Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

1 files changed, 666 insertions, 0 deletions

diff --git a/vendor/exr/src/compression/mod.rs b/vendor/exr/src/compression/mod.rs
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+
+//! Contains the compression attribute definition
+//! and methods to compress and decompress data.
+
+
+// private modules make non-breaking changes easier
+mod zip;
+mod rle;
+mod piz;
+mod pxr24;
+mod b44;
+
+
+use std::convert::TryInto;
+use std::mem::size_of;
+use half::f16;
+use crate::meta::attribute::{IntegerBounds, SampleType, ChannelList};
+use crate::error::{Result, Error, usize_to_i32};
+use crate::meta::header::Header;
+
+
+/// A byte vector.
+pub type ByteVec = Vec<u8>;
+
+/// A byte slice.
+pub type Bytes<'s> = &'s [u8];
+
+/// Specifies which compression method to use.
+/// Use uncompressed data for fastest loading and writing speeds.
+/// Use RLE compression for fast loading and writing with slight memory savings.
+/// Use ZIP compression for slow processing with large memory savings.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum Compression {
+
+    /// Store uncompressed values.
+    /// Produces large files that can be read and written very quickly.
+    /// Consider using RLE instead, as it provides some compression with almost equivalent speed.
+    Uncompressed,
+
+    /// Produces slightly smaller files
+    /// that can still be read and written rather quickly.
+    /// The compressed file size is usually between 60 and 75 percent of the uncompressed size.
+    /// Works best for images with large flat areas, such as masks and abstract graphics.
+    /// This compression method is lossless.
+    RLE,
+
+    /// Uses ZIP compression to compress each line. Slowly produces small images
+    /// which can be read with moderate speed. This compression method is lossless.
+    /// Might be slightly faster but larger than `ZIP16´.
+    ZIP1,  // TODO ZIP { individual_lines: bool, compression_level: Option<u8> }  // TODO specify zip compression level?
+
+    /// Uses ZIP compression to compress blocks of 16 lines. Slowly produces small images
+    /// which can be read with moderate speed. This compression method is lossless.
+    /// Might be slightly slower but smaller than `ZIP1´.
+    ZIP16, // TODO collapse with ZIP1
+
+    /// PIZ compression works well for noisy and natural images. Works better with larger tiles.
+    /// Only supported for flat images, but not for deep data.
+    /// This compression method is lossless.
+    // A wavelet transform is applied to the pixel data, and the result is Huffman-
+    // encoded. This scheme tends to provide the best compression ratio for the types of
+    // images that are typically processed at Industrial Light & Magic. Files are
+    // compressed and decompressed at roughly the same speed. For photographic
+    // images with film grain, the files are reduced to between 35 and 55 percent of their
+    // uncompressed size.
+    // PIZ compression works well for scan-line based files, and also for tiled files with
+    // large tiles, but small tiles do not shrink much. (PIZ-compressed data start with a
+    // relatively long header; if the input to the compressor is short, adding the header
+    // tends to offset any size reduction of the input.)
+    PIZ,
+
+    /// Like `ZIP1`, but reduces precision of `f32` images to `f24`.
+    /// Therefore, this is lossless compression for `f16` and `u32` data, lossy compression for `f32` data.
+    /// This compression method works well for depth
+    /// buffers and similar images, where the possible range of values is very large, but
+    /// where full 32-bit floating-point accuracy is not necessary. Rounding improves
+    /// compression significantly by eliminating the pixels' 8 least significant bits, which
+    /// tend to be very noisy, and therefore difficult to compress.
+    /// This produces really small image files. Only supported for flat images, not for deep data.
+    // After reducing 32-bit floating-point data to 24 bits by rounding (while leaving 16-bit
+    // floating-point data unchanged), differences between horizontally adjacent pixels
+    // are compressed with zlib, similar to ZIP. PXR24 compression preserves image
+    // channels of type HALF and UINT exactly, but the relative error of FLOAT data
+    // increases to about ???.
+    PXR24, // TODO specify zip compression level?
+
+    /// This is a lossy compression method for f16 images.
+    /// It's the predecessor of the `B44A` compression,
+    /// which has improved compression rates for uniformly colored areas.
+    /// You should probably use `B44A` instead of the plain `B44`.
+    ///
+    /// Only supported for flat images, not for deep data.
+    // lossy 4-by-4 pixel block compression,
+    // flat fields are compressed more
+    // Channels of type HALF are split into blocks of four by four pixels or 32 bytes. Each
+    // block is then packed into 14 bytes, reducing the data to 44 percent of their
+    // uncompressed size. When B44 compression is applied to RGB images in
+    // combination with luminance/chroma encoding (see below), the size of the
+    // compressed pixels is about 22 percent of the size of the original RGB data.
+    // Channels of type UINT or FLOAT are not compressed.
+    // Decoding is fast enough to allow real-time playback of B44-compressed OpenEXR
+    // image sequences on commodity hardware.
+    // The size of a B44-compressed file depends on the number of pixels in the image,
+    // but not on the data in the pixels. All images with the same resolution and the same
+    // set of channels have the same size. This can be advantageous for systems that
+    // support real-time playback of image sequences; the predictable file size makes it
+    // easier to allocate space on storage media efficiently.
+    // B44 compression is only supported for flat images.
+    B44, // TODO B44 { optimize_uniform_areas: bool }
+
+    /// This is a lossy compression method for f16 images.
+    /// All f32 and u32 channels will be stored without compression.
+    /// All the f16 pixels are divided into 4x4 blocks.
+    /// Each block is then compressed as a whole.
+    ///
+    /// The 32 bytes of a block will require only ~14 bytes after compression,
+    /// independent of the actual pixel contents. With chroma subsampling,
+    /// a block will be compressed to ~7 bytes.
+    /// Uniformly colored blocks will be compressed to ~3 bytes.
+    ///
+    /// The 512 bytes of an f32 block will not be compressed at all.
+    ///
+    /// Should be fast enough for realtime playback.
+    /// Only supported for flat images, not for deep data.
+    B44A, // TODO collapse with B44
+
+    /// __This lossy compression is not yet supported by this implementation.__
+    // lossy DCT based compression, in blocks
+    // of 32 scanlines. More efficient for partial buffer access.
+    DWAA(Option<f32>), // TODO does this have a default value? make this non optional? default Compression Level setting is 45.0
+
+    /// __This lossy compression is not yet supported by this implementation.__
+    // lossy DCT based compression, in blocks
+    // of 256 scanlines. More efficient space
+    // wise and faster to decode full frames
+    // than DWAA_COMPRESSION.
+    DWAB(Option<f32>), // TODO collapse with B44. default Compression Level setting is 45.0
+}
+
+impl std::fmt::Display for Compression {
+    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(formatter, "{} compression", match self {
+            Compression::Uncompressed => "no",
+            Compression::RLE => "rle",
+            Compression::ZIP1 => "zip line",
+            Compression::ZIP16 => "zip block",
+            Compression::B44 => "b44",
+            Compression::B44A => "b44a",
+            Compression::DWAA(_) => "dwaa",
+            Compression::DWAB(_) => "dwab",
+            Compression::PIZ => "piz",
+            Compression::PXR24 => "pxr24",
+        })
+    }
+}
+
+
+
+impl Compression {
+
+    /// Compress the image section of bytes.
+    pub fn compress_image_section(self, header: &Header, uncompressed_native_endian: ByteVec, pixel_section: IntegerBounds) -> Result<ByteVec> {
+        let max_tile_size = header.max_block_pixel_size();
+
+        assert!(pixel_section.validate(Some(max_tile_size)).is_ok(), "decompress tile coordinate bug");
+        if header.deep { assert!(self.supports_deep_data()) }
+
+        use self::Compression::*;
+        let compressed_little_endian = match self {
+            Uncompressed => {
+                return Ok(convert_current_to_little_endian(
+                    uncompressed_native_endian, &header.channels, pixel_section
+                ))
+            },
+
+            // we need to clone here, because we might have to fallback to the uncompressed data later (when compressed data is larger than raw data)
+            ZIP16 => zip::compress_bytes(&header.channels, uncompressed_native_endian.clone(), pixel_section),
+            ZIP1 => zip::compress_bytes(&header.channels, uncompressed_native_endian.clone(), pixel_section),
+            RLE => rle::compress_bytes(&header.channels, uncompressed_native_endian.clone(), pixel_section),
+            PIZ => piz::compress(&header.channels, uncompressed_native_endian.clone(), pixel_section),
+            PXR24 => pxr24::compress(&header.channels, uncompressed_native_endian.clone(), pixel_section),
+            B44 => b44::compress(&header.channels, uncompressed_native_endian.clone(), pixel_section, false),
+            B44A => b44::compress(&header.channels, uncompressed_native_endian.clone(), pixel_section, true),
+            _ => return Err(Error::unsupported(format!("yet unimplemented compression method: {}", self)))
+        };
+
+        let compressed_little_endian = compressed_little_endian.map_err(|_|
+            Error::invalid(format!("pixels cannot be compressed ({})", self))
+        )?;
+
+        if self == Uncompressed || compressed_little_endian.len() < uncompressed_native_endian.len() {
+            // only write compressed if it actually is smaller than raw
+            Ok(compressed_little_endian)
+        }
+        else {
+            // if we do not use compression, manually convert uncompressed data
+            Ok(convert_current_to_little_endian(uncompressed_native_endian, &header.channels, pixel_section))
+        }
+    }
+
+    /// Decompress the image section of bytes.
+    pub fn decompress_image_section(self, header: &Header, compressed: ByteVec, pixel_section: IntegerBounds, pedantic: bool) -> Result<ByteVec> {
+        let max_tile_size = header.max_block_pixel_size();
+
+        assert!(pixel_section.validate(Some(max_tile_size)).is_ok(), "decompress tile coordinate bug");
+        if header.deep { assert!(self.supports_deep_data()) }
+
+        let expected_byte_size = pixel_section.size.area() * header.channels.bytes_per_pixel; // FIXME this needs to account for subsampling anywhere
+
+        // note: always true where self == Uncompressed
+        if compressed.len() == expected_byte_size {
+            // the compressed data was larger than the raw data, so the small raw data has been written
+            Ok(convert_little_endian_to_current(compressed, &header.channels, pixel_section))
+        }
+        else {
+            use self::Compression::*;
+            let bytes = match self {
+                Uncompressed => Ok(convert_little_endian_to_current(compressed, &header.channels, pixel_section)),
+                ZIP16 => zip::decompress_bytes(&header.channels, compressed, pixel_section, expected_byte_size, pedantic),
+                ZIP1 => zip::decompress_bytes(&header.channels, compressed, pixel_section, expected_byte_size, pedantic),
+                RLE => rle::decompress_bytes(&header.channels, compressed, pixel_section, expected_byte_size, pedantic),
+                PIZ => piz::decompress(&header.channels, compressed, pixel_section, expected_byte_size, pedantic),
+                PXR24 => pxr24::decompress(&header.channels, compressed, pixel_section, expected_byte_size, pedantic),
+                B44 | B44A => b44::decompress(&header.channels, compressed, pixel_section, expected_byte_size, pedantic),
+                _ => return Err(Error::unsupported(format!("yet unimplemented compression method: {}", self)))
+            };
+
+            // map all errors to compression errors
+            let bytes = bytes
+                .map_err(|decompression_error| match decompression_error {
+                    Error::NotSupported(message) =>
+                        Error::unsupported(format!("yet unimplemented compression special case ({})", message)),
+
+                    error => Error::invalid(format!(
+                        "compressed {:?} data ({})",
+                        self, error.to_string()
+                    )),
+                })?;
+
+            if bytes.len() != expected_byte_size {
+                Err(Error::invalid("decompressed data"))
+            }
+
+            else { Ok(bytes) }
+        }
+    }
+
+    /// For scan line images and deep scan line images, one or more scan lines may be
+    /// stored together as a scan line block. The number of scan lines per block
+    /// depends on how the pixel data are compressed.
+    pub fn scan_lines_per_block(self) -> usize {
+        use self::Compression::*;
+        match self {
+            Uncompressed | RLE   | ZIP1    => 1,
+            ZIP16 | PXR24                  => 16,
+            PIZ   | B44   | B44A | DWAA(_) => 32,
+            DWAB(_)                        => 256,
+        }
+    }
+
+    /// Deep data can only be compressed using RLE or ZIP compression.
+    pub fn supports_deep_data(self) -> bool {
+        use self::Compression::*;
+        match self {
+            Uncompressed | RLE | ZIP1 => true,
+            _ => false,
+        }
+    }
+
+    /// Most compression methods will reconstruct the exact pixel bytes,
+    /// but some might throw away unimportant data for specific types of samples.
+    pub fn is_lossless_for(self, sample_type: SampleType) -> bool {
+        use self::Compression::*;
+        match self {
+            PXR24 => sample_type != SampleType::F32, // pxr reduces f32 to f24
+            B44 | B44A => sample_type != SampleType::F16, // b44 only compresses f16 values, others are left uncompressed
+            Uncompressed | RLE | ZIP1 | ZIP16 | PIZ => true,
+            DWAB(_) | DWAA(_) => false,
+        }
+    }
+
+    /// Most compression methods will reconstruct the exact pixel bytes,
+    /// but some might throw away unimportant data in some cases.
+    pub fn may_loose_data(self) -> bool {
+        use self::Compression::*;
+        match self {
+            Uncompressed | RLE | ZIP1 | ZIP16 | PIZ => false,
+            PXR24 | B44 | B44A | DWAB(_) | DWAA(_)  => true,
+        }
+    }
+
+    /// Most compression methods will reconstruct the exact pixel bytes,
+    /// but some might replace NaN with zeroes.
+    pub fn supports_nan(self) -> bool {
+        use self::Compression::*;
+        match self {
+            B44 | B44A | DWAB(_) | DWAA(_) => false, // TODO dwa might support it?
+            _ => true
+        }
+    }
+
+}
+
+// see https://github.com/AcademySoftwareFoundation/openexr/blob/6a9f8af6e89547bcd370ae3cec2b12849eee0b54/OpenEXR/IlmImf/ImfMisc.cpp#L1456-L1541
+
+#[allow(unused)] // allows the extra parameters to be unused
+fn convert_current_to_little_endian(mut bytes: ByteVec, channels: &ChannelList, rectangle: IntegerBounds) -> ByteVec {
+    #[cfg(target = "big_endian")]
+    reverse_block_endianness(&mut byte_vec, channels, rectangle);
+
+    bytes
+}
+
+#[allow(unused)] // allows the extra parameters to be unused
+fn convert_little_endian_to_current(mut bytes: ByteVec, channels: &ChannelList, rectangle: IntegerBounds) -> ByteVec {
+    #[cfg(target = "big_endian")]
+    reverse_block_endianness(&mut bytes, channels, rectangle);
+
+    bytes
+}
+
+#[allow(unused)] // unused when on little endian system
+fn reverse_block_endianness(bytes: &mut [u8], channels: &ChannelList, rectangle: IntegerBounds){
+    let mut remaining_bytes: &mut [u8] = bytes;
+
+    for y in rectangle.position.y() .. rectangle.end().y() {
+        for channel in &channels.list {
+            let line_is_subsampled = mod_p(y, usize_to_i32(channel.sampling.y())) != 0;
+            if line_is_subsampled { continue; }
+
+            let sample_count = rectangle.size.width() / channel.sampling.x();
+
+            match channel.sample_type {
+                SampleType::F16 => remaining_bytes = chomp_convert_n::<f16>(reverse_2_bytes, remaining_bytes, sample_count),
+                SampleType::F32 => remaining_bytes = chomp_convert_n::<f32>(reverse_4_bytes, remaining_bytes, sample_count),
+                SampleType::U32 => remaining_bytes = chomp_convert_n::<u32>(reverse_4_bytes, remaining_bytes, sample_count),
+            }
+        }
+    }
+
+    #[inline]
+    fn chomp_convert_n<T>(convert_single_value: fn(&mut[u8]), mut bytes: &mut [u8], count: usize) -> &mut [u8] {
+        let type_size = size_of::<T>();
+        let (line_bytes, rest) = bytes.split_at_mut(count * type_size);
+        let value_byte_chunks = line_bytes.chunks_exact_mut(type_size);
+
+        for value_bytes in value_byte_chunks {
+            convert_single_value(value_bytes);
+        }
+
+        rest
+    }
+
+    debug_assert!(remaining_bytes.is_empty(), "not all bytes were converted to little endian");
+}
+
+#[inline]
+fn reverse_2_bytes(bytes: &mut [u8]){
+    // this code seems like it could be optimized easily by the compiler
+    let two_bytes: [u8; 2] = bytes.try_into().expect("invalid byte count");
+    bytes.copy_from_slice(&[two_bytes[1], two_bytes[0]]);
+}
+
+#[inline]
+fn reverse_4_bytes(bytes: &mut [u8]){
+    let four_bytes: [u8; 4] = bytes.try_into().expect("invalid byte count");
+    bytes.copy_from_slice(&[four_bytes[3], four_bytes[2], four_bytes[1], four_bytes[0]]);
+}
+
+#[inline]
+fn div_p (x: i32, y: i32) -> i32 {
+    if x >= 0 {
+        if y >= 0 { x  / y }
+        else { -(x  / -y) }
+    }
+    else {
+        if y >= 0 { -((y-1-x) / y) }
+        else { (-y-1-x) / -y }
+    }
+}
+
+#[inline]
+fn mod_p(x: i32, y: i32) -> i32 {
+    x - y * div_p(x, y)
+}
+
+/// A collection of functions used to prepare data for compression.
+mod optimize_bytes {
+
+    /// Integrate over all differences to the previous value in order to reconstruct sample values.
+    pub fn differences_to_samples(buffer: &mut [u8]) {
+        // The naive implementation is very simple:
+        //
+        // for index in 1..buffer.len() {
+        //    buffer[index] = (buffer[index - 1] as i32 + buffer[index] as i32 - 128) as u8;
+        // }
+        //
+        // But we process elements in pairs to take advantage of instruction-level parallelism.
+        // When computations within a pair do not depend on each other, they can be processed in parallel.
+        // Since this function is responsible for a very large chunk of execution time,
+        // this tweak alone improves decoding performance of RLE images by 20%.
+        if let Some(first) = buffer.get(0) {
+            let mut previous = *first as i16;
+            for chunk in &mut buffer[1..].chunks_exact_mut(2) {
+                // no bounds checks here due to indices and chunk size being constant
+                let diff0 = chunk[0] as i16;
+                let diff1 = chunk[1] as i16;
+                // these two computations do not depend on each other, unlike in the naive version,
+                // so they can be executed by the CPU in parallel via instruction-level parallelism
+                let sample0 = (previous + diff0 - 128) as u8;
+                let sample1 = (previous + diff0 + diff1 - 128 * 2) as u8;
+                chunk[0] = sample0;
+                chunk[1] = sample1;
+                previous = sample1 as i16;
+            }
+            // handle the remaining element at the end not processed by the loop over pairs, if present
+            for elem in &mut buffer[1..].chunks_exact_mut(2).into_remainder().iter_mut() {
+                let sample = (previous + *elem as i16 - 128) as u8;
+                *elem = sample;
+                previous = sample as i16;
+            }
+        }
+    }
+
+    /// Derive over all values in order to produce differences to the previous value.
+    pub fn samples_to_differences(buffer: &mut [u8]){
+        // naive version:
+        // for index in (1..buffer.len()).rev() {
+        //     buffer[index] = (buffer[index] as i32 - buffer[index - 1] as i32 + 128) as u8;
+        // }
+        //
+        // But we process elements in batches to take advantage of autovectorization.
+        // If the target platform has no vector instructions (e.g. 32-bit ARM without `-C target-cpu=native`)
+        // this will instead take advantage of instruction-level parallelism.
+        if let Some(first) = buffer.get(0) {
+            let mut previous = *first as i16;
+            // Chunk size is 16 because we process bytes (8 bits),
+            // and 8*16 = 128 bits is the size of a typical SIMD register.
+            // Even WASM has 128-bit SIMD registers.
+            for chunk in &mut buffer[1..].chunks_exact_mut(16) {
+                // no bounds checks here due to indices and chunk size being constant
+                let sample0 = chunk[0] as i16;
+                let sample1 = chunk[1] as i16;
+                let sample2 = chunk[2] as i16;
+                let sample3 = chunk[3] as i16;
+                let sample4 = chunk[4] as i16;
+                let sample5 = chunk[5] as i16;
+                let sample6 = chunk[6] as i16;
+                let sample7 = chunk[7] as i16;
+                let sample8 = chunk[8] as i16;
+                let sample9 = chunk[9] as i16;
+                let sample10 = chunk[10] as i16;
+                let sample11 = chunk[11] as i16;
+                let sample12 = chunk[12] as i16;
+                let sample13 = chunk[13] as i16;
+                let sample14 = chunk[14] as i16;
+                let sample15 = chunk[15] as i16;
+                // Unlike in decoding, computations in here are truly independent from each other,
+                // which enables the compiler to vectorize this loop.
+                // Even if the target platform has no vector instructions,
+                // so using more parallelism doesn't imply doing more work,
+                // and we're not really limited in how wide we can go.
+                chunk[0] = (sample0 - previous + 128) as u8;
+                chunk[1] = (sample1 - sample0 + 128) as u8;
+                chunk[2] = (sample2 - sample1 + 128) as u8;
+                chunk[3] = (sample3 - sample2 + 128) as u8;
+                chunk[4] = (sample4 - sample3 + 128) as u8;
+                chunk[5] = (sample5 - sample4 + 128) as u8;
+                chunk[6] = (sample6 - sample5 + 128) as u8;
+                chunk[7] = (sample7 - sample6 + 128) as u8;
+                chunk[8] = (sample8 - sample7 + 128) as u8;
+                chunk[9] = (sample9 - sample8 + 128) as u8;
+                chunk[10] = (sample10 - sample9 + 128) as u8;
+                chunk[11] = (sample11 - sample10 + 128) as u8;
+                chunk[12] = (sample12 - sample11 + 128) as u8;
+                chunk[13] = (sample13 - sample12 + 128) as u8;
+                chunk[14] = (sample14 - sample13 + 128) as u8;
+                chunk[15] = (sample15 - sample14 + 128) as u8;
+                previous = sample15;
+            }
+            // Handle the remaining element at the end not processed by the loop over batches, if present
+            // This is what the iterator-based version of this function would look like without vectorization
+            for elem in &mut buffer[1..].chunks_exact_mut(16).into_remainder().iter_mut() {
+                let diff = (*elem as i16 - previous + 128) as u8;
+                previous = *elem as i16;
+                *elem = diff;
+            }
+        }
+    }
+
+    use std::cell::Cell;
+    thread_local! {
+        // A buffer for reusing between invocations of interleaving and deinterleaving.
+        // Allocating memory is cheap, but zeroing or otherwise initializing it is not.
+        // Doing it hundreds of times (once per block) would be expensive.
+        // This optimization brings down the time spent in interleaving from 15% to 5%.
+        static SCRATCH_SPACE: Cell<Vec<u8>> = Cell::new(Vec::new());
+    }
+
+    fn with_reused_buffer<F>(length: usize, mut func: F) where F: FnMut(&mut [u8]) {
+        SCRATCH_SPACE.with(|scratch_space| {
+            // reuse a buffer if we've already initialized one
+            let mut buffer = scratch_space.take();
+            if buffer.len() < length {
+                // Efficiently create a zeroed Vec by requesting zeroed memory from the OS.
+                // This is slightly faster than a `memcpy()` plus `memset()` that would happen otherwise,
+                // but is not a big deal either way since it's not a hot codepath.
+                buffer = vec![0u8; length];
+            }
+
+            // call the function
+            func(&mut buffer[..length]);
+
+            // save the internal buffer for reuse
+            scratch_space.set(buffer);
+        });
+    }
+
+    /// Interleave the bytes such that the second half of the array is every other byte.
+    pub fn interleave_byte_blocks(separated: &mut [u8]) {
+        with_reused_buffer(separated.len(), |interleaved| {
+
+            // Split the two halves that we are going to interleave.
+            let (first_half, second_half) = separated.split_at((separated.len() + 1) / 2);
+            // The first half can be 1 byte longer than the second if the length of the input is odd,
+            // but the loop below only processes numbers in pairs.
+            // To handle it, preserve the last element of the first slice, to be handled after the loop.
+            let first_half_last = first_half.last();
+            // Truncate the first half to match the lenght of the second one; more optimizer-friendly
+            let first_half_iter = &first_half[..second_half.len()];
+
+            // Main loop that performs the interleaving
+            for ((first, second), interleaved) in first_half_iter.iter().zip(second_half.iter())
+                .zip(interleaved.chunks_exact_mut(2)) {
+                    // The length of each chunk is known to be 2 at compile time,
+                    // and each index is also a constant.
+                    // This allows the compiler to remove the bounds checks.
+                    interleaved[0] = *first;
+                    interleaved[1] = *second;
+            }
+
+            // If the length of the slice was odd, restore the last element of the first half that we saved
+            if interleaved.len() % 2 == 1 {
+                if let Some(value) = first_half_last {
+                    // we can unwrap() here because we just checked that the lenght is non-zero:
+                    // `% 2 == 1` will fail for zero
+                    *interleaved.last_mut().unwrap() = *value;
+                }
+            }
+
+            // write out the results
+            separated.copy_from_slice(&interleaved);
+        });
+    }
+
+/// Separate the bytes such that the second half contains every other byte.
+/// This performs deinterleaving - the inverse of interleaving.
+pub fn separate_bytes_fragments(source: &mut [u8]) {
+    with_reused_buffer(source.len(), |separated| {
+
+        // Split the two halves that we are going to interleave.
+        let (first_half, second_half) = separated.split_at_mut((source.len() + 1) / 2);
+        // The first half can be 1 byte longer than the second if the length of the input is odd,
+        // but the loop below only processes numbers in pairs.
+        // To handle it, preserve the last element of the input, to be handled after the loop.
+        let last = source.last();
+        let first_half_iter = &mut first_half[..second_half.len()];
+
+        // Main loop that performs the deinterleaving
+        for ((first, second), interleaved) in first_half_iter.iter_mut().zip(second_half.iter_mut())
+            .zip(source.chunks_exact(2)) {
+                // The length of each chunk is known to be 2 at compile time,
+                // and each index is also a constant.
+                // This allows the compiler to remove the bounds checks.
+                *first = interleaved[0];
+                *second = interleaved[1];
+        }
+
+        // If the length of the slice was odd, restore the last element of the input that we saved
+        if source.len() % 2 == 1 {
+            if let Some(value) = last {
+                // we can unwrap() here because we just checked that the lenght is non-zero:
+                // `% 2 == 1` will fail for zero
+                *first_half.last_mut().unwrap() = *value;
+            }
+        }
+
+        // write out the results
+        source.copy_from_slice(&separated);
+    });
+}
+
+
+    #[cfg(test)]
+    pub mod test {
+
+        #[test]
+        fn roundtrip_interleave(){
+            let source = vec![ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ];
+            let mut modified = source.clone();
+
+            super::separate_bytes_fragments(&mut modified);
+            super::interleave_byte_blocks(&mut modified);
+
+            assert_eq!(source, modified);
+        }
+
+        #[test]
+        fn roundtrip_derive(){
+            let source = vec![ 0, 1, 2, 7, 4, 5, 6, 7, 13, 9, 10 ];
+            let mut modified = source.clone();
+
+            super::samples_to_differences(&mut modified);
+            super::differences_to_samples(&mut modified);
+
+            assert_eq!(source, modified);
+        }
+
+    }
+}
+
+
+#[cfg(test)]
+pub mod test {
+    use super::*;
+    use crate::meta::attribute::ChannelDescription;
+    use crate::block::samples::IntoNativeSample;
+
+    #[test]
+    fn roundtrip_endianness_mixed_channels(){
+        let a32 = ChannelDescription::new("A", SampleType::F32, true);
+        let y16 = ChannelDescription::new("Y", SampleType::F16, true);
+        let channels = ChannelList::new(smallvec![ a32, y16 ]);
+
+        let data = vec![
+            23582740683_f32.to_ne_bytes().as_slice(),
+            35827420683_f32.to_ne_bytes().as_slice(),
+            27406832358_f32.to_f16().to_ne_bytes().as_slice(),
+            74062358283_f32.to_f16().to_ne_bytes().as_slice(),
+
+            52582740683_f32.to_ne_bytes().as_slice(),
+            45827420683_f32.to_ne_bytes().as_slice(),
+            15406832358_f32.to_f16().to_ne_bytes().as_slice(),
+            65062358283_f32.to_f16().to_ne_bytes().as_slice(),
+        ].into_iter().flatten().map(|x| *x).collect();
+
+        roundtrip_convert_endianness(
+            data, &channels,
+            IntegerBounds::from_dimensions((2, 2))
+        );
+    }
+
+    fn roundtrip_convert_endianness(
+        current_endian: ByteVec, channels: &ChannelList, rectangle: IntegerBounds
+    ){
+        let little_endian = convert_current_to_little_endian(
+            current_endian.clone(), channels, rectangle
+        );
+
+        let current_endian_decoded = convert_little_endian_to_current(
+            little_endian.clone(), channels, rectangle
+        );
+
+        assert_eq!(current_endian, current_endian_decoded, "endianness conversion failed");
+    }
+}
\ No newline at end of file