain the path to a clone of
the repository https://github.com/lemire/base64data.

Instructions are similar for Visual Studio users.

To use the library as a CMake dependency in your project, please see `tests/installation_tests/from_fetch` for
an example.

Since ICU is so common and popular, we assume that you may have it already on your system. When
it is not found, it is simply omitted from the benchmarks. Thus, to benchmark against ICU, make
sure you have ICU installed on your machine and that cmake can find it. For macOS, you may
install it with brew using `brew install icu4c`. If you have ICU on your system but cmake cannot
find it, you may need to provide cmake with a path to ICU, such as `ICU_ROOT=/usr/local/opt/icu4c cmake -B build`.

You may also use a package manager. E.g.,  [we have a complete example using vcpkg](https://github.com/simdutf/simdutf-vcpkg).


Single-header version
----------------------

You can create a single-header version of the library where
all of the code is put into two files (`simdutf.h` and `simdutf.cpp`).
We publish a zip archive containing these files, e.g., see
https://github.com/simdutf/simdutf/releases/download/v7.4.0/singleheader.zip

You may generate it on your own using a Python script.

```
python3 ./singleheader/amalgamate.py
```

We require Python 3 or better.

Under Linux and macOS, you may test it as follows:

```
cd singleheader
c++ -o amalgamation_demo amalgamation_demo.cpp -std=c++17
./amalgamation_demo
```

Single-header version with limited features
-------------------------------------------

When creating a single-header version, it is possible to limit which
features are enabled. Then the API of library is limited too and the
amalgamated sources do not include code related to disabled features.

The script `singleheader/amalgamate.py` accepts the following parameters:

* `--with-utf8` - procedures related only to UTF-8 encoding (like string validation);
* `--with-utf16` - likewise: only UTF-16 encoding;
* `--with-utf32` - likewise: only UTF-32 encoding;
* `--with-ascii` - procedures related to ASCII encoding;
* `--with-latin1` - convert between selected UTF encodings and Latin1;
* `--with-base64` - procedures related to Base64 encoding, includes 'find';
* `--with-detect-enc` - enable detect encoding.

If we need conversion between different encodings, like UTF-8 and UTF-32, then
these two features have to be enabled.

The amalgamated sources set to 1 the following preprocesor defines:

* `SIMDUTF_FEATURE_UTF8`,
* `SIMDUTF_FEATURE_UTF16`,
* `SIMDUTF_FEATURE_UTF32`,
* `SIMDUTF_FEATURE_ASCII`,
* `SIMDUTF_FEATURE_LATIN1`,
* `SIMDUTF_FEATURE_BASE64`,
* `SIMDUTF_FEATURE_DETECT_ENCODING`.

Thus, when it is needed to make sure the correct set of features are
enabled, we may test it using preprocessor:

```cpp
#if SIMDUTF_FEATURE_UTF16 || SIMDUTF_FEATURE_UTF32
    #error "Please amalagamate simdutf without UTF-16 and UTF-32"
#endif
```


Packages
------

[![Packaging status](https://repology.org/badge/vertical-allrepos/simdutf.svg)](https://repology.org/project/simdutf/versions)


Example
---------

Using the single-header version, you could compile the following program.

```cpp
#include <iostream>
#include <memory>

#include "simdutf.cpp"
#include "simdutf.h"

int main(int argc, char *argv[]) {
  const char *source = "1234";
  // 4 == strlen(source)
  bool validutf8 = simdutf::validate_utf8(source, 4);
  if (validutf8) {
    std::cout << "valid UTF-8" << std::endl;
  } else {
    std::cerr << "invalid UTF-8" << std::endl;
    return EXIT_FAILURE;
  }
  // We need a buffer of size where to write the UTF-16LE code units.
  size_t expected_utf16words = simdutf::utf16_length_from_utf8(source, 4);
  std::unique_ptr<char16_t[]> utf16_output{new char16_t[expected_utf16words]};
  // convert to UTF-16LE
  size_t utf16words =
      simdutf::convert_utf8_to_utf16le(source, 4, utf16_output.get());
  std::cout << "wrote " << utf16words << " UTF-16LE code units." << std::endl;
  // It wrote utf16words * sizeof(char16_t) bytes.
  bool validutf16 = simdutf::validate_utf16le(utf16_output.get(), utf16words);
  if (validutf16) {
    std::cout << "valid UTF-16LE" << std::endl;
  } else {
    std::cerr << "invalid UTF-16LE" << std::endl;
    return EXIT_FAILURE;
  }
  // convert it back:
  // We need a buffer of size where to write the UTF-8 code units.
  size_t expected_utf8words =
      simdutf::utf8_length_from_utf16le(utf16_output.get(), utf16words);
  std::unique_ptr<char[]> utf8_output{new char[expected_utf8words]};
  // convert to UTF-8
  size_t utf8words = simdutf::convert_utf16le_to_utf8(
      utf16_output.get(), utf16words, utf8_output.get());
  std::cout << "wrote " << utf8words << " UTF-8 code units." << std::endl;
  std::string final_string(utf8_output.get(), utf8words);
  std::cout << final_string << std::endl;
  if (final_string != source) {
    std::cerr << "bad conversion" << std::endl;
    return EXIT_FAILURE;
  } else {
    std::cerr << "perfect round trip" << std::endl;
  }
  return EXIT_SUCCESS;
}
```

API
-----

Our API is made of a few non-allocating functions. They typically take a pointer and a length as a parameter,
and they sometimes take a pointer to an output buffer. Users are responsible for memory allocation.

We use three types of data pointer types:
- `char*` for UTF-8 or indeterminate Unicode formats,
- `char16_t*` for UTF-16 (both UTF-16LE and UTF-16BE),
- `char32_t*` for UTF-32. UTF-32 is primarily used for internal use, not data interchange. Thus, unless otherwise stated, `char32_t` refers to the native type and is typically UTF-32LE since virtually all systems are little-endian today.
In generic terms, we refer to `char`, `char16_t`,  and `char32_t` as *code units*. A *character* may use several *code units*: between 1 and 4 code units in UTF-8, and between
1 and 2 code units in UTF-16LE and UTF-16BE.

Our functions and declarations are all in the `simdutf` namespace. Thus you should prefix our functions
and types with `simdutf::` as required.

If using C++20, all functions which take a pointer and a size (which is almost all of them)
also have a span overload. Here is an example:

```cpp
std::vector<char> data{1, 2, 3, 4, 5};
// C++11 API
auto cpp11 = simdutf::autodetect_encoding(data.data(), data.size());
// C++20 API
auto cpp20 = simdutf::autodetect_encoding(data);
```

The span overloads use std::span for UTF-16 and UTF-32. For latin1, UTF-8,
"binary" (used by the base64 functions) anything that has a `.size()` and
`.data()` that returns a pointer to a byte-like type will be accepted as a
span. This makes it possible to directly pass std::string, std::string_view,
std::vector, std::array and std::span to the functions. The reason for allowing
all byte-like types in the api (as opposed to only `std::span<char>`) is to
make it easy to interface with whatever data the user may have, without having
to resort to casting.

We have basic functions to detect the type of an input. They return an integer defined by
the following `enum`.

```cpp
enum encoding_type {
        UTF8 = 1,       // BOM 0xef 0xbb 0xbf
        UTF16_LE = 2,   // BOM 0xff 0xfe
        UTF16_BE = 4,   // BOM 0xfe 0xff
        UTF32_LE = 8,   // BOM 0xff 0xfe 0x00 0x00
        UTF32_BE = 16,  // BOM 0x00 0x00 0xfe 0xff

        unspecified = 0
};
```

```cpp

/**
 * Autodetect the encoding of the input, a single encoding is recommended.
 * E.g., the function might return simdutf::encoding_type::UTF8,
 * simdutf::encoding_type::UTF16_LE, simdutf::encoding_type::UTF16_BE, or
 * simdutf::encoding_type::UTF32_LE.
 *
 * @param input the string to analyze.
 * @param length the length of the string in bytes.
 * @return the detected encoding type
 */
simdutf_warn_unused simdutf::encoding_type autodetect_encoding(const char *input, size_t length) noexcept;

/**
 * Autodetect the possible encodings of the input in one pass.
 * E.g., if the input might be UTF-16LE or UTF-8, this function returns
 * the value (simdutf::encoding_type::UTF8 | simdutf::encoding_type::UTF16_LE).
 *
 * Overridden by each implementation.
 *
 * @param input the string to analyze.
 * @param length the length of the string in bytes.
 * @return the detected encoding type
 */
simdutf_warn_unused int detect_encodings(const char *input, size_t length) noexcept;
```



For validation and transcoding, we also provide functions that will stop on error and return a result struct which is a pair of two fields:
```cpp
struct result {
  error_code error; // see `struct error_code`.
  size_t count; // In case of error, indicates the position of the error in the input in code units.
  // In case of success, indicates the number of code units validated/written.
};
```
On error, the `error` field indicates the type of error encountered and the `count` field indicates the position of the error in the input in code units or the number of characters validated/written.
We report six types of errors related to Latin1, UTF-8, UTF-16 and UTF-32 encodings:
```cpp
enum error_code {
  SUCCESS = 0,
  HEADER_BITS, // Any byte must have fewer than 5 header bits.
  TOO_SHORT,   // The leading byte must be followed by N-1 continuation bytes,
               // where N is the UTF-8 character length This is also the error
               // when the input is truncated.
  TOO_LONG,    // We either have too many consecutive continuation bytes or the
               // string starts with a continuation byte.
  OVERLONG, // The decoded character must be above U+7F for two-byte characters,
            // U+7FF for three-byte characters, and U+FFFF for four-byte
            // characters.
  TOO_LARGE, // The decoded character must be less than or equal to
             // U+10FFFF,less than or equal than U+7F for ASCII OR less than
             // equal than U+FF for Latin1
  SURROGATE, // The decoded character must be not be in U+D800...DFFF (UTF-8 or
             // UTF-32) OR a high surrogate must be followed by a low surrogate
             // and a low surrogate must be preceded by a high surrogate
             // (UTF-16) OR there must be no surrogate at all (Latin1)
  INVALID_BASE64_CHARACTER, // Found a character that cannot be part of a valid
                            // base64 string. This may include a misplaced padding character ('=').
  BASE64_INPUT_REMAINDER,   // The base64 input terminates with a single
                            // character, excluding padding (=). It is also used
                            // in strict mode when padding is not adequate.
  BASE64_EXTRA_BITS,        // The base64 input terminates with non-zero
                            // padding bits.
  OUTPUT_BUFFER_TOO_SMALL,  // The provided buffer is too small.
  OTHER                     // Not related to validation/transcoding.
};

```
On success, the `error` field is set to `SUCCESS` and the `position` field indicates either the number of code units validated for validation functions or the number of written
code units in the output format for transcoding functions. In ASCII, Latin1 and UTF-8, code units occupy 8 bits (they are bytes); in UTF-16LE and UTF-16BE, code units occupy 16 bits; in UTF-32, code units occupy 32 bits.

Generally speaking, functions that report errors always stop soon after an error is
encountered and might therefore be faster on inputs where an error occurs early in the input.
The functions that return a boolean indicating whether or not an error has been encountered
are meant to be used in an *optimistic setting*---when we expect that inputs will almost always
be correct.

You may use functions that report an error to indicate where the problem happens during, as follows:

```cpp
  std::string bad_ascii = "\x20\x20\x20\x20\x20\xff\x20\x20\x20";
  simdutf::result res = implementation.validate_ascii_with_errors(bad_ascii.data(), bad_ascii.size());
  if(res.error != simdutf::error_code::SUCCESS) {
    std::cerr << "error at index " << res.count << std::endl;
  }
```

Or as follows:

```cpp
  std::string bad_utf8 = "\xc3\xa9\xc3\xa9\x20\xff\xc3\xa9";
  simdutf::result res = implementation.validate_utf8_with_errors(bad_utf8.data(), bad_utf8.size());
  if(res.error != simdutf::error_code::SUCCESS) {
    std::cerr << "error at index " << res.count << std::endl;
  }
  res = implementation.validate_utf8_with_errors(bad_utf8.data(), res.count);
  // will be successful in this case
  if(res.error == simdutf::error_code::SUCCESS) {
    std::cerr << "we have " << res.count << "valid bytes" << std::endl;
  }
```


We have fast validation functions.

```cpp
/**
 * Validate the ASCII string.
 *
 * Overridden by each implementation.
 *
 * @param buf the ASCII string to validate.
 * @param len the length of the string in bytes.
 * @return true if and only if the string is valid ASCII.
 */
simdutf_warn_unused bool validate_ascii(const char *buf, size_t len) noexcept;

/**
 * Validate the ASCII string and stop on error.
 *
 * Overridden by each implementation.
 *
 * @param buf the ASCII string to validate.
 * @param len the length of the string in bytes.
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of code units validated if successful.
 */
simdutf_warn_unused result validate_ascii_with_errors(const char *buf, size_t len) noexcept;

/**
 * Validate the ASCII string as a UTF-16 sequence.
 * An UTF-16 sequence is considered an ASCII sequence
 * if it could be converted to an ASCII string losslessly.
 *
 * Overridden by each implementation.
 *
 * @param buf the UTF-16 string to validate.
 * @param len the length of the string in bytes.
 * @return true if and only if the string is valid ASCII.
 */
simdutf_warn_unused bool validate_utf16_as_ascii(const char16_t *buf,
                                                 size_t len) noexcept;

/**
 * Validate the ASCII string as a UTF-16BE sequence.
 * An UTF-16 sequence is considered an ASCII sequence
 * if it could be converted to an ASCII string losslessly.
 *
 * Overridden by each implementation.
 *
 * @param buf the UTF-16BE string to validate.
 * @param len the length of the string in bytes.
 * @return true if and only if the string is valid ASCII.
 */
simdutf_warn_unused bool validate_utf16be_as_ascii(const char16_t *buf,
                                                   size_t len) noexcept;
/**
 * Validate the ASCII string as a UTF-16LE sequence.
 * An UTF-16 sequence is considered an ASCII sequence
 * if it could be converted to an ASCII string losslessly.
 *
 * Overridden by each implementation.
 *
 * @param buf the UTF-16LE string to validate.
 * @param len the length of the string in bytes.
 * @return true if and only if the string is valid ASCII.
 */
simdutf_warn_unused bool validate_utf16le_as_ascii(const char16_t *buf,
                                                   size_t len) noexcept;

/**
 * Validate the UTF-8 string. This function may be best when you expect
 * the input to be almost always valid. Otherwise, consider using
 * validate_utf8_with_errors.
 *
 * Overridden by each implementation.
 *
 * @param buf the UTF-8 string to validate.
 * @param len the length of the string in bytes.
 * @return true if and only if the string is valid UTF-8.
 */
simdutf_warn_unused bool validate_utf8(const char *buf, size_t len) noexcept;

/**
 * Validate the UTF-8 string and stop on error. It might be faster than
 * validate_utf8 when an error is expected to occur early.
 *
 * Overridden by each implementation.
 *
 * @param buf the UTF-8 string to validate.
 * @param len the length of the string in bytes.
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of code units validated if successful.
 */
simdutf_warn_unused result validate_utf8_with_errors(const char *buf, size_t len) noexcept;

/**
 * Using native endianness; Validate the UTF-16 string.
 * This function may be best when you expect the input to be almost always valid.
 * Otherwise, consider using validate_utf16_with_errors.
 *
 * Overridden by each implementation.
 *
 * This function is not BOM-aware.
 *
 * @param buf the UTF-16 string to validate.
 * @param len the length of the string in number of 2-byte code units (char16_t).
 * @return true if and only if the string is valid UTF-16.
 */
simdutf_warn_unused bool validate_utf16(const char16_t *buf, size_t len) noexcept;

/**
 * Validate the UTF-16LE string. This function may be best when you expect
 * the input to be almost always valid. Otherwise, consider using
 * validate_utf16le_with_errors.
 *
 * Overridden by each implementation.
 *
 * This function is not BOM-aware.
 *
 * @param buf the UTF-16LE string to validate.
 * @param len the length of the string in number of 2-byte code units (char16_t).
 * @return true if and only if the string is valid UTF-16LE.
 */
simdutf_warn_unused bool validate_utf16le(const char16_t *buf, size_t len) noexcept;

/**
 * Validate the UTF-16BE string. This function may be best when you expect
 * the input to be almost always valid. Otherwise, consider using
 * validate_utf16be_with_errors.
 *
 * Overridden by each implementation.
 *
 * This function is not BOM-aware.
 *
 * @param buf the UTF-16BE string to validate.
 * @param len the length of the string in number of 2-byte code units (char16_t).
 * @return true if and only if the string is valid UTF-16BE.
 */
simdutf_warn_unused bool validate_utf16be(const char16_t *buf, size_t len) noexcept;

/**
 * Using native endianness; Validate the UTF-16 string and stop on error.
 * It might be faster than validate_utf16 when an error is expected to occur early.
 *
 * Overridden by each implementation.
 *
 * This function is not BOM-aware.
 *
 * @param buf the UTF-16 string to validate.
 * @param len the length of the string in number of 2-byte code units (char16_t).
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of code units validated if successful.
 */
simdutf_warn_unused result validate_utf16_with_errors(const char16_t *buf, size_t len) noexcept;

/**
 * Validate the UTF-16LE string and stop on error. It might be faster than
 * validate_utf16le when an error is expected to occur early.
 *
 * Overridden by each implementation.
 *
 * This function is not BOM-aware.
 *
 * @param buf the UTF-16LE string to validate.
 * @param len the length of the string in number of 2-byte code units (char16_t).
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of code units validated if successful.
 */
simdutf_warn_unused result validate_utf16le_with_errors(const char16_t *buf, size_t len) noexcept;

/**
 * Validate the UTF-16BE string and stop on error. It might be faster than
 * validate_utf16be when an error is expected to occur early.
 *
 * Overridden by each implementation.
 *
 * This function is not BOM-aware.
 *
 * @param buf the UTF-16BE string to validate.
 * @param len the length of the string in number of 2-byte code units (char16_t).
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of code units validated if successful.
 */
simdutf_warn_unused result validate_utf16be_with_errors(const char16_t *buf, size_t len) noexcept;

/**
 * Validate the UTF-32 string.
 *
 * Overridden by each implementation.
 *
 * This function is not BOM-aware.
 *
 * @param buf the UTF-32 string to validate.
 * @param len the length of the string in number of 4-byte code units (char32_t).
 * @return true if and only if the string is valid UTF-32.
 */
simdutf_warn_unused bool validate_utf32(const char32_t *buf, size_t len) noexcept;

/**
 * Validate the UTF-32 string and stop on error.
 *
 * Overridden by each implementation.
 *
 * This function is not BOM-aware.
 *
 * @param buf the UTF-32 string to validate.
 * @param len the length of the string in number of 4-byte code units (char32_t).
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of code units validated if successful.
 */
simdutf_warn_unused result validate_utf32_with_errors(const char32_t *buf, size_t len) noexcept;

```

Given a potentially invalid UTF-16 input, you may want to make it correct, by using
a replacement character whenever needed. We have fast functions for this purpose
(`to_well_formed_utf16`, `to_well_formed_utf16le`, and `to_well_formed_utf16be`).
They can either copy the string while fixing it, or they can be used to fix
a string in-place.

```cpp

/**
 * Fixes an ill-formed UTF-16LE string by replacing mismatched surrogates with
 * the Unicode replacement character U+FFFD. If input and output points to
 * different memory areas, the procedure copies string, and it's expected that
 * output memory is at least as big as the input. It's also possible to set
 * input equal output, that makes replacements an in-place operation.
 *
 * @param input the UTF-16LE string to correct.
 * @param len the length of the string in number of 2-byte code units
 * (char16_t).
 * @param output the output buffer.
 */
void to_well_formed_utf16le(const char16_t *input, size_t len,
                            char16_t *output) noexcept;

/**
 * Fixes an ill-formed UTF-16BE string by replacing mismatched surrogates with
 * the Unicode replacement character U+FFFD. If input and output points to
 * different memory areas, the procedure copies string, and it's expected that
 * output memory is at least as big as the input. It's also possible to set
 * input equal output, that makes replacements an in-place operation.
 *
 * @param input the UTF-16BE string to correct.
 * @param len the length of the string in number of 2-byte code units
 * (char16_t).
 * @param output the output buffer.
 */
void to_well_formed_utf16be(const char16_t *input, size_t len,
                            char16_t *output) noexcept;

/**
 * Fixes an ill-formed UTF-16 string by replacing mismatched surrogates with the
 * Unicode replacement character U+FFFD. If input and output points to different
 * memory areas, the procedure copies string, and it's expected that output
 * memory is at least as big as the input. It's also possible to set input equal
 * output, that makes replacements an in-place operation.
 *
 * @param input the UTF-16 string to correct.
 * @param len the length of the string in number of 2-byte code units
 * (char16_t).
 * @param output the output buffer.
 */
void to_well_formed_utf16(const char16_t *input, size_t len,
                          char16_t *output) noexcept;
```

Given a valid UTF-8 or UTF-16 input, you may count the number Unicode characters using
fast functions. For UTF-32, there is no need for a function given that each character
requires a flat 4 bytes. Likewise for Latin1: one byte will always equal one character.

```cpp
/**
 * Count the number of code points (characters) in the string assuming that
 * it is valid.
 *
 * This function assumes that the input string is valid UTF-16 (native endianness).
 * It is acceptable to pass invalid UTF-16 strings but in such cases
 * the result is implementation defined.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16 string to process
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @return number of code points
 */
simdutf_warn_unused size_t count_utf16(const char16_t * input, size_t length) noexcept;

/**
 * Count the number of code points (characters) in the string assuming that
 * it is valid.
 *
 * This function assumes that the input string is valid UTF-16LE.
 * It is acceptable to pass invalid UTF-16 strings but in such cases
 * the result is implementation defined.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16LE string to process
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @return number of code points
 */
simdutf_warn_unused size_t count_utf16le(const char16_t * input, size_t length) noexcept;

/**
 * Count the number of code points (characters) in the string assuming that
 * it is valid.
 *
 * This function assumes that the input string is valid UTF-16BE.
 * It is acceptable to pass invalid UTF-16 strings but in such cases
 * the result is implementation defined.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16BE string to process
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @return number of code points
 */
simdutf_warn_unused size_t count_utf16be(const char16_t * input, size_t length) noexcept;

/**
 * Count the number of code points (characters) in the string assuming that
 * it is valid.
 *
 * This function assumes that the input string is valid UTF-8.
 * It is acceptable to pass invalid UTF-8 strings but in such cases
 * the result is implementation defined.
 *
 * @param input         the UTF-8 string to process
 * @param length        the length of the string in bytes
 * @return number of code points
 */
simdutf_warn_unused size_t count_utf8(const char * input, size_t length) noexcept;

```

Prior to transcoding an input, you need to allocate enough memory to receive the result.
We have fast function that scan the input and compute the size of the output. These functions
are fast and non-validating.



```cpp

/**
 * Return the number of bytes that this Latin1 string would require in UTF-8 format.
 *
 * @param input         the Latin1 string to convert
 * @param length        the length of the string bytes
 * @return the number of bytes required to encode the Latin1 string as UTF-8
 */
simdutf_warn_unused size_t utf8_length_from_latin1(const char * input, size_t length) noexcept;

/**
 * Compute the number of bytes that this UTF-8 string would require in Latin1 format.
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-8 strings but in such cases
 * the result is implementation defined.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-8 string to convert
 * @param length        the length of the string in byte
 * @return the number of bytes required to encode the UTF-8 string as Latin1
 */
simdutf_warn_unused size_t latin1_length_from_utf8(const char * input, size_t length) noexcept;

/**
 * Compute the number of bytes that this UTF-16 string would require in Latin1 format.
 *
 * @param length        the length of the string in Latin1 code units (char)
 * @return the length of the string in Latin1 code units (char) required to encode the UTF-16 string as Latin1
 */
simdutf_warn_unused size_t latin1_length_from_utf16(size_t length) noexcept;

/*
 * Compute the number of bytes that this UTF-16LE/BE string would require in Latin1 format.
 *
 * This function does not validate the input.  It is acceptable to pass invalid UTF-16 strings but in such cases
 * the result is implementation defined.
 *
 * This function is not BOM-aware.
 *
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @return the number of bytes required to encode the UTF-16LE string as Latin1
 */
simdutf_warn_unused size_t latin1_length_from_utf16(size_t length) noexcept;

/**
 * Compute the number of bytes that this UTF-32 string would require in Latin1 format.
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-32 strings but in such cases
 * the result is implementation defined.
 *
 * This function is not BOM-aware.
 *
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @return the number of bytes required to encode the UTF-32 string as Latin1
 */
simdutf_warn_unused size_t latin1_length_from_utf32(size_t length) noexcept;

/**
 * Compute the number of 2-byte code units that this UTF-8 string would require in UTF-16 format.
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-8 strings but in such cases
 * the result is implementation defined.
 *
 * @param input         the UTF-8 string to process
 * @param length        the length of the string in bytes
 * @return the number of char16_t code units required to encode the UTF-8 string as UTF-16
 */
simdutf_warn_unused size_t utf16_length_from_utf8(const char * input, size_t length) noexcept;


/**
 * Compute the number of 4-byte code units that this UTF-8 string would require in UTF-32 format.
 *
 * This function is equivalent to count_utf8
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-8 strings but in such cases
 * the result is implementation defined.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-8 string to process
 * @param length        the length of the string in bytes
 * @return the number of char32_t code units required to encode the UTF-8 string as UTF-32
 */
simdutf_warn_unused size_t utf32_length_from_utf8(const char * input, size_t length) noexcept;

/**
 * Using native endianness; Compute the number of bytes that this UTF-16
 * string would require in UTF-8 format.
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-16 strings but in such cases
 * the result is implementation defined.
 *
 * @param input         the UTF-16 string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @return the number of bytes required to encode the UTF-16LE string as UTF-8
 */
simdutf_warn_unused size_t utf8_length_from_utf16(const char16_t * input, size_t length) noexcept;

/**
 * Compute the number of bytes that this UTF-16LE string would require in UTF-8 format.
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-16 strings but in such cases
 * the result is implementation defined.
 *
 * @param input         the UTF-16LE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @return the number of bytes required to encode the UTF-16LE string as UTF-8
 */
simdutf_warn_unused size_t utf8_length_from_utf16le(const char16_t * input, size_t length) noexcept;

/**
 * Compute the number of bytes that this UTF-16BE string would require in UTF-8 format.
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-16 strings but in such cases
 * the result is implementation defined.
 *
 * @param input         the UTF-16BE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @return the number of bytes required to encode the UTF-16BE string as UTF-8
 */
simdutf_warn_unused size_t utf8_length_from_utf16be(const char16_t * input, size_t length) noexcept;

/**
 * Compute the number of bytes that this UTF-32 string would require in UTF-8 format.
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-32 strings but in such cases
 * the result is implementation defined.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @return the number of bytes required to encode the UTF-32 string as UTF-8
 */
simdutf_warn_unused size_t utf8_length_from_utf32(const char32_t * input, size_t length) noexcept;

/**
 * Compute the number of two-byte code units that this UTF-32 string would require in UTF-16 format.
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-32 strings but in such cases
 * the result is implementation defined.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @return the number of bytes required to encode the UTF-32 string as UTF-16
 */
simdutf_warn_unused size_t utf16_length_from_utf32(const char32_t * input, size_t length) noexcept;

/**
 * Compute the number of code units that this Latin1 string would require in UTF-16 format.
 *
 * @param length        the length of the string in Latin1 code units (char)
 * @return the length of the string in 2-byte code units (char16_t) required to encode the Latin1 string as UTF-16
 */
simdutf_warn_unused size_t utf16_length_from_latin1(size_t length) noexcept;

/**
 * Using native endianness; Compute the number of bytes that this UTF-16
 * string would require in UTF-32 format.
 *
 * This function is equivalent to count_utf16.
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-16 strings but in such cases
 * the result is implementation defined.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16 string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @return the number of bytes required to encode the UTF-16LE string as UTF-32
 */
simdutf_warn_unused size_t utf32_length_from_utf16(const char16_t * input, size_t length) noexcept;

/**
 * Compute the number of bytes that this UTF-16LE string would require in UTF-32 format.
 *
 * This function is equivalent to count_utf16le.
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-16 strings but in such cases
 * the result is implementation defined.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16LE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @return the number of bytes required to encode the UTF-16LE string as UTF-32
 */
simdutf_warn_unused size_t utf32_length_from_utf16le(const char16_t * input, size_t length) noexcept;

/**
 * Compute the number of bytes that this UTF-16BE string would require in UTF-32 format.
 *
 * This function is equivalent to count_utf16be.
 *
 * This function does not validate the input. It is acceptable to pass invalid UTF-16 strings but in such cases
 * the result is implementation defined.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16BE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @return the number of bytes required to encode the UTF-16BE string as UTF-32
 */
simdutf_warn_unused size_t utf32_length_from_utf16be(const char16_t * input, size_t length) noexcept;

/**
 * Compute the number of bytes that this Latin1 string would require in UTF-32 format.
 *
 * @param length        the length of the string in Latin1 code units (char)
 * @return the length of the string in 4-byte code units (char32_t) required to encode the Latin1 string as UTF-32
 */
simdutf_warn_unused size_t utf32_length_from_latin1(size_t length) noexcept;
```



We have a wide range of conversion between Latin1, UTF-8, UTF-16 and UTF-32. They assume
that you are allocated sufficient memory for the input. The simplest conversion
function output a single integer representing the size of the input, with a value of zero
indicating an error (e.g., `convert_utf8_to_utf16le`). They are well suited in the
scenario where you expect the input to be valid most of the time.



```cpp
/**
 * Convert Latin1 string into UTF-8 string.
 *
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the Latin1 string to convert
 * @param length        the length of the string in bytes
 * @param utf8_output  the pointer to buffer that can hold conversion result
 * @return the number of written char; 0 if conversion is not possible
 */
simdutf_warn_unused size_t convert_latin1_to_utf8(const char * input, size_t length, char* utf8_output) noexcept;

/**
 * Convert Latin1 string into UTF-8 string with output limit.
 *
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the Latin1 string to convert
 * @param length        the length of the string in bytes
 * @param utf8_output  	the pointer to buffer that can hold conversion result
 * @param utf8_len      the maximum output length
 * @return the number of written char; 0 if conversion is not possible
 */
simdutf_warn_unused size_t convert_latin1_to_utf8_safe(const char * input, size_t length, char* utf8_output, size_t utf8_len) noexcept;

/**
 * Using native endianness, convert a Latin1 string into a UTF-16 string.
 *
 * @param input         the Latin1 string to convert
 * @param length        the length of the string in bytes
 * @param utf16_buffer  the pointer to buffer that can hold conversion result
 * @return the number of written char16_t.
 */
simdutf_warn_unused size_t convert_latin1_to_utf16(const char * input, size_t length, char16_t* utf16_output) noexcept;

/**
 * Convert possibly Latin1 string into UTF-16LE string.
 *
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the Latin1 string to convert
 * @param length        the length of the string in bytes
 * @param utf16_buffer  the pointer to buffer that can hold conversion result
 * @return the number of written char16_t; 0 if conversion is not possible
 */
simdutf_warn_unused size_t convert_latin1_to_utf16le(const char * input, size_t length, char16_t* utf16_output) noexcept;

/**
 * Convert Latin1 string into UTF-16BE string.
 *
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the Latin1 string to convert
 * @param length        the length of the string in bytes
 * @param utf16_buffer  the pointer to buffer that can hold conversion result
 * @return the number of written char16_t; 0 if conversion is not possible
 */
simdutf_warn_unused size_t convert_latin1_to_utf16be(const char * input, size_t length, char16_t* utf16_output) noexcept;

/**
 * Convert Latin1 string into UTF-32 string.
 *
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the Latin1 string to convert
 * @param length        the length of the string in bytes
 * @param utf32_buffer  the pointer to buffer that can hold conversion result
 * @return the number of written char32_t; 0 if conversion is not possible
 */
simdutf_warn_unused size_t convert_latin1_to_utf32(const char * input, size_t length, char32_t* utf32_buffer) noexcept;

/**
 * Convert possibly broken UTF-8 string into Latin1 string.
 * If the string cannot be represented as Latin1, an error
 * code is returned.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the UTF-8 string to convert
 * @param length        the length of the string in bytes
 * @param latin1_output  the pointer to buffer that can hold conversion result
 * @return the number of written char; 0 if the input was not valid UTF-8 string or if it cannot be represented as Latin1
 */
simdutf_warn_unused size_t convert_utf8_to_latin1(const char * input, size_t length, char* latin1_output) noexcept;

/**
 * Using native endianness; Convert possibly broken UTF-8 string into a UTF-16 string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the UTF-8 string to convert
 * @param length        the length of the string in bytes
 * @param utf16_buffer  the pointer to buffer that can hold conversion result
 * @return the number of written char16_t; 0 if the input was not valid UTF-8 string
 */
simdutf_warn_unused size_t convert_utf8_to_utf16(const char * input, size_t length, char16_t* utf16_output) noexcept;

/**
 * Convert possibly broken UTF-8 string into UTF-16LE string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the UTF-8 string to convert
 * @param length        the length of the string in bytes
 * @param utf16_buffer  the pointer to buffer that can hold conversion result
 * @return the number of written char16_t; 0 if the input was not valid UTF-8 string
 */
simdutf_warn_unused size_t convert_utf8_to_utf16le(const char * input, size_t length, char16_t* utf16_output) noexcept;

/**
 * Convert possibly broken UTF-8 string into UTF-16BE string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the UTF-8 string to convert
 * @param length        the length of the string in bytes
 * @param utf16_buffer  the pointer to buffer that can hold conversion result
 * @return the number of written char16_t; 0 if the input was not valid UTF-8 string
 */
simdutf_warn_unused size_t convert_utf8_to_utf16be(const char * input, size_t length, char16_t* utf16_output) noexcept;

/**
 * Convert possibly broken UTF-8 string into UTF-32 string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the UTF-8 string to convert
 * @param length        the length of the string in bytes
 * @param utf32_buffer  the pointer to buffer that can hold conversion result
 * @return the number of written char32_t; 0 if the input was not valid UTF-8 string
 */
simdutf_warn_unused size_t convert_utf8_to_utf32(const char * input, size_t length, char32_t* utf32_output) noexcept;

/**
 * Using native endianness, convert possibly broken UTF-16 string into UTF-8
 * string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16 string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param utf8_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-16LE
 * string
 */
simdutf_warn_unused size_t convert_utf16_to_utf8(const char16_t *input,
                                                 size_t length,
                                                 char *utf8_buffer) noexcept;


/**
 * Using native endianness, convert possibly broken UTF-16 string into UTF-8
 * string with output limit.
 *
 * We write as many characters as possible into the output buffer,
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 *
 * @param input         the UTF-16 string to convert
 * @param length        the length of the string in 16-bit code units (char16_t)
 * @param utf8_output  	the pointer to buffer that can hold conversion result
 * @param utf8_len      the maximum output length
 * @return the number of written char; 0 if conversion is not possible
 */
simdutf_warn_unused size_t
convert_utf16_to_utf8_safe(const char16_t *input, size_t length, char *utf8_output,
                            size_t utf8_len) noexcept;

/**
 * Using native endianness, convert possibly broken UTF-16 string into Latin1 string.
 * If the string cannot be represented as Latin1, an error
 * is returned.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16 string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param latin1_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-16 string or if it cannot be represented as Latin1
 */
simdutf_warn_unused size_t convert_utf16_to_latin1(const char16_t * input, size_t length, char* latin1_buffer) noexcept;

/**
 * Convert possibly broken UTF-16LE string into Latin1 string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16LE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param latin1_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-16LE string or if it cannot be represented as Latin1
 */
simdutf_warn_unused size_t convert_utf16le_to_latin1(const char16_t * input, size_t length, char* latin1_buffer) noexcept;

/**
 * Convert possibly broken UTF-16BE string into Latin1 string.
 * If the string cannot be represented as Latin1, an error
 * is returned.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16BE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param latin1_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-16BE string or if it cannot be represented as Latin1
 */
simdutf_warn_unused size_t convert_utf16be_to_latin1(const char16_t * input, size_t length, char* latin1_buffer) noexcept;

/**
 * Convert possibly broken UTF-16LE string into UTF-8 string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16LE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param utf8_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-16LE string
 */
simdutf_warn_unused size_t convert_utf16le_to_utf8(const char16_t * input, size_t length, char* utf8_buffer) noexcept;

/**
 * Convert possibly broken UTF-16BE string into UTF-8 string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16BE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param utf8_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-16LE string
 */
simdutf_warn_unused size_t convert_utf16be_to_utf8(const char16_t * input, size_t length, char* utf8_buffer) noexcept;


/**
 * Convert possibly broken UTF-32 string into Latin1 string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @param latin1_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-32 string or if it cannot be represented as Latin1
 */
simdutf_warn_unused size_t convert_utf32_to_latin1(const char32_t * input, size_t length, char* latin1_buffer) noexcept;

/**
 * Convert possibly broken UTF-32 string into UTF-8 string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @param utf8_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-32 string
 */
simdutf_warn_unused size_t convert_utf32_to_utf8(const char32_t * input, size_t length, char* utf8_buffer) noexcept;

/**
 * Using native endianness; Convert possibly broken UTF-32 string into a UTF-16 string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @param utf16_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-32 string
 */
simdutf_warn_unused size_t convert_utf32_to_utf16(const char32_t * input, size_t length, char16_t* utf16_buffer) noexcept;

/**
 * Convert possibly broken UTF-32 string into UTF-16LE string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @param utf16_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-32 string
 */
simdutf_warn_unused size_t convert_utf32_to_utf16le(const char32_t * input, size_t length, char16_t* utf16_buffer) noexcept;

/**
 * Convert possibly broken UTF-32 string into UTF-16BE string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @param utf16_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-32 string
 */
simdutf_warn_unused size_t convert_utf32_to_utf16be(const char32_t * input, size_t length, char16_t* utf16_buffer) noexcept;

/**
 * Using native endianness; Convert possibly broken UTF-16 string into UTF-32 string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16 string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param utf32_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-16LE string
 */
simdutf_warn_unused size_t convert_utf16_to_utf32(const char16_t * input, size_t length, char32_t* utf32_buffer) noexcept;

/**
 * Convert possibly broken UTF-16LE string into UTF-32 string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16LE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param utf32_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-16LE string
 */
simdutf_warn_unused size_t convert_utf16le_to_utf32(const char16_t * input, size_t length, char32_t* utf32_buffer) noexcept;

/**
 * Convert possibly broken UTF-16BE string into UTF-32 string.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16BE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param utf32_buffer   the pointer to buffer that can hold conversion result
 * @return number of written code units; 0 if input is not a valid UTF-16LE string
 */
simdutf_warn_unused size_t convert_utf16be_to_utf32(const char16_t * input, size_t length, char32_t* utf32_buffer) noexcept;
```

In some cases, you need to transcode UTF-8 or UTF-16 inputs, but you may have a truncated
string, meaning that the last character might be incomplete. In such cases, we recommend
trimming the end of your input so you do not encounter an error.

```cpp

/**
 * Given a valid UTF-8 string having a possibly truncated last character,
 * this function checks the end of string. If the last character is truncated (or partial),
 * then it returns a shorter length (shorter by 1 to 3 bytes) so that the short UTF-8
 * strings only contain complete characters. If there is no truncated character,
 * the original length is returned.
 *
 * This function assumes that the input string is valid UTF-8, but possibly truncated.
 *
 * @param input         the UTF-8 string to process
 * @param length        the length of the string in bytes
 * @return the length of the string in bytes, possibly shorter by 1 to 3 bytes
 */
simdutf_warn_unused size_t trim_partial_utf8(const char *input, size_t length);

/**
 * Given a valid UTF-16BE string having a possibly truncated last character,
 * this function checks the end of string. If the last character is truncated (or partial),
 * then it returns a shorter length (shorter by 1 unit) so that the short UTF-16BE
 * strings only contain complete characters. If there is no truncated character,
 * the original length is returned.
 *
 * This function assumes that the input string is valid UTF-16BE, but possibly truncated.
 *
 * @param input         the UTF-16BE string to process
 * @param length        the length of the string in bytes
 * @return the length of the string in bytes, possibly shorter by 1 unit
 */
simdutf_warn_unused size_t trim_partial_utf16be(const char16_t* input, size_t length);

/**
 * Given a valid UTF-16LE string having a possibly truncated last character,
 * this function checks the end of string. If the last character is truncated (or partial),
 * then it returns a shorter length (shorter by 1 unit) so that the short UTF-16LE
 * strings only contain complete characters. If there is no truncated character,
 * the original length is returned.
 *
 * This function assumes that the input string is valid UTF-16LE, but possibly truncated.
 *
 * @param input         the UTF-16LE string to process
 * @param length        the length of the string in bytes
 * @return the length of the string in unit, possibly shorter by 1 unit
 */
simdutf_warn_unused size_t trim_partial_utf16le(const char16_t* input, size_t length);


/**
 * Given a valid UTF-16 string having a possibly truncated last character,
 * this function checks the end of string. If the last character is truncated (or partial),
 * then it returns a shorter length (shorter by 1 unit) so that the short UTF-16
 * strings only contain complete characters. If there is no truncated character,
 * the original length is returned.
 *
 * This function assumes that the input string is valid UTF-16, but possibly truncated.
 * We use the native endianness.
 *
 * @param input         the UTF-16 string to process
 * @param length        the length of the string in bytes
 * @return the length of the string in unit, possibly shorter by 1 unit
 */
simdutf_warn_unused size_t trim_partial_utf16(const char16_t* input, size_t length);
```

You may use these `trim_` functions to decode inputs piece by piece, as in the following
examples. First a case where you want to decode a UTF-8 strings in two steps:

```cpp
  const char unicode[] = "\xc3\xa9\x63ole d'\xc3\xa9t\xc3\xa9";
  // suppose you want to decode only the start of this string.
  size_t length = 10;
  // Picking 10 bytes is problematic because we might end up in the middle of a
  // code point. But we can rewind to the previous code point.
  length = simdutf::trim_partial_utf8(unicode, length);
  // Now we can transcode safely
  size_t budget_utf16 = simdutf::utf16_length_from_utf8(unicode, length);
  std::unique_ptr<char16_t[]> utf16{new char16_t[budget_utf16]};
  size_t utf16words =
      simdutf::convert_utf8_to_utf16le(unicode, length, utf16.get());
  // We can then transcode the next batch
  const char * next = unicode + length;
  size_t next_length = sizeof(unicode) - length;
  size_t next_budget_utf16 = simdutf::utf16_length_from_utf8(next, next_length);
  std::unique_ptr<char16_t[]> next_utf16{new char16_t[next_budget_utf16]};
  size_t next_utf16words =
      simdutf::convert_utf8_to_utf16le(next, next_length, next_utf16.get());
```

You can use the same approach with UTF-16:

```cpp  // We have three sequences of surrogate pairs (UTF-16).
  const char16_t unicode[] = u"\x3cd8\x10df\x3cd8\x10df\x3cd8\x10df";
  // suppose you want to decode only the start of this string.
  size_t length = 3;
  // Picking 3 units is problematic because we might end up in the middle of a
  // surrogate pair. But we can rewind to the previous code point.
  length = simdutf::trim_partial_utf16(unicode, length);
  // Now we can transcode safely
  size_t budget_utf8 = simdutf::utf8_length_from_utf16(unicode, length);
  std::unique_ptr<char[]> utf8{new char[budget_utf8]};
  size_t utf8words =
      simdutf::convert_utf16_to_utf8(unicode, length, utf8.get());
  // We can then transcode the next batch
  const char16_t * next = unicode + length;
  size_t next_length = 6 - length;
  size_t next_budget_utf8 = simdutf::utf8_length_from_utf16(next, next_length);
  std::unique_ptr<char[]> next_utf8{new char[next_budget_utf8]};
  size_t next_utf8words =
      simdutf::convert_utf16_to_utf8(next, next_length, next_utf8.get());
```


We have more advanced conversion functions which output a `simdutf::result` structure with
an indication of the error type and a `count` entry (e.g., `convert_utf8_to_utf16le_with_errors`).
They are well suited when you expect that there might be errors in the input that require
further investigation. The `count` field contains the location of the error in the input in code units,
if there is an error, or otherwise the number of code units written. You may use these functions as follows:

```cpp
  // this UTF-8 string has a bad byte at index 5
  std::string bad_utf8 = "\xc3\xa9\xc3\xa9\x20\xff\xc3\xa9";
  size_t budget_utf16 = simdutf::utf16_length_from_utf8(bad_utf8.data(), bad_utf8.size());
  std::unique_ptr<char16_t[]> utf16{new char16_t[budget_utf16]};
  simdutf::result res = simdutf::convert_utf8_to_utf16_with_errors(bad_utf8.data(), bad_utf8.size(), utf16.get());
  if(res.error != simdutf::error_code::SUCCESS) {
    std::cerr << "error at index " << res.count << std::endl;
  }
  // the following will be successful
  res = simdutf::convert_utf8_to_utf16_with_errors(bad_utf8.data(), res.count, utf16.get());
  if(res.error == simdutf::error_code::SUCCESS) {
    std::cerr << "we have transcoded " << res.count << " characters" << std::endl;
  }
```

We have several transcoding functions returning `simdutf::error` results:

```cpp
/**
 * Convert possibly broken UTF-8 string into Latin1 string with errors.
 * If the string cannot be represented as Latin1, an error
 * code is returned.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the UTF-8 string to convert
 * @param length        the length of the string in bytes
 * @param latin1_output  the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of code units validated if successful.
 */
simdutf_warn_unused result convert_utf8_to_latin1_with_errors(const char * input, size_t length, char* latin1_output) noexcept;

/**
 * Convert possibly broken UTF-16LE string into Latin1 string.
 * If the string cannot be represented as Latin1, an error
 * is returned.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16LE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param latin1_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char written if successful.
 */
simdutf_warn_unused result convert_utf16le_to_latin1_with_errors(const char16_t * input, size_t length, char* latin1_buffer) noexcept;

/**
 * Convert possibly broken UTF-16BE string into Latin1 string.
 * If the string cannot be represented as Latin1, an error
 * is returned.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16BE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param latin1_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char written if successful.
 */
simdutf_warn_unused result convert_utf16be_to_latin1_with_errors(const char16_t * input, size_t length, char* latin1_buffer) noexcept;

/**
 * Using native endianness, convert possibly broken UTF-16 string into Latin1 string.
 * If the string cannot be represented as Latin1, an error
 * is returned.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16 string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param latin1_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char written if successful.
 */
simdutf_warn_unused result convert_utf16_to_latin1_with_errors(const char16_t * input, size_t length, char* latin1_buffer) noexcept;


/**
 * Using native endianness; Convert possibly broken UTF-8 string into UTF-16
 * string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the UTF-8 string to convert
 * @param length        the length of the string in bytes
 * @param utf16_buffer  the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char16_t written if successful.
 */
simdutf_warn_unused result convert_utf8_to_utf16_with_errors(const char * input, size_t length, char16_t* utf16_output) noexcept;

/**
 * Convert possibly broken UTF-8 string into UTF-16LE string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the UTF-8 string to convert
 * @param length        the length of the string in bytes
 * @param utf16_buffer  the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char16_t written if successful.
 */
simdutf_warn_unused result convert_utf8_to_utf16le_with_errors(const char * input, size_t length, char16_t* utf16_output) noexcept;

/**
 * Convert possibly broken UTF-8 string into UTF-16BE string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the UTF-8 string to convert
 * @param length        the length of the string in bytes
 * @param utf16_buffer  the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char16_t written if successful.
 */
simdutf_warn_unused result convert_utf8_to_utf16be_with_errors(const char * input, size_t length, char16_t* utf16_output) noexcept;

/**
 * Convert possibly broken UTF-8 string into UTF-32 string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * @param input         the UTF-8 string to convert
 * @param length        the length of the string in bytes
 * @param utf32_buffer  the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char32_t written if successful.
 */
simdutf_warn_unused result convert_utf8_to_utf32_with_errors(const char * input, size_t length, char32_t* utf32_output) noexcept;


/**
 * Convert possibly broken UTF-16LE string into UTF-8 string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16LE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param utf8_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char written if successful.
 */
simdutf_warn_unused result convert_utf16le_to_utf8_with_errors(const char16_t * input, size_t length, char* utf8_buffer) noexcept;

/**
 * Convert possibly broken UTF-16BE string into UTF-8 string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16BE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param utf8_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char written if successful.
 */
simdutf_warn_unused result convert_utf16be_to_utf8_with_errors(const char16_t * input, size_t length, char* utf8_buffer) noexcept;


/**
 * Convert possibly broken UTF-32 string into Latin1 string and stop on error.
 * If the string cannot be represented as Latin1, an error is returned.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @param latin1_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char written if successful.
 */
simdutf_warn_unused result convert_utf32_to_latin1_with_errors(const char32_t * input, size_t length, char* latin1_buffer) noexcept;


/**
 * Convert possibly broken UTF-32 string into UTF-8 string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @param utf8_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char written if successful.
 */
simdutf_warn_unused result convert_utf32_to_utf8_with_errors(const char32_t * input, size_t length, char* utf8_buffer) noexcept;

/**
 * Using native endianness; Convert possibly broken UTF-32 string into UTF-16
 * string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @param utf16_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char16_t written if successful.
 */
simdutf_warn_unused result convert_utf32_to_utf16_with_errors(const char32_t * input, size_t length, char16_t* utf16_buffer) noexcept;

/**
 * Convert possibly broken UTF-32 string into UTF-16LE string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @param utf16_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char16_t written if successful.
 */
simdutf_warn_unused result convert_utf32_to_utf16le_with_errors(const char32_t * input, size_t length, char16_t* utf16_buffer) noexcept;

/**
 * Convert possibly broken UTF-32 string into UTF-16BE string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-32 string to convert
 * @param length        the length of the string in 4-byte code units (char32_t)
 * @param utf16_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char16_t written if successful.
 */
simdutf_warn_unused result convert_utf32_to_utf16be_with_errors(const char32_t * input, size_t length, char16_t* utf16_buffer) noexcept;

/**
 * Using native endianness; Convert possibly broken UTF-16 string into
 * UTF-32 string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16 string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param utf32_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char32_t written if successful.
 */
simdutf_warn_unused result convert_utf16_to_utf32_with_errors(const char16_t * input, size_t length, char32_t* utf32_buffer) noexcept;

/**
 * Convert possibly broken UTF-16LE string into UTF-32 string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16LE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param utf32_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char32_t written if successful.
 */
simdutf_warn_unused result convert_utf16le_to_utf32_with_errors(const char16_t * input, size_t length, char32_t* utf32_buffer) noexcept;

/**
 * Convert possibly broken UTF-16BE string into UTF-32 string and stop on error.
 *
 * During the conversion also validation of the input string is done.
 * This function is suitable to work with inputs from untrusted sources.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16BE string to convert
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param utf32_buffer   the pointer to buffer that can hold conversion result
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the error (in the input in code units) if any, or the number of char32_t written if successful.
 */
simdutf_warn_unused result convert_utf16be_to_utf32_with_errors(const char16_t * input, size_t length, char32_t* utf32_buffer) noexcept;

```

If you have a UTF-16 input, you may change its endianness with a fast function.

```cpp
/**
 * Change the endianness of the input. Can be used to go from UTF-16LE to UTF-16BE or
 * from UTF-16BE to UTF-16LE.
 *
 * This function does not validate the input.
 *
 * This function is not BOM-aware.
 *
 * @param input         the UTF-16 string to process
 * @param length        the length of the string in 2-byte code units (char16_t)
 * @param output        the pointer to a buffer that can hold the conversion result
 */
void change_endianness_utf16(const char16_t * input, size_t length, char16_t * output) noexcept;

```

Base64
-----

The WHATWG (Web Hypertext Application Technology Working Group) defines a "forgiving" base64 decoding algorithm in its Infra Standard, which is used in web contexts like the JavaScript atob() function. This algorithm is more lenient than strict RFC 4648 base64, primarily to handle common web data variations. It ignores all ASCII whitespace (spaces, tabs, newlines, etc.), allows omitting padding characters (=), and decodes inputs as long as they meet certain length and character validity rules. However, it still rejects inputs that could lead to ambiguous or incomplete byte formation.

We also converting from [WHATWG forgiving-base64](https://infra.spec.whatwg.org/#forgiving-base64-decode) to binary, and back. In particular, you can convert base64 inputs which contain ASCII spaces (' ', '\t', '\n', '\r', '\f') to binary. We also support the base64 URL encoding alternative. These functions are part of the Node.js JavaScript runtime: in particular `atob` in Node.js relies on simdutf.


The key steps in this algorithm are:
- Remove all whitespace from the input string.
- If the resulting string's length is a multiple of 4 and it ends with one or two '=' characters, remove those '=' from the end (treating them as optional padding).
- If the length (after any padding removal) modulo 4 equals 1, the input is invalid— this prevents cases where the bit count wouldn't align properly to form whole bytes.
- Check that all remaining characters are valid base64 symbols (A-Z, a-z, 0-9, +, /, or =); otherwise, invalid.
- Decode by converting each character to its 6-bit value, concatenating the bits, and grouping them into 8-bit bytes. At the end, if there are leftover bits (12 or 18), form as many full bytes as possible and discard the trailing bits (4 or 2, respectively), assuming they are padding zeros.

This forgiving approach makes base64 decoding robust for web use, but it enforces rules to avoid data corruption.

The conversion of binary data to base64 always succeeds and is relatively simple. Suppose
that you have an original input of binary data `source` (e.g., `std::vector<char>`).
```C++
std::vector<char> buffer(simdutf::base64_length_from_binary(source.size()));
simdutf::binary_to_base64(source.data(), source.size(), buffer.data());
```

Decoding base64 requires validation and, thus, error handling. Furthermore, because
we prune ASCII spaces, we may need to adjust the result size afterward.

```C++
std::vector<char> buffer(simdutf::maximal_binary_length_from_base64(base64.data(), base64.size()));
simdutf::result r = simdutf::base64_to_binary(base64.data(), base64.size(), buffer.data());
if(r.error) {
  // We have some error, r.count tells you where the error was encountered in the input if
  // the error is INVALID_BASE64_CHARACTER. If the error is BASE64_INPUT_REMAINDER, then
  // a single valid base64 character remained, and r.count contains the number of bytes decoded.
} else {
  buffer.resize(r.count); // resize the buffer according to actual number of bytes
}
```

Let us consider concrete examples.  Take the following strings:
`"  A  A  "`, `"  A  A  G  A  /  v  8  "`, `"  A  A  G  A  /  v  8  =  "`, `"  A  A  G  A  /  v  8  =  =  "`.
They are all valid WHATWG base64 inputs, except for the last one.

- The first string, `"  A  A  "`, becomes "AA" after whitespace removal. Its length is 2, and 2 % 4 = 2 (not 1), so it's valid. Decoding: 'A' is 000000 and 'A' is 000000, giving 12 bits (000000000000). Form one byte from the first 8 bits (00000000 = 0x00) and discard the last 4 bits (0000). Result: a single byte value of 0.
- The second string, `"  A  A  G  A  /  v  8  "`, becomes "AAGA/v8" (length 7, 7 % 4 = 3, not 1—valid). Decoding the 42 bits yields the byte sequence 0x00, 0x01, 0x80, 0xFE, 0xFF (as you noted; the process groups full 24-bit chunks into three bytes each, then handles the remaining 18 bits as two bytes, discarding the last 2 bits).
- The third string, `"  A  A  G  A  /  v  8  =  "`, becomes "AAGA/v8=" (length 8, 8 % 4 = 0). It ends with one '=', so remove it, leaving "AAGA/v8" (same as the second example). Valid, and decodes to the same byte sequence: 0x00, 0x01, 0x80, 0xFE, 0xFF.
- The fourth string, `"  A  A  G  A  /  v  8  =  =  "`, becomes "AAGA/v8==" (length 9, 9 % 4 = 1). The length isn't a multiple of 4, so the algorithm doesn't remove the trailing '=='. Since the length modulo 4 is 1, it's invalid. This rule exists because a remainder of 1 would leave only 6 leftover bits after full bytes, which can't form a complete byte (unlike remainders of 2 or 3, which leave 12 or 18 bits and allow discarding 4 or 2 bits). Adding extra '=' here disrupts the expected alignment without qualifying for padding removal.

Let us process them with actual code.

```C++
  std::vector<std::string> sources = {
      "  A  A  ", "  A  A  G  A  /  v  8  ", "  A  A  G  A  /  v  8  =  ", "  A  A  G  A  /  v  8  =  =  "};
  std::vector<std::vector<uint8_t>> expected = {
      {0}, {0, 0x1, 0x80, 0xfe, 0xff}, {0, 0x1, 0x80, 0xfe, 0xff}, {}}; // last one is in error
  for(size_t i = 0; i < sources.size(); i++) {
    const std::string &source = sources[i];
    std::cout << "source: '" << source << "'" << std::endl;
    // allocate enough memory for the maximal binary length
    std::vector<uint8_t> buffer(simdutf::maximal_binary_length_from_base64(
       source.data(), source.size()));
    // convert to binary and check for errors
    simdutf::result r = simdutf::base64_to_binary(
        source.data(), source.size(), (char*)buffer.data());
    if(r.error != simdutf::error_code::SUCCESS) {
      // We have that expected[i].empty().
      std::cout << "output: error" << std::endl;
    } else {
      buffer.resize(r.count); // in case of success, r.count contains the output length
      // We have that buffer == expected[i]
      std::cout << "output: " << r.count << " bytes" << std::endl;
    }
  }
```

This code should print the following:

```
source: '  A  A  '
output: 1 bytes
source: '  A  A  G  A  /  v  8  '
output: 5 bytes
source: '  A  A  G  A  /  v  8  =  '
output: 5 bytes
source: '  A  A  G  A  /  v  8  =  =  '
output: error
```

As you can see, the result is as expected.

In some instances, you may want to limit the size of the output further when decoding base64.
For this purpose, you may use the `base64_to_binary_safe` functions. The functions may also
be useful if you seek to decode the input into segments having a maximal capacity.
Another benefit of the `base64_to_binary_safe` functions is that they inform you
about how much data was written to the output buffer, even when there is a fatal
error.
This number might not be 'maximal': our fast functions may leave some data that could
have been decoded prior to a bad character undecode. With the
`base64_to_binary_safe` function, you also have the option of requesting that as much
of the data as possible is decoded despite the error by setting the `decode_up_to_bad_char`
parameter to true (it defaults to false for best performance).


```C++
  size_t len = 72; // for simplicity we chose len divisible by 3
  std::vector<char> base64(len, 'a'); // we want to decode 'aaaaa....'
  std::vector<char> back((len + 3) / 4 * 3);
  size_t limited_length = back.size() / 2; // Intentionally too small
  // We proceed to decode half:
  simdutf::result r = simdutf::base64_to_binary_safe(
            base64.data(), base64.size(), back.data(), limited_length);
  assert(r.error == simdutf::error_code::OUTPUT_BUFFER_TOO_SMALL);
  // We decoded r.count base64 8-bit units to limited_length bytes
  // Now let us decode the rest !!!
  //
  // We have read up to r.count in the input buffer and we have
  // produced limited_length bytes.
  //
  size_t input_index = r.count;
  size_t limited_length2 = back.size();
  r = simdutf::base64_to_binary_safe(base64.data() + input_index,
                                           base64.size() - input_index,
                                           back.data(), limited_length2);
  assert(r.error == simdutf::error_code::SUCCESS);
  // We decoded r.count base64 8-bit units to limited_length2 bytes
  // We are done
  assert(limited_length2 + limited_length == (len + 3) / 4 * 3);
```

We can repeat our previous examples with the various spaced strings using
`base64_to_binary_safe`. It works much the same except that the convention
for the content of `result.count` differs. The output size is stored
by reference in the output length parameter.

```cpp

 std::vector<std::string> sources = {
      "  A  A  ", "  A  A  G  A  /  v  8  ", "  A  A  G  A  /  v  8  =  ", "  A  A  G  A  /  v  8  =  =  "};
  std::vector<std::vector<uint8_t>> expected = {
      {0}, {0, 0x1, 0x80, 0xfe, 0xff}, {0, 0x1, 0x80, 0xfe, 0xff}, {}}; // last one is in error
  for(size_t i = 0; i < sources.size(); i++) {
    const std::string &source = sources[i];
    std::cout << "source: '" << source << "'" << std::endl;
    // allocate enough memory for the maximal binary length
    std::vector<uint8_t> buffer(simdutf::maximal_binary_length_from_base64(
       source.data(), source.size()));
    // convert to binary and check for errors
    size_t output_length = buffer.size();
    simdutf::result r = simdutf::base64_to_binary_safe(
        source.data(), source.size(), (char*)buffer.data(), output_length);
    if(r.error != simdutf::error_code::SUCCESS) {
      // We have expected[i].empty()
      std::cout << "output: error" << std::endl;
    } else {
      buffer.resize(output_length); // in case of success, output_length contains the output length
      // We have buffer == expected[i])
      std::cout << "output: " << output_length << " bytes" << std::endl;
      std::cout << "input (consumed): " << r.count << " bytes" << std::endl;
    }
```

This code should output the following:

```
source: '  A  A  '
output: 1 bytes
input (consumed): 8 bytes
source: '  A  A  G  A  /  v  8  '
output: 5 bytes
input (consumed): 23 bytes
source: '  A  A  G  A  /  v  8  =  '
output: 5 bytes
input (consumed): 26 bytes
source: '  A  A  G  A  /  v  8  =  =  '
output: error
```

See our function specifications for more details.

In other instances, you may receive your base64 inputs in 16-bit units (e.g., from UTF-16 strings):
we have function overloads for these cases as well.

Some users may want to decode the base64 inputs in chunks, especially when doing
file or networking programming. These users should see `tools/fastbase64.cpp`, a command-line
utility designed for as an example. It reads and writes base64 files using chunks of at most
a few tens of kilobytes.


We support two conventions: `base64_default` and `base64_url`:
* The default (`base64_default`) includes the characters `+` and `/` as part of its alphabet. It also
  pads the output with the padding character (`=`) so that the output is divisible by 4. Thus, we have
  that the string `"Hello, World!"` is encoded to `"SGVsbG8sIFdvcmxkIQ=="` with an expression such as
  `simdutf::binary_to_base64(source, size, out, simdutf::base64_default)`.
  When using the default, you can omit the option parameter for simplicity:
  `simdutf::binary_to_base64(source, size, out, buffer.data())`. When decoding, white space
  characters are omitted as per the [WHATWG forgiving-base64](https://infra.spec.whatwg.org/#forgiving-base64-decode) standard. Further, if padding characters are present at the end of the
  stream, there must be no more than two, and if there are any, the total number of characters (excluding
  ASCII spaces ' ', '\t', '\n', '\r', '\f' but including padding characters) must be divisible by four.
* The URL convention (`base64_url`) uses the characters `-` and `_` as part of its alphabet. It does
  not pad its output. Thus, we have that the string `"Hello, World!"` is encoded to `"SGVsbG8sIFdvcmxkIQ"` instead of `"SGVsbG8sIFdvcmxkIQ=="`. To specify the URL convention, you can pass the appropriate option to our decoding and encoding functions: e.g., `simdutf::base64_to_binary(source, size, out, simdutf::base64_url)`.

When we encounter a character that is neither an ASCII space nor a base64 character (a garbage character), we detect an error. To tolerate 'garbage' characters, you can use `base64_default_accept_garbage` or `base64_url_accept_garbage` instead of `base64_default` or `base64_url`.

Thus we follow the convention of systems such as the Node or Bun JavaScript runtimes with respect to padding. The
default base64 uses padding whereas the URL variant does not.

```JavaScript
> console.log(Buffer.from("Hello World").toString('base64'));
SGVsbG8gV29ybGQ=
undefined
> console.log(Buffer.from("Hello World").toString('base64url'));
SGVsbG8gV29ybGQ
```

This is justified as per [RFC 4648](https://www.rfc-editor.org/rfc/rfc4648):

> The pad character "=" is typically percent-encoded when used in an URI, but if the data length is known implicitly, this can be avoided by skipping the padding; see section 3.2.

Nevertheless, some users may want to use padding with the URL variant
and omit it with the default variant. These users can
'reverse' the convention by using `simdutf::base64_url |  simdutf::base64_reverse_padding` or `simdutf::base64_default | simdutf::base64_reverse_padding`.
For greater convenience, you may use `simdutf::base64_default_no_padding` and
`simdutf::base64_url_with_padding`, as shorthands.

When decoding, by default we use a loose approach: the padding character may be omitted.
Advanced users may use the `last_chunk_options` parameter to use either a `strict` approach,
where precise padding must be used or an error is generated, or the `stop_before_partial`
option which discards leftover base64 characters when the padding is not appropriate.
The `stop_before_partial` option might be appropriate for streaming applications
where you expect to get part of the base64 stream.
The `strict` approach is useful if you want to have one-to-one correspondence between
the base64 code and the binary data. If the default setting is used (`last_chunk_handling_options::loose`),
then `"ZXhhZg=="`, `"ZXhhZg"`, `"ZXhhZh=="` all decode to the same binary content.
If `last_chunk_options` is set to `last_chunk_handling_options::strict`, then
decoding `"ZXhhZg=="` succeeds, but decoding `"ZXhhZg"` fails with `simdutf::error_code::BASE64_INPUT_REMAINDER` while `"ZXhhZh=="` fails with
`simdutf::error_code::BASE64_EXTRA_BITS`. If `last_chunk_options` is set to `last_chunk_handling_options::stop_before_partial`,
then decoding `"ZXhhZg"` decodes into `exa` (and `Zg` is left over).

The specification of our base64 functions is as follows:

```C++

// base64_options are used to specify the base64 encoding options.
// ASCII spaces are ' ', '\t', '\n', '\r', '\f'
// garbage characters are characters that are not part of the base64 alphabet nor ASCII spaces.
using base64_options = uint64_t;
enum base64_options : uint64_t {
  base64_default = 0, /* standard base64 format (with padding) */
  base64_url = 1,     /* base64url format (no padding) */
  base64_default_no_padding =
      base64_default |
      base64_reverse_padding, /* standard base64 format without padding */
  base64_url_with_padding =
      base64_url | base64_reverse_padding, /* base64url with padding */
  base64_default_accept_garbage =
      4, /* standard base64 format accepting garbage characters, the input stops with the first '=' if any */
  base64_url_accept_garbage =
      5, /* base64url format accepting garbage characters, the input stops with the first '=' if any */
  base64_default_or_url =
      8, /* standard/base64url hybrid format (only meaningful for decoding!) */
  base64_default_or_url_accept_garbage =
      12, /* standard/base64url hybrid format accepting garbage characters
             (only meaningful for decoding!), the input stops with the first '=' if any */
};

// last_chunk_handling_options are used to specify the handling of the last
// chunk in base64 decoding.
// https://tc39.es/proposal-arraybuffer-base64/spec/#sec-frombase64
enum last_chunk_handling_options : uint64_t {
  loose = 0,               /* standard base64 format, decode partial final chunk */
  strict = 1,              /* error when the last chunk is partial, 2 or 3 chars, and unpadded, or non-zero bit padding */
  stop_before_partial = 2, /* if the last chunk is partial , ignore it (no error) */
  only_full_chunks = 3 /* only decode full blocks (4 base64 characters, no padding) */
};

/**
 * Provide the maximal binary length in bytes given the base64 input.
 * In general, if the input contains ASCII spaces, the result will be less than
 * the maximum length.
 *
 * @param input         the base64 input to process
 * @param length        the length of the base64 input in bytes
 * @return maximal number of binary bytes
 */
simdutf_warn_unused size_t maximal_binary_length_from_base64(const char * input, size_t length) noexcept;

/**
 * Provide the maximal binary length in bytes given the base64 input.
 * In general, if the input contains ASCII spaces, the result will be less than
 * the maximum length.
 *
 * @param input         the base64 input to process, in ASCII stored as 16-bit units
 * @param length        the length of the base64 input in 16-bit units
 * @return maximal number of binary bytes
 */
simdutf_warn_unused size_t maximal_binary_length_from_base64(const char16_t * input, size_t length) noexcept;


/**
 * Convert a base64 input to a binary output.
 *
 * This function follows the WHATWG forgiving-base64 format, which means that it
 * will ignore any ASCII spaces in the input. You may provide a padded input
 * (with one or two equal signs at the end) or an unpadded input (without any
 * equal signs at the end).
 *
 * See https://infra.spec.whatwg.org/#forgiving-base64-decode
 *
 * This function will fail in case of invalid input. When last_chunk_options = loose,
 * there are two possible reasons for failure: the input contains a number of
 * base64 characters that when divided by 4, leaves a single remainder character
 * (BASE64_INPUT_REMAINDER), or the input contains a character that is not a
 * valid base64 character (INVALID_BASE64_CHARACTER).
 *
 * When the error is INVALID_BASE64_CHARACTER, r.count contains the index in the
 * input where the invalid character was found. When the error is
 * BASE64_INPUT_REMAINDER, then r.count contains the number of bytes decoded.
 *
 * The default option (simdutf::base64_default) expects the characters `+` and
 * `/` as part of its alphabet. The URL option (simdutf::base64_url) expects the
 * characters `-` and `_` as part of its alphabet.
 *
 * The padding (`=`) is validated if present. There may be at most two padding
 * characters at the end of the input. If there are any padding characters, the
 * total number of characters (excluding spaces but including padding
 * characters) must be divisible by four.
 *
 * You should call this function with a buffer that is at least
 * maximal_binary_length_from_base64(input, length) bytes long. If you fail to
 * provide that much space, the function may cause a buffer overflow.
 *
 * Advanced users may want to tailor how the last chunk is handled. By default,
 * we use a loose (forgiving) approach but we also support a strict approach
 * as well as a stop_before_partial approach, as per the following proposal:
 *
 * https://tc39.es/proposal-arraybuffer-base64/spec/#sec-frombase64
 *
 * @param input         the base64 string to process
 * @param length        the length of the string in bytes
 * @param output        the pointer to a buffer that can hold the conversion
 * result (should be at least maximal_binary_length_from_base64(input, length)
 * bytes long).
 * @param options       the base64 options to use, usually base64_default or
 * base64_url, and base64_default by default.
 * @param last_chunk_options the last chunk handling options,
 * last_chunk_handling_options::loose by default
 * but can also be last_chunk_handling_options::strict or
 * last_chunk_handling_options::stop_before_partial.
 * @return a result pair struct (of type simdutf::result containing the two
 * fields error and count) with an error code and either position of the error
 * (in the input in bytes) if any, or the number of bytes written if successful.
 */
simdutf_warn_unused result
base64_to_binary(const char *input, size_t length, char *output,
                 base64_options options = base64_default,
                 last_chunk_handling_options last_chunk_options = loose) noexcept;

/**
 * Provide the base64 length in bytes given the length of a binary input.
 *
 * @param length        the length of the input in bytes
 * @parem options       the base64 options to use, can be base64_default or base64_url, is base64_default by default.
 * @return number of base64 bytes
 */
simdutf_warn_unused size_t base64_length_from_binary(size_t length, base64_options options = base64_default) noexcept;


/**
 * Convert a binary input to a base64 output.
 *
 * The default option (simdutf::base64_default) uses the characters `+` and `/` as part of its alphabet.
 * Further, it adds padding (`=`) at the end of the output to ensure that the output length is a multiple of four.
 *
 * The URL option (simdutf::base64_url) uses the characters `-` and `_` as part of its alphabet. No padding
 * is added at the end of the output.
 *
 * This function always succeeds.
 *
 * @param input         the binary to process
 * @param length        the length of the input in bytes
 * @param output        the pointer to a buffer that can hold the conversion result (should be at least base64_length_from_binary(length) bytes long)
 * @param options       the base64 options to use, can be base64_default or base64_url, is base64_default by default.
 * @return number of written bytes, will be equal to base64_length_from_binary(length, options)
 */
size_t binary_to_base64(const char * input, size_t length, char* output, base64_options options = base64_default) noexcept;

/**
 * Convert a base64 input to a binary output.
 *
 * This function follows the WHATWG forgiving-base64 format, which means that it will
 * ignore any ASCII spaces in the input. You may provide a padded input (with one or two
 * equal signs at the end) or an unpadded input (without any equal signs at the end).
 *
 * See https://infra.spec.whatwg.org/#forgiving-base64-decode
 *
 * This function will fail in case of invalid input. When last_chunk_options = loose,
 * there are two possible reasons for failure: the input contains a number of
 * base64 characters that when divided by 4, leaves a single remainder character
 * (BASE64_INPUT_REMAINDER), or the input contains a character that is not a
 * valid base64 character (INVALID_BASE64_CHARACTER).
 *
 * When the error is INVALID_BASE64_CHARACTER, r.count contains the index in the input
 * where the invalid character was found. When the error is BASE64_INPUT_REMAINDER, then
 * r.count contains the number of bytes decoded.
 *
 * You should call this function with a buffer that is at least maximal_binary_length_from_base64(input, length) bytes long.
 * If you fail to provide that much space, the function may cause a buffer overflow.
 *
 * Advanced users may want to tailor how the last chunk is handled. By default,
 * we use a loose (forgiving) approach but we also support a strict approach
 * as well as a stop_before_partial approach, as per the following proposal:
 *
 * https://tc39.es/proposal-arraybuffer-base64/spec/#sec-frombase64
 *
 * @param input         the base64 string to process, in ASCII stored as 16-bit units
 * @param length        the length of the string in 16-bit units
 * @param output        the pointer to a buffer that can hold the conversion result (should be at least maximal_binary_length_from_base64(input, length) bytes long).
 * @param options       the base64 options to use, can be base64_default or base64_url, is base64_default by default.
 * @param last_chunk_options the last chunk handling options,
 * last_chunk_handling_options::loose by default
 * but can also be last_chunk_handling_options::strict or
 * last_chunk_handling_options::stop_before_partial.
 * @return a result pair struct (of type simdutf::result containing the two fields error and count) with an error code and either position of the INVALID_BASE64_CHARACTER error (in the input in units) if any, or the number of bytes written if successful.
 */
simdutf_warn_unused result base64_to_binary(const char16_t * input, size_t length, char* output, base64_options options = base64_default, last_chunk_handling_options last_chunk_options =
                     last_chunk_handling_options::loose)  noexcept;


/**
 * Convert a base64 input to a binary output.
 *
 * This function follows the WHATWG forgiving-base64 format, which means that it
 * will ignore any ASCII spaces in the input. You may provide a padded input
 * (with one or two equal signs at the end) or an unpadded input (without any
 * equal signs at the end).
 *
 * See https://infra.spec.whatwg.org/#forgiving-base64-decode
 *
 * This function will fail in case of invalid input. When last_chunk_options =
 * loose, there are three possible reasons for failure: the input contains a
 * number of base64 characters that when divided by 4, leaves a single remainder
 * character (BASE64_INPUT_REMAINDER), the input contains a character that is
 * not a valid base64 character (INVALID_BASE64_CHARACTER), or the output buffer
 * is too small (OUTPUT_BUFFER_TOO_SMALL).
 *
 * When OUTPUT_BUFFER_TOO_SMALL, we return both the number of bytes written
 * and the number of units processed, see description of the parameters and
 * returned value.
 *
 * When the error is INVALID_BASE64_CHARACTER, r.count contains the index in the
 * input where the invalid character was found. When the error is
 * BASE64_INPUT_REMAINDER, then r.count contains the number of bytes decoded.
 *
 * The default option (simdutf::base64_default) expects the characters `+` and
 * `/` as part of its alphabet. The URL option (simdutf::base64_url) expects the
 * characters `-` and `_` as part of its alphabet.
 *
 * The padding (`=`) is validated if present. There may be at most two padding
 * characters at the end of the input. If there are any padding characters, the
 * total number of characters (excluding spaces but including padding
 * characters) must be divisible by four.
 *
 * The INVALID_BASE64_CHARACTER cases are considered fatal and you are expected
 * to discard the output unless the parameter decode_up_to_bad_char is set to
 * true. In that case, the function will decode up to the first invalid character.
 * Extra padding characters ('=') are considered invalid characters.
 *
 * Advanced users may want to tailor how the last chunk is handled. By default,
 * we use a loose (forgiving) approach but we also support a strict approach
 * as well as a stop_before_partial approach, as per the following proposal:
 *
 * https://tc39.es/proposal-arraybuffer-base64/spec/#sec-frombase64
 *
 * @param input         the base64 string to process, in ASCII stored as 8-bit
 * or 16-bit units
 * @param length        the length of the string in 8-bit or 16-bit units.
 * @param output        the pointer to a buffer that can hold the conversion
 * result.
 * @param outlen        the number of bytes that can be written in the output
 * buffer. Upon return, it is modified to reflect how many bytes were written.
 * @param options       the base64 options to use, can be base64_default or
 * base64_url, is base64_default by default.
 * @param last_chunk_options the last chunk handling options,
 * last_chunk_handling_options::loose by default
 * but can also be last_chunk_handling_options::strict or
 * last_chunk_handling_options::stop_before_partial.
 * @param decode_up_to_bad_char if true, the function will decode up to the
 * first invalid character. By default (false), it is assumed that the output
 * buffer is to be discarded. When there are multiple errors in the input,
 * using decode_up_to_bad_char might trigger a different error.
 * @return a result pair struct (of type simdutf::result containing the two
 * fields error and count) with an error code and position of the
 * INVALID_BASE64_CHARACTER error (in the input in units) if any, or the number
 * of units processed if successful.
 */
simdutf_warn_unused result base64_to_binary_safe(const char * input, size_t length, char* output, size_t& outlen, base64_options options = base64_default,
      last_chunk_handling_options last_chunk_options = loose,
      bool decode_up_to_bad_char = false) noexcept;
simdutf_warn_unused result base64_to_binary_safe(const char16_t * input, size_t length, char* output, size_t& outlen, base64_options options = base64_default,
      last_chunk_handling_options last_chunk_options = loose,
      bool decode_up_to_bad_char = false) noexcept;
```

Find
-----

The C++ standard library provides `std::find` for locating a character in a string, but its performance can be suboptimal on modern hardware. To address this, we introduce `simdutf::find`, a high-performance alternative optimized for recent processors using SIMD instructions. It operates on raw pointers (`char` or `char16_t`) for maximum efficiency.



```cpp
  std::string input = "abc";

  const char* result =
      simdutf::find(input.data(), input.data() + input.size(), 'c');
  // result should point at the letter 'c'
```

The `simdutf::find` interface is straightforward and efficient.


```cpp
/**
  * Find the first occurrence of a character in a string. If the character is
  * not found, return a pointer to the end of the string.
  * @param start        the start of the string
  * @param end          the end of the string
  * @param character    the character to find
  * @return a pointer to the first occurrence of the character in the string,
  * or a pointer to the end of the string if the character is not found.
  *
  */
simdutf_warn_unused const char *find(const char *start, const char *end,
                          char character) noexcept;
simdutf_warn_unused const char16_t *find(const char16_t *start, const char16_t *end,
                              char16_t character) noexcept;
```

# C++20 and std::span usage in simdutf

If you are compiling with C++20 or later, span support is enabled. This allows you to use simdutf in a safer and more expressive way, without manually handling pointers and sizes.

The span interface is easy to use. If you have a container like `std::vector` or `std::array`, you can pass the container directly. If you have a pointer and a size, construct a `std::span` and pass it.
When dealing with ranges of bytes (like `char`), anything that has a `std::span-like` interface (has appopriate `data()` and `size()` member functions) is accepted. Ranges of larger types are accepted as `std::span` arguments.

## Example

Suppose you want to convert a UTF-16 string to UTF-8:

```cpp
#include <simdutf.h>
#include <vector>
#include <span>
#include <string>

std::u16string utf16_input = u"Bonjour le monde";
std::vector<char> utf8_output(64); // ensure sufficient size

// Use std::span for input and output
size_t written = simdutf::convert_utf16_to_utf8_safe(utf16_input, utf8_output);
```


## Note
- You are still responsible for providing a sufficiently large output buffer, just as with the pointer/size API.


The sutf command-line tool
------
We also provide a command-line tool which can be build as follows:
```
cmake -B build && cmake --build build --target sutf
```
This command builds the executable in `./build/tool/` under most platforms. The sutf tool enables the user to easily transcode files from one encoding to another directly from the command line.
The usage is similar to [iconv](https://www.gnu.org/software/libiconv/) (see `sutf --help` for more details). The sutf command-line tool relies on the simdutf library functions for fast transcoding of supported
formats (UTF-8, UTF-16LE, UTF-16BE and UTF-32). If iconv is found on the system and simdutf does not support a conversion, the sutf tool falls back on iconv: a message lets the user know if iconv is available
during compilation. The following is an example of transcoding two input files to an output file, from UTF-8 to UTF-16LE:
```
sutf -f UTF-8 -t UTF-16LE -o output_file.txt first_input_file.txt second_input_file.txt
```

Manual implementation selection
-------------------------------

When compiling the llibrary for x64 processors, we build several implementations of each functions. At runtime, the best
implementation is picked automatically. Advanced users may want to pick a particular implementation, thus bypassing our
runtime detection. It is possible and even relatively convenient to do so. The following C++ program checks all the available
implementation, and selects one as the default:

```cpp
#include "simdutf.h"
#include <cstdlib>
#include <iostream>
#include <string>

int main() {
  // This is just a demonstration, not actual testing required.
  std::string source = "La vie est belle.";
  std::string chosen_implementation;
  for (auto &implementation : simdutf::get_available_implementations()) {
    if (!implementation->supported_by_runtime_system()) {
      continue;
    }
    bool validutf8 = implementation->validate_utf8(source.c_str(), source.size());
    if (!validutf8) {
      return EXIT_FAILURE;
    }
    std::cout << implementation->name() << ": " << implementation->description()
              << std::endl;
    chosen_implementation = implementation->name();
  }
  auto my_implementation =
      simdutf::get_available_implementations()[chosen_implementation];
  if (!my_implementation) {
    return EXIT_FAILURE;
  }
  if (!my_implementation->supported_by_runtime_system()) {
    return EXIT_FAILURE;
  }
  simdutf::get_active_implementation() = my_implementation;
  bool validutf8 = simdutf::validate_utf8(source.c_str(), source.size());
  if (!validutf8) {
    return EXIT_FAILURE;
  }
  if (simdutf::get_active_implementation()->name() != chosen_implementation) {
    return EXIT_FAILURE;
  }
  std::cout << "I have manually selected: " << simdutf::get_active_implementation()->name() << std::endl;
  return EXIT_SUCCESS;
}
```

Thread safety
-----------

We built simdutf with thread safety in mind. The simdutf library is single-threaded throughout.
The CPU detection, which runs the first time parsing is attempted and switches to the fastest parser for your CPU, is transparent and thread-safe. Our runtime dispatching is based on global objects that are instantiated at the beginning of the main thread and may be discarded at the end of the main thread. If you have multiple threads running and some threads use the library while the main thread is cleaning up ressources, you may encounter issues. If you expect such problems, you may consider using [std::quick_exit](https://en.cppreference.com/w/cpp/utility/program/quick_exit).


References
-----------

* Robert Clausecker, Daniel Lemire, [Transcoding Unicode Characters with AVX-512 Instructions](https://arxiv.org/abs/2212.05098),  Software: Practice and Experience 53 (12), 2023.
* Daniel Lemire, Wojciech Muła,  [Transcoding Billions of Unicode Characters per Second with SIMD Instructions](https://arxiv.org/abs/2109.10433), Software: Practice and Experience 52 (2), 2022.
* John Keiser, Daniel Lemire, [Validating UTF-8 In Less Than One Instruction Per Byte](https://arxiv.org/abs/2010.03090), Software: Practice and Experience 51 (5), 2021.
* Wojciech Muła, Daniel Lemire, [Base64 encoding and decoding at almost the speed of a memory copy](https://arxiv.org/abs/1910.05109), Software: Practice and Experience 50 (2), 2020.
* Wojciech Muła, Daniel Lemire, [Faster Base64 Encoding and Decoding using AVX2 Instructions](https://arxiv.org/abs/1704.00605), ACM Transactions on the Web 12 (3), 2018.


License
-------

This code is made available under the [Apache License 2.0](https://www.apache.org/licenses/LICENSE-2.0.html) as well as the MIT license. As a user, you can pick the license you prefer.

We include a few competitive solutions under the benchmarks/competition directory. They are provided for
research purposes only.
