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//! Experimental entropy coding algorithm for advanced variants of bitsback coding.
//!
//! This module provides the [`ChainCoder`], an experimental entropy coder that is similar
//! to an [`AnsCoder`] in that it operates as a stack (i.e., a last-in-first-out data
//! structure). However, different to an `AnsCoder`, a `ChainCoder` treats each symbol
//! independently. Thus, when decoding some bit string into a sequence of symbols, any
//! modification to the entropy model for one symbol does not affect decoding for any other
//! symbol (by contrast, when decoding with an `AnsCoder` then changing the entropy model
//! for one symbol can affect *all* subsequently decoded symbols too, see
//! [Motivation](#motivation) below).
//!
//! This property of treating symbols independently upon decoding can be useful for advanced
//! compression methods that combine inference, quantization, and bits-back coding.
//!
//! # Motivation
//!
//! The following example illustrates how decoding differs between an [`AnsCoder`] and a
//! [`ChainCoder`]. We decode the same bitstring `data` twice with each coder: once with a
//! sequence of toy entropy models, and then a second time with slightly different sequence
//! of entropy models. Importantly, only the entropy model for the first decoded symbol
//! differs between the two applications of each coder. We then observe that
//! - with the `AnsCoder`, changing the first entropy model affects not only the first
//! decoded symbol but also has a ripple effect that can affect subsequently decoded
//! symbols; while
//! - with the `ChainCoder`, changing the first entropy model affects only the first decoded
//! symbol; all subsequently decoded symbols remain unchanged.
//!
//! ```
//! use constriction::stream::{
//! model::DefaultContiguousCategoricalEntropyModel,
//! stack::DefaultAnsCoder, chain::DefaultChainCoder, Decode
//! };
//!
//! /// Shorthand for decoding a sequence of symbols with categorical entropy models.
//! fn decode_categoricals<Decoder: Decode<24, Word = u32>>(
//! decoder: &mut Decoder,
//! probabilities: &[[f64; 4]],
//! ) -> Vec<usize> {
//! let entropy_models = probabilities
//! .iter()
//! .map(
//! |probs| DefaultContiguousCategoricalEntropyModel
//! ::from_floating_point_probabilities(probs).unwrap()
//! );
//! decoder.decode_symbols(entropy_models).collect::<Result<Vec<_>, _>>().unwrap()
//! }
//!
//! // Let's define some sample binary data and some probabilities for our entropy models
//! let data = vec![0x80d1_4131, 0xdda9_7c6c, 0x5017_a640, 0x0117_0a3d];
//! let mut probabilities = [
//! [0.1, 0.7, 0.1, 0.1], // Probabilities for the entropy model of the first decoded symbol.
//! [0.2, 0.2, 0.1, 0.5], // Probabilities for the entropy model of the second decoded symbol.
//! [0.2, 0.1, 0.4, 0.3], // Probabilities for the entropy model of the third decoded symbol.
//! ];
//!
//! // Decoding the binary data with an `AnsCoder` results in the symbols `[0, 0, 1]`.
//! let mut ans_coder = DefaultAnsCoder::from_binary(data.clone()).unwrap();
//! let symbols = decode_categoricals(&mut ans_coder, &probabilities);
//! assert_eq!(symbols, [0, 0, 1]);
//!
//! // Even if we change only the first entropy model (slightly), *all* decoded symbols can change:
//! probabilities[0] = [0.09, 0.71, 0.1, 0.1]; // was: `[0.1, 0.7, 0.1, 0.1]`
//! let mut ans_coder = DefaultAnsCoder::from_binary(data.clone()).unwrap();
//! let symbols = decode_categoricals(&mut ans_coder, &probabilities);
//! assert_eq!(symbols, [1, 0, 3]); // (instead of `[0, 0, 1]` from above)
//! // It's no surprise that the first symbol changed since we changed its entropy model. But
//! // note that the third symbol changed too even though we hadn't changed its entropy model.
//! // --> Changes to entropy models (and also to compressed bits) have a *global* effect.
//!
//! // Let's try the same with a `ChainCoder`:
//! probabilities[0] = [0.1, 0.7, 0.1, 0.1]; // Restore original entropy model for first symbol.
//! let mut chain_coder = DefaultChainCoder::from_binary(data.clone()).unwrap();
//! let symbols = decode_categoricals(&mut chain_coder, &probabilities);
//! assert_eq!(symbols, [0, 3, 3]);
//! // We get different symbols than for the `AnsCoder`, of course, but that's not the point here.
//!
//! probabilities[0] = [0.09, 0.71, 0.1, 0.1]; // Change the first entropy model again slightly.
//! let mut chain_coder = DefaultChainCoder::from_binary(data).unwrap();
//! let symbols = decode_categoricals(&mut chain_coder, &probabilities);
//! assert_eq!(symbols, [1, 3, 3]); // (instead of `[0, 3, 3]` from above)
//! // The only symbol that changed was the one whose entropy model we had changed.
//! // --> In a `ChainCoder`, changes to entropy models (and also to compressed bits)
//! // only have a *local* effect on the decompressed symbols.
//! ```
//!
//! # How does this work?
//!
//! TODO
//!
//! [`AnsCoder`]: super::stack::AnsCoder
use alloc::vec::Vec;
use core::{borrow::Borrow, convert::Infallible, fmt::Display};
use num_traits::AsPrimitive;
use super::{
model::{DecoderModel, EncoderModel},
Code, Decode, Encode, TryCodingError,
};
use crate::{
backends::{ReadWords, WriteWords},
BitArray, CoderError, DefaultEncoderFrontendError, NonZeroBitArray, Pos, PosSeek, Seek, Stack,
};
/// Experimental entropy coder for advanced variants of bitsback coding.
///
/// See [module level documentation](super) for motivation and explanation of the
/// implemented entropy coding algorithm.
///
/// # Intended Usage
///
/// A typical usage cycle goes along the following steps:
///
/// ## When compressing data using the bits-back trick
///
/// 0. Start with some stack of (typically already compressed) binary data, which you want
/// to piggy-back into the choice of certain latent variables.
/// 1. Create a `ChainCoder` by calling [`ChainCoder::from_binary`] or
/// [`ChainCoder::from_compressed`] (depending on whether you can guarantee that the
/// stack of binary data has a nonzero word on top).
/// 2. Use the `ChainCoder` and a sequence of entropy models to decode some symbols.
/// 3. Export the remainders data on the `ChainCoder` by calling [`.into_remainders()`].
///
/// ## When decompressing the data
///
/// 1. Create a `ChainCoder` by calling [`ChainCoder::from_remainders`].
/// 2. Encode the symbols you obtained in Step 2 above back onto the new chain coder (in
/// reverse order) using the same entropy models.
/// 3. Recover the original binary data from Step 0 above by calling [`.into_binary()`] or
/// [`.into_compressed()`] (using the `analogous choice as in Step 1 above).
///
/// # Examples
///
/// The following two examples show two variants of the typical usage cycle described above.
///
/// ```
/// use constriction::stream::{model::DefaultLeakyQuantizer, Decode, chain::DefaultChainCoder};
/// use probability::distribution::Gaussian;
///
/// // Step 0 of the compressor: Generate some sample binary data for demonstration purpose.
/// let original_data = (0..100u32).map(
/// |i| i.wrapping_mul(0xad5f_b2ed).wrapping_add(0xed55_4892)
/// ).collect::<Vec<_>>();
///
/// // Step 1 of the compressor: obtain a `ChainCoder` from the original binary data.
/// let mut coder = DefaultChainCoder::from_binary(original_data.clone()).unwrap();
///
/// // Step 2 of the compressor: decode data into symbols using some entropy models.
/// let quantizer = DefaultLeakyQuantizer::new(-100..=100);
/// let models = (0..50u32).map(|i| quantizer.quantize(Gaussian::new(i as f64, 10.0)));
/// let symbols = coder.decode_symbols(models.clone()).collect::<Result<Vec<_>, _>>().unwrap();
///
/// // Step 3 of the compressor: export the remainders data.
/// let (remainders_prefix, remainders_suffix) = coder.into_remainders().unwrap();
/// // (verify that we've indeed reduced the amount of data:)
/// assert!(remainders_prefix.len() + remainders_suffix.len() < original_data.len());
///
/// // ... do something with the `symbols`, then recover them later ...
///
/// // Step 1 of the decompressor: create a `ChainCoder` from the remainders data. We only really
/// // need the `remainders_suffix` here, but it would also be legal to use the concatenation of
/// // `remainders_prefix` with `remainders_suffix` (see other example below).
/// let mut coder = DefaultChainCoder::from_remainders(remainders_suffix).unwrap();
///
/// // Step 2 of the decompressor: re-encode the symbols in reverse order.
/// coder.encode_symbols_reverse(symbols.into_iter().zip(models));
///
/// // Step 3 of the decompressor: recover the original data.
/// let (recovered_prefix, recovered_suffix) = coder.into_binary().unwrap();
/// assert!(recovered_prefix.is_empty()); // Empty because we discarded `remainders_prefix` above.
/// let mut recovered = remainders_prefix; // But we have to prepend it to the recovered data now.
/// recovered.extend_from_slice(&recovered_suffix);
///
/// assert_eq!(recovered, original_data);
/// ```
///
/// In Step 3 of the compressor in the example above, calling `.into_remainders()` on a
/// `ChainCoder` returns a tuple of a `remainders_prefix` and a `remainders_suffix`. The
/// `remainders_prefix` contains superflous data that we didn't need when decoding the
/// `symbols` (`remainders_prefix` is an unaltered prefix of the original `data`). We
/// therefore don't need `remainders_prefix` for re-encoding the symbols, so we didn't pass
/// it to `ChainCoder::from_remainders` in Step 1 of the decompressor above.
///
/// If we were to write out `remainders_prefix` and `remainders_suffix` to a file then it
/// would be tedious to keep track of where the prefix ends and where the suffix begins.
/// Luckily, we don't have to do this. We can just as well concatenate `remainders_prefix`
/// and `remainders_suffix` right away. The only additional change this will cause is that
/// the call to `.into_binary()` in Step 3 of the decompressor will then return a non-empty
/// `recovered_prefix` because the second `ChainCoder` will then also have some superflous
/// data. So we'll have to again concatenate the two returned buffers. The following example
/// shows how this works:
///
/// ```
/// # use constriction::stream::{model::DefaultLeakyQuantizer, Decode, chain::DefaultChainCoder};
/// # use probability::distribution::Gaussian;
/// # let original_data = (0..100u32).map(
/// # |i| i.wrapping_mul(0xad5f_b2ed).wrapping_add(0xed55_4892)
/// # ).collect::<Vec<_>>();
/// # let mut coder = DefaultChainCoder::from_binary(original_data.clone()).unwrap();
/// # let quantizer = DefaultLeakyQuantizer::new(-100..=100);
/// # let models = (0..50u32).map(|i| quantizer.quantize(Gaussian::new(i as f64, 10.0)));
/// # let symbols = coder.decode_symbols(models.clone()).collect::<Result<Vec<_>, _>>().unwrap();
/// # let (remainders_prefix, remainders_suffix) = coder.into_remainders().unwrap();
/// // ... compressor same as in the previous example above ...
///
/// // Alternative Step 1 of the decompressor: concatenate `remainders_prefix` with
/// // `remainders_suffix` before creating a `ChainCoder` from them.
/// let mut remainders = remainders_prefix;
/// remainders.extend_from_slice(&remainders_suffix);
/// let mut coder = DefaultChainCoder::from_remainders(remainders).unwrap();
///
/// // Step 2 of the decompressor: re-encode symbols in reverse order (same as in previous example).
/// coder.encode_symbols_reverse(symbols.into_iter().zip(models));
///
/// // Alternative Step 3 of the decompressor: recover the original data by another concatenation.
/// let (recovered_prefix, recovered_suffix) = coder.into_binary().unwrap();
/// assert!(!recovered_prefix.is_empty()); // No longer empty because there was superflous data.
/// let mut recovered = recovered_prefix; // So we have to concatenate `recovered_{pre,suf}fix`.
/// recovered.extend_from_slice(&recovered_suffix);
///
/// assert_eq!(recovered, original_data);
/// ```
///
/// [`.into_remainders()`]: Self::into_remainders
/// [`.into_binary()`]: Self::into_binary
/// [`.into_compressed()`]: Self::into_compressed
#[derive(Debug, Clone)]
pub struct ChainCoder<Word, State, CompressedBackend, RemaindersBackend, const PRECISION: usize>
where
Word: BitArray + Into<State>,
State: BitArray + AsPrimitive<Word>,
{
/// The compressed bit string. Written to by encoder, read from by decoder.
compressed: CompressedBackend,
/// Leftover information from decoding. Read from by encoder, written to by decoder.
remainders: RemaindersBackend,
heads: ChainCoderHeads<Word, State, PRECISION>,
}
/// Type of the internal state used by [`ChainCoder<Word, State>`]. Relevant for
/// [`Seek`]ing.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct ChainCoderHeads<Word: BitArray, State: BitArray, const PRECISION: usize> {
/// All bits following the highest order bit (which is a given in a `NonZero`) are
/// leftover bits from previous reads from `compressed` that still need to be consumed.
/// Thus, there are at most `Word::BITS - 1` leftover bits at any time.
compressed: Word::NonZero,
/// Satisfies invariants:
/// - `heads.remainders >= 1 << (State::BITS - Word::BITS - PRECISION)`; and
/// - `heads.remainders < 1 << (State::BITS - PRECISION)`
remainders: State,
}
impl<Word: BitArray, State: BitArray, const PRECISION: usize>
ChainCoderHeads<Word, State, PRECISION>
{
/// Returns `true` iff there's currently an integer amount of `Words` on `compressed`
#[inline(always)]
pub fn is_whole(self) -> bool {
self.compressed.get() == Word::one()
}
/// Private on purpose.
fn new<B: ReadWords<Word, Stack>>(
source: &mut B,
push_one: bool,
) -> Result<ChainCoderHeads<Word, State, PRECISION>, CoderError<(), B::ReadError>>
where
Word: Into<State>,
{
assert!(State::BITS >= Word::BITS + PRECISION);
assert!(PRECISION > 0);
assert!(PRECISION <= Word::BITS);
let threshold = State::one() << (State::BITS - Word::BITS - PRECISION);
let mut remainders_head = if push_one {
State::one()
} else {
match source.read()? {
Some(word) if word != Word::zero() => word.into(),
_ => return Err(CoderError::Frontend(())),
}
};
while remainders_head < threshold {
remainders_head = remainders_head << Word::BITS
| source.read()?.ok_or(CoderError::Frontend(()))?.into();
}
Ok(ChainCoderHeads {
compressed: Word::one().into_nonzero().expect("1 != 0"),
remainders: remainders_head,
})
}
}
pub type DefaultChainCoder = ChainCoder<u32, u64, Vec<u32>, Vec<u32>, 24>;
pub type SmallChainCoder = ChainCoder<u16, u32, Vec<u16>, Vec<u16>, 12>;
impl<Word, State, CompressedBackend, RemaindersBackend, const PRECISION: usize>
ChainCoder<Word, State, CompressedBackend, RemaindersBackend, PRECISION>
where
Word: BitArray + Into<State>,
State: BitArray + AsPrimitive<Word>,
{
/// Creates a new `ChainCoder` for decoding from the provided `data`.
///
/// The reader `data` must have enough words to initialize the chain heads but can
/// otherwise be arbitrary. In particualar, `data` doesn't necessary have to come from
/// an [`AnsCoder`]. If you know that `data` comes from an `AnsCoder` then it's slightly
/// better to call [`from_compressed`] instead.
///
/// Retuns an error if `data` does not have enough words to initialize the chain heads
/// or if reading from `data` lead to an error.
///
/// [`AnsCoder`]: super::stack::AnsCoder
/// [`from_compressed`]: Self::from_compressed
pub fn from_binary(
mut data: CompressedBackend,
) -> Result<Self, CoderError<CompressedBackend, CompressedBackend::ReadError>>
where
CompressedBackend: ReadWords<Word, Stack>,
RemaindersBackend: Default,
{
let heads = match ChainCoderHeads::new(&mut data, true) {
Ok(heads) => heads,
Err(CoderError::Frontend(())) => return Err(CoderError::Frontend(data)),
Err(CoderError::Backend(err)) => return Err(CoderError::Backend(err)),
};
let remainders = RemaindersBackend::default();
Ok(Self {
compressed: data,
remainders,
heads,
})
}
/// Creates a new `ChainCoder` for decoding from the compressed data of an [`AnsCoder`]
///
/// The provided read backend `compressed`, must have enough words to initialize the
/// chain heads and must not have a zero word at the current read position. The latter
/// is always satisfied for (nonempty) data returned from [`AnsCoder::into_compressed`].
///
/// Retuns an error if `compressed` does not have enough words, if reading from
/// `compressed` lead to an error, or if the first word read from `compressed` is zero.
///
/// [`AnsCoder`]: super::stack::AnsCoder
/// [`AnsCoder::into_compressed`]: super::stack::AnsCoder::into_compressed
pub fn from_compressed(
mut compressed: CompressedBackend,
) -> Result<Self, CoderError<CompressedBackend, CompressedBackend::ReadError>>
where
CompressedBackend: ReadWords<Word, Stack>,
RemaindersBackend: Default,
{
let heads = match ChainCoderHeads::new(&mut compressed, false) {
Ok(heads) => heads,
Err(CoderError::Frontend(())) => return Err(CoderError::Frontend(compressed)),
Err(CoderError::Backend(err)) => return Err(CoderError::Backend(err)),
};
let remainders = RemaindersBackend::default();
Ok(Self {
compressed,
remainders,
heads,
})
}
/// Terminates decoding and returns the remainders bit string as a tuple `(prefix,
/// suffix)`.
///
/// - The `prefix` is a shortened but otherwise unaltered variant of the data from which
/// you created this `ChainCoder` when you called [`ChainCoder::from_binary`] or
/// [`ChainCoder::from_compressed`].
/// - The `suffix` is a stack with at least two nonzero words on top.
///
/// You can use the returned tuple `(prefix, suffix)` in either of the following two
/// ways (see examples in the [struct level documentation](ChainCoder)):
/// - Either put `prefix` away and continue only with `suffix` as follows:
/// 1. obtain a new `ChainCoder` by calling [`ChainCoder::from_remainders(suffix)`];
/// 2. encode the same symbols that you decoded from the original `ChainCoder` back
/// onto the new `ChainCoder` (in reverse order);
/// 3. call [`.into_binary()`] or [`.into_compressed()`] on the new `ChainCoder` to
/// obatain another tuple `(prefix2, suffix2)`.
/// 4. concatenate `prefix`, `prefix2`, and `suffix2` to recover the data from which
/// you created the original `ChainCoder` when you constructed it with
/// [`ChainCoder::from_binary`] or [`ChainCoder::from_compressed`], respectively.
/// - Or you can concatenate `prefix` with `suffix`, create a new `ChainCoder` from the
/// concatenation by calling `ChainCoder::from_remainders(concatenation)`, continue
/// with steps 2 and 3 above, and then just concatenate `prefix2` with `suffix2` to
/// recover the original data.
///
/// [`ChainCoder::from_remainders(suffix)`]: Self::from_remainders
/// [`.into_binary()`]: Self::into_binary
/// [`.into_compressed()`]: Self::into_compressed
pub fn into_remainders(
mut self,
) -> Result<(CompressedBackend, RemaindersBackend), RemaindersBackend::WriteError>
where
RemaindersBackend: WriteWords<Word>,
{
// Flush remainders head.
while self.heads.remainders != State::zero() {
self.remainders.write(self.heads.remainders.as_())?;
self.heads.remainders = self.heads.remainders >> Word::BITS;
}
// Transfer compressed head onto `remainders`.
self.remainders.write(self.heads.compressed.get())?;
Ok((self.compressed, self.remainders))
}
/// Creates a new `ChainCoder` for encoding some symbols together with the data
/// previously obtained from [`into_remainders`].
///
/// See [`into_remainders`] for detailed explanation.
///
/// [`into_remainders`]: Self::into_remainders
pub fn from_remainders(
mut remainders: RemaindersBackend,
) -> Result<Self, CoderError<RemaindersBackend, RemaindersBackend::ReadError>>
where
RemaindersBackend: ReadWords<Word, Stack>,
CompressedBackend: Default,
{
let compressed_head = match remainders.read()?.and_then(Word::into_nonzero) {
Some(word) => word,
_ => return Err(CoderError::Frontend(remainders)),
};
let mut heads = match ChainCoderHeads::new(&mut remainders, false) {
Ok(heads) => heads,
Err(CoderError::Frontend(())) => return Err(CoderError::Frontend(remainders)),
Err(CoderError::Backend(err)) => return Err(CoderError::Backend(err)),
};
heads.compressed = compressed_head;
let compressed = CompressedBackend::default();
Ok(Self {
compressed,
remainders,
heads,
})
}
/// Terminates encoding if possible and returns the compressed data as a tuple `(prefix,
/// suffix)`
///
/// Call this method only if the original `ChainCoder` used for decoding was constructed
/// with [`ChainCoder::from_compressed`] (typically if the original data came from an
/// [`AnsCoder`]). If the original `ChainCoder` was instead constructed with
/// [`ChainCoder::from_binary`] then call [`.into_binary()`] instead.
///
/// Returns an error unless there's currently an integer amount of `Words` in the
/// compressed data (which will be the case if you've used the `ChainCoder` correctly,
/// see also [`is_whole`]).
///
/// See [`into_remainders`] for usage instructions.
///
/// [`is_whole`]: Self::is_whole
/// [`AnsCoder`]: super::stack::AnsCoder
/// [`.into_binary()`]: Self::into_binary
/// [`into_remainders`]: Self::into_remainders
pub fn into_compressed(
mut self,
) -> Result<
(RemaindersBackend, CompressedBackend),
CoderError<Self, CompressedBackend::WriteError>,
>
where
CompressedBackend: WriteWords<Word>,
{
if !self.is_whole() {
return Err(CoderError::Frontend(self));
}
// Transfer remainders head onto `compressed`.
while self.heads.remainders != State::zero() {
self.compressed.write(self.heads.remainders.as_())?;
self.heads.remainders = self.heads.remainders >> Word::BITS;
}
Ok((self.remainders, self.compressed))
}
/// Terminates encoding if possible and returns the compressed data as a tuple `(prefix,
/// suffix)`
///
/// Call this method only if the original `ChainCoder` used for decoding was constructed
/// with [`ChainCoder::from_binary`]. If the original `ChainCoder` was instead
/// constructed with [`ChainCoder::from_compressed`] then call [`.into_compressed()`]
/// instead.
///
/// Returns an error unless there's currently an integer amount of `Words` in the both
/// the compressed data and the remainders data (which will be the case if you've used
/// the `ChainCoder` correctly and if the original chain coder was constructed with
/// `from_binary` rather than `from_compressed`).
///
/// See [`into_remainders`] for usage instructions.
///
/// [`is_whole`]: Self::is_whole
/// [`AnsCoder`]: super::stack::AnsCoder
/// [`.into_compressed()`]: Self::into_compressed
/// [`into_remainders`]: Self::into_remainders
pub fn into_binary(
mut self,
) -> Result<
(RemaindersBackend, CompressedBackend),
CoderError<Self, CompressedBackend::WriteError>,
>
where
CompressedBackend: WriteWords<Word>,
{
if !self.is_whole()
|| (State::BITS - self.heads.remainders.leading_zeros() as usize - 1) % Word::BITS != 0
{
return Err(CoderError::Frontend(self));
}
// Transfer remainders head onto `compressed`.
while self.heads.remainders > State::one() {
self.compressed.write(self.heads.remainders.as_())?;
self.heads.remainders = self.heads.remainders >> Word::BITS;
}
debug_assert!(self.heads.remainders == State::one());
Ok((self.remainders, self.compressed))
}
/// Returns `true` iff there's currently an integer amount of `Words` in the compressed data
#[inline(always)]
pub fn is_whole(&self) -> bool {
self.heads.compressed.get() == Word::one()
}
pub fn encode_symbols_reverse<S, M, I>(
&mut self,
symbols_and_models: I,
) -> Result<(), EncoderError<Word, CompressedBackend, RemaindersBackend>>
where
S: Borrow<M::Symbol>,
M: EncoderModel<PRECISION>,
M::Probability: Into<Word>,
Word: AsPrimitive<M::Probability>,
I: IntoIterator<Item = (S, M)>,
I::IntoIter: DoubleEndedIterator,
CompressedBackend: WriteWords<Word>,
RemaindersBackend: ReadWords<Word, Stack>,
{
self.encode_symbols(symbols_and_models.into_iter().rev())
}
pub fn try_encode_symbols_reverse<S, M, E, I>(
&mut self,
symbols_and_models: I,
) -> Result<(), TryCodingError<EncoderError<Word, CompressedBackend, RemaindersBackend>, E>>
where
S: Borrow<M::Symbol>,
M: EncoderModel<PRECISION>,
M::Probability: Into<Word>,
Word: AsPrimitive<M::Probability>,
I: IntoIterator<Item = core::result::Result<(S, M), E>>,
I::IntoIter: DoubleEndedIterator,
CompressedBackend: WriteWords<Word>,
RemaindersBackend: ReadWords<Word, Stack>,
{
self.try_encode_symbols(symbols_and_models.into_iter().rev())
}
#[inline(always)]
pub fn encode_iid_symbols_reverse<S, M, I>(
&mut self,
symbols: I,
model: M,
) -> Result<(), EncoderError<Word, CompressedBackend, RemaindersBackend>>
where
S: Borrow<M::Symbol>,
M: EncoderModel<PRECISION> + Copy,
M::Probability: Into<Word>,
Word: AsPrimitive<M::Probability>,
I: IntoIterator<Item = S>,
I::IntoIter: DoubleEndedIterator,
CompressedBackend: WriteWords<Word>,
RemaindersBackend: ReadWords<Word, Stack>,
{
self.encode_iid_symbols(symbols.into_iter().rev(), model)
}
#[allow(clippy::type_complexity)]
pub fn increase_precision<const NEW_PRECISION: usize>(
mut self,
) -> Result<
ChainCoder<Word, State, CompressedBackend, RemaindersBackend, NEW_PRECISION>,
CoderError<Infallible, BackendError<Infallible, RemaindersBackend::WriteError>>,
>
where
RemaindersBackend: WriteWords<Word>,
{
assert!(NEW_PRECISION >= PRECISION);
assert!(NEW_PRECISION <= Word::BITS);
assert!(State::BITS >= Word::BITS + NEW_PRECISION);
if self.heads.remainders >= State::one() << (State::BITS - NEW_PRECISION) {
self.flush_remainders_head()?;
}
Ok(ChainCoder {
compressed: self.compressed,
remainders: self.remainders,
heads: ChainCoderHeads {
compressed: self.heads.compressed,
remainders: self.heads.remainders,
},
})
}
#[allow(clippy::type_complexity)]
pub fn decrease_precision<const NEW_PRECISION: usize>(
mut self,
) -> Result<
ChainCoder<Word, State, CompressedBackend, RemaindersBackend, NEW_PRECISION>,
CoderError<EncoderFrontendError, BackendError<Infallible, RemaindersBackend::ReadError>>,
>
where
RemaindersBackend: ReadWords<Word, Stack>,
{
assert!(NEW_PRECISION <= PRECISION);
assert!(NEW_PRECISION > 0);
if self.heads.remainders < State::one() << (State::BITS - NEW_PRECISION - Word::BITS) {
// Won't truncate since, from the above check it follows that we satisfy the contract
// `self.heads.remainders < 1 << (State::BITS - Word::BITS)`.
self.refill_remainders_head()?
}
Ok(ChainCoder {
compressed: self.compressed,
remainders: self.remainders,
heads: ChainCoderHeads {
compressed: self.heads.compressed,
remainders: self.heads.remainders,
},
})
}
/// Converts the `stable::Decoder` into a new `stable::Decoder` that accepts entropy
/// models with a different fixed-point precision.
///
/// Here, "precision" refers to the number of bits with which probabilities are
/// represented in entropy models passed to the `decode_XXX` methods.
///
/// The generic argument `NEW_PRECISION` can usually be omitted because the compiler
/// can infer its value from the first time the new `stable::Decoder` is used for
/// decoding. The recommended usage pattern is to store the returned
/// `stable::Decoder` in a variable that shadows the old `stable::Decoder` (since
/// the old one gets consumed anyway), i.e.,
/// `let mut stable_decoder = stable_decoder.change_precision()`. See example below.
///
/// # Failure Case
///
/// The conversion can only fail if *all* of the following conditions are true
///
/// - `NEW_PRECISION < PRECISION`; and
/// - the `ChainCoder` has already been used incorrectly: it
/// must have encoded too many symbols or used the wrong sequence of entropy
/// models, causing it to use up just a few more bits of `remainders` than available
/// (but also not exceeding the capacity enough for this to be detected during
/// encoding).
///
/// In the event of this failure, `change_precision` returns `Err(self)`.
///
/// # Example
///
/// ```
/// use constriction::stream::{model::LeakyQuantizer, Decode, chain::DefaultChainCoder};
///
/// // Construct two entropy models with 24 bits and 20 bits of precision, respectively.
/// let continuous_distribution = probability::distribution::Gaussian::new(0.0, 10.0);
/// let quantizer24 = LeakyQuantizer::<_, _, u32, 24>::new(-100..=100);
/// let quantizer20 = LeakyQuantizer::<_, _, u32, 20>::new(-100..=100);
/// let distribution24 = quantizer24.quantize(continuous_distribution);
/// let distribution20 = quantizer20.quantize(continuous_distribution);
///
/// // Construct a `ChainCoder` and decode some data with the 24 bit precision entropy model.
/// let data = vec![0x0123_4567u32, 0x89ab_cdef];
/// let mut coder = DefaultChainCoder::from_binary(data).unwrap();
/// let _symbol_a = coder.decode_symbol(distribution24);
///
/// // Change `coder`'s precision and decode data with the 20 bit precision entropy model.
/// // The compiler can infer the new precision based on how `coder` will be used.
/// let mut coder = coder.change_precision().unwrap();
/// let _symbol_b = coder.decode_symbol(distribution20);
/// ```
#[inline(always)]
pub fn change_precision<const NEW_PRECISION: usize>(
self,
) -> Result<
ChainCoder<Word, State, CompressedBackend, RemaindersBackend, NEW_PRECISION>,
ChangePrecisionError<Word, RemaindersBackend>,
>
where
RemaindersBackend: WriteWords<Word> + ReadWords<Word, Stack>,
{
if NEW_PRECISION > PRECISION {
self.increase_precision()
.map_err(ChangePrecisionError::Increase)
} else {
self.decrease_precision()
.map_err(ChangePrecisionError::Decrease)
}
}
#[inline(always)]
/// This would flush meaningless zero bits if `self.heads.remainders < 1 << Word::BITS`.
fn flush_remainders_head<FrontendError, ReadError>(
&mut self,
) -> Result<(), CoderError<FrontendError, BackendError<ReadError, RemaindersBackend::WriteError>>>
where
RemaindersBackend: WriteWords<Word>,
{
self.remainders
.write(self.heads.remainders.as_())
.map_err(|err| CoderError::Backend(BackendError::Remainders(err)))?;
self.heads.remainders = self.heads.remainders >> Word::BITS;
Ok(())
}
/// This truncates if `self.heads.remainders >= 1 << (State::BITS - Word::BITS)`.
#[inline(always)]
fn refill_remainders_head<WriteError>(
&mut self,
) -> Result<
(),
CoderError<EncoderFrontendError, BackendError<WriteError, RemaindersBackend::ReadError>>,
>
where
RemaindersBackend: ReadWords<Word, Stack>,
{
let word = self
.remainders
.read()
.map_err(|err| CoderError::Backend(BackendError::Remainders(err)))?
.ok_or(CoderError::Frontend(EncoderFrontendError::OutOfRemainders))?;
self.heads.remainders = (self.heads.remainders << Word::BITS) | word.into();
Ok(())
}
}
impl<Word, State, CompressedBackend, RemaindersBackend, const PRECISION: usize> Code
for ChainCoder<Word, State, CompressedBackend, RemaindersBackend, PRECISION>
where
Word: BitArray + Into<State>,
State: BitArray + AsPrimitive<Word>,
{
type Word = Word;
type State = ChainCoderHeads<Word, State, PRECISION>;
fn state(&self) -> Self::State {
self.heads
}
}
#[allow(type_alias_bounds)]
pub type DecoderError<
Word,
CompressedBackend: ReadWords<Word, Stack>,
RemaindersBackend: WriteWords<Word>,
> = CoderError<
DecoderFrontendError,
BackendError<CompressedBackend::ReadError, RemaindersBackend::WriteError>,
>;
#[allow(type_alias_bounds)]
pub type EncoderError<
Word,
CompressedBackend: WriteWords<Word>,
RemaindersBackend: ReadWords<Word, Stack>,
> = CoderError<
EncoderFrontendError,
BackendError<CompressedBackend::WriteError, RemaindersBackend::ReadError>,
>;
/// Frontend error type for misuse of a [`ChainCoder`] for decoding.
#[derive(Debug, PartialEq, Eq)]
pub enum DecoderFrontendError {
OutOfCompressedData,
}
impl core::fmt::Display for DecoderFrontendError {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
match self {
Self::OutOfCompressedData => {
write!(f, "Out of compressed data.")
}
}
}
}
#[cfg(feature = "std")]
impl std::error::Error for DecoderFrontendError {}
/// Frontend error type for misuse of a [`ChainCoder`] for encoding.
#[derive(Debug, PartialEq, Eq)]
pub enum EncoderFrontendError {
OutOfRemainders,
ImpossibleSymbol,
}
impl core::fmt::Display for EncoderFrontendError {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
match self {
Self::OutOfRemainders => {
write!(f, "Out of remainders information from previous decoding.")
}
Self::ImpossibleSymbol => DefaultEncoderFrontendError::ImpossibleSymbol.fmt(f),
}
}
}
#[cfg(feature = "std")]
impl std::error::Error for EncoderFrontendError {}
/// Error type for backend errors in a [`ChainCoder`].
#[derive(Debug, PartialEq, Eq)]
pub enum BackendError<CompressedBackendError, RemaindersBackendError> {
Compressed(CompressedBackendError),
Remainders(RemaindersBackendError),
}
impl<CompressedBackendError: Display, RemaindersBackendError: Display> core::fmt::Display
for BackendError<CompressedBackendError, RemaindersBackendError>
{
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
match self {
Self::Compressed(err) => {
write!(f, "Read/write error when accessing compressed: {err}")
}
Self::Remainders(err) => {
write!(f, "Read/write error when accessing remainders: {err}")
}
}
}
}
#[cfg(feature = "std")]
impl<
CompressedBackendError: std::error::Error + 'static,
RemaindersBackendError: std::error::Error + 'static,
> std::error::Error for BackendError<CompressedBackendError, RemaindersBackendError>
{
fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
match self {
Self::Compressed(err) => Some(err),
Self::Remainders(err) => Some(err),
}
}
}
#[derive(Debug, PartialEq, Eq)]
pub enum ChangePrecisionError<Word, RemaindersBackend>
where
RemaindersBackend: WriteWords<Word> + ReadWords<Word, Stack>,
{
Increase(CoderError<Infallible, BackendError<Infallible, RemaindersBackend::WriteError>>),
Decrease(
CoderError<EncoderFrontendError, BackendError<Infallible, RemaindersBackend::ReadError>>,
),
}
impl<Word, RemaindersBackend> Display for ChangePrecisionError<Word, RemaindersBackend>
where
RemaindersBackend: WriteWords<Word> + ReadWords<Word, Stack>,
RemaindersBackend::WriteError: Display,
RemaindersBackend::ReadError: Display,
{
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
match self {
ChangePrecisionError::Increase(err) => {
write!(f, "Error while increasing precision of chain coder: {err}")
}
ChangePrecisionError::Decrease(err) => {
write!(f, "Error while decreasing precision of chain coder: {err}")
}
}
}
}
#[cfg(feature = "std")]
impl<Word, RemaindersBackend> std::error::Error for ChangePrecisionError<Word, RemaindersBackend>
where
Self: core::fmt::Debug,
RemaindersBackend: WriteWords<Word> + ReadWords<Word, Stack>,
RemaindersBackend::WriteError: std::error::Error + 'static,
RemaindersBackend::ReadError: std::error::Error + 'static,
{
fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
match self {
Self::Increase(err) => Some(err),
Self::Decrease(err) => Some(err),
}
}
}
impl<Word, State, CompressedBackend, RemaindersBackend, const PRECISION: usize> PosSeek
for ChainCoder<Word, State, CompressedBackend, RemaindersBackend, PRECISION>
where
Word: BitArray + Into<State>,
State: BitArray + AsPrimitive<Word>,
CompressedBackend: PosSeek,
RemaindersBackend: PosSeek,
{
type Position = (
BackendPosition<CompressedBackend::Position, RemaindersBackend::Position>,
ChainCoderHeads<Word, State, PRECISION>,
);
}
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct BackendPosition<CompressedPosition, RemaindersPosition> {
pub compressed: CompressedPosition,
pub remainders: RemaindersPosition,
}
impl<Word, State, CompressedBackend, RemaindersBackend, const PRECISION: usize> Pos
for ChainCoder<Word, State, CompressedBackend, RemaindersBackend, PRECISION>
where
Word: BitArray + Into<State>,
State: BitArray + AsPrimitive<Word>,
CompressedBackend: Pos,
RemaindersBackend: Pos,
{
fn pos(&self) -> Self::Position {
(
BackendPosition {
compressed: self.compressed.pos(),
remainders: self.remainders.pos(),
},
self.state(),
)
}
}
impl<Word, State, CompressedBackend, RemaindersBackend, const PRECISION: usize> Seek
for ChainCoder<Word, State, CompressedBackend, RemaindersBackend, PRECISION>
where
Word: BitArray + Into<State>,
State: BitArray + AsPrimitive<Word>,
CompressedBackend: Seek,
RemaindersBackend: Seek,
{
fn seek(&mut self, (pos, state): Self::Position) -> Result<(), ()> {
self.compressed.seek(pos.compressed)?;
self.remainders.seek(pos.remainders)?;
// `state` is valid since we don't provide a public API to modify fields of
// `ChainCoderHeads` individually.
self.heads = state;
Ok(())
}
}
impl<Word, State, CompressedBackend, RemaindersBackend, const PRECISION: usize> Decode<PRECISION>
for ChainCoder<Word, State, CompressedBackend, RemaindersBackend, PRECISION>
where
Word: BitArray + Into<State>,
State: BitArray + AsPrimitive<Word>,
CompressedBackend: ReadWords<Word, Stack>,
RemaindersBackend: WriteWords<Word>,
{
type FrontendError = DecoderFrontendError;
type BackendError = BackendError<CompressedBackend::ReadError, RemaindersBackend::WriteError>;
fn decode_symbol<M>(
&mut self,
model: M,
) -> Result<M::Symbol, DecoderError<Word, CompressedBackend, RemaindersBackend>>
where
M: DecoderModel<PRECISION>,
M::Probability: Into<Self::Word>,
Self::Word: AsPrimitive<M::Probability>,
{
assert!(PRECISION <= Word::BITS);
assert!(PRECISION != 0);
assert!(State::BITS >= Word::BITS + PRECISION);
let word = if PRECISION == Word::BITS
|| self.heads.compressed.get() < Word::one() << PRECISION
{
let word = self
.compressed
.read()
.map_err(BackendError::Compressed)?
.ok_or(CoderError::Frontend(
DecoderFrontendError::OutOfCompressedData,
))?;
if PRECISION != Word::BITS {
self.heads.compressed = unsafe {
// SAFETY:
// - `0 < PRECISION < Word::BITS` as per our assertion and the above check,
// therefore `Word::BITS - PRECISION > 0` and both the left-shift and
// the right-shift are valid;
// - `heads.compressed.get() != 0` sinze `heads.compressed` is a `NonZero`.
// - `heads.compressed.get() < 1 << PRECISION`, so all its "one" bits are
// in the `PRECISION` lowest significant bits; since it we have
// `Word::BITS` bits available, shifting left by `Word::BITS - PRECISION`
// doesn't truncate, and thus the result is also nonzero.
Word::NonZero::new_unchecked(
self.heads.compressed.get() << (Word::BITS - PRECISION) | word >> PRECISION,
)
};
}
word
} else {
let word = self.heads.compressed.get();
self.heads.compressed = unsafe {
// SAFETY: `heads.compressed.get() >= 1 << PRECISION`, so shifting right by
// `PRECISION` doesn't result in zero.
Word::NonZero::new_unchecked(self.heads.compressed.get() >> PRECISION)
};
word
};
let quantile = if PRECISION == Word::BITS {
word
} else {
word % (Word::one() << PRECISION)
};
let quantile = quantile.as_();
let (symbol, left_sided_cumulative, probability) = model.quantile_function(quantile);
let remainder = quantile - left_sided_cumulative;
// This can't truncate because
// - we maintain the invariant `heads.remainders < 1 << (State::BITS - PRECISION)`; and
// - `probability <= 1 << PRECISION` and `remainder < probability`.
// Thus, `remainders * proability + remainder < (remainders + 1) * probability`
// which is `<= (1 << (State::BITS - PRECISION)) << PRECISION = 1 << State::BITS`.
self.heads.remainders =
self.heads.remainders * probability.get().into().into() + remainder.into().into();
if self.heads.remainders >= State::one() << (State::BITS - PRECISION) {
// The invariant on `self.heads.remainders` (see its doc comment) is violated and must
// be restored.
self.flush_remainders_head()?;
}
Ok(symbol)
}
fn maybe_exhausted(&self) -> bool {
self.compressed.maybe_exhausted() || self.remainders.maybe_full()
}
}
impl<Word, State, CompressedBackend, RemaindersBackend, const PRECISION: usize> Encode<PRECISION>
for ChainCoder<Word, State, CompressedBackend, RemaindersBackend, PRECISION>
where
Word: BitArray + Into<State>,
State: BitArray + AsPrimitive<Word>,
CompressedBackend: WriteWords<Word>,
RemaindersBackend: ReadWords<Word, Stack>,
{
type FrontendError = EncoderFrontendError;
type BackendError = BackendError<CompressedBackend::WriteError, RemaindersBackend::ReadError>;
fn encode_symbol<M>(
&mut self,
symbol: impl Borrow<M::Symbol>,
model: M,
) -> Result<(), EncoderError<Word, CompressedBackend, RemaindersBackend>>
where
M: EncoderModel<PRECISION>,
M::Probability: Into<Self::Word>,
Self::Word: AsPrimitive<M::Probability>,
{
// assert!(State::BITS >= Word::BITS + PRECISION);
assert!(PRECISION <= Word::BITS);
assert!(PRECISION > 0);
let (left_sided_cumulative, probability) = model
.left_cumulative_and_probability(symbol)
.ok_or(CoderError::Frontend(EncoderFrontendError::ImpossibleSymbol))?;
if self.heads.remainders
< probability.get().into().into() << (State::BITS - Word::BITS - PRECISION)
{
self.refill_remainders_head()?;
// At this point, the invariant on `self.heads.remainders` (see its doc comment) is
// temporarily violated (but it will be restored below). This is how
// `decode_symbol` can detect that it has to flush `remainders.state`.
}
let remainder = (self.heads.remainders % probability.get().into().into())
.as_()
.as_();
let quantile = (left_sided_cumulative + remainder).into();
self.heads.remainders = self.heads.remainders / probability.get().into().into();
if PRECISION != Word::BITS
&& self.heads.compressed.get() < Word::one() << (Word::BITS - PRECISION)
{
unsafe {
// SAFETY:
// - `heads.compressed` is nonzero because it is a `NonZero`
// - `heads.compressed`, has `Word::BITS` bits and we checked above that all its one
// bits are within theleast significant `Word::BITS - PRECISION` bits. Thus, the
// most significant `PRECISION` bits are 0 and the left-shift doesn't truncate.
// Thus, the result of the left-shift is also noznero.
self.heads.compressed =
(self.heads.compressed.get() << PRECISION | quantile).into_nonzero_unchecked();
}
} else {
let word = if PRECISION == Word::BITS {
quantile
} else {
let word = self.heads.compressed.get() << PRECISION | quantile;
unsafe {
// SAFETY: if we're here then `heads.compressed >= 1 << (Word::BITS - PRECISION).
// Thus, shifting right by this amount of bits leaves at least one 1 bit.
self.heads.compressed = (self.heads.compressed.get()
>> (Word::BITS - PRECISION))
.into_nonzero_unchecked();
}
word
};
self.compressed
.write(word)
.map_err(BackendError::Compressed)?;
}
Ok(())
}
fn maybe_full(&self) -> bool {
self.remainders.maybe_exhausted() || self.compressed.maybe_full()
}
}
#[cfg(test)]
mod tests {
use super::super::model::LeakyQuantizer;
use super::*;
use probability::distribution::Gaussian;
use rand_xoshiro::{
rand_core::{RngCore, SeedableRng},
Xoshiro256StarStar,
};
use alloc::vec;
#[test]
fn restore_none() {
generic_restore_many::<u32, u64, u32, 24>(4, 0);
}
#[test]
fn restore_one() {
generic_restore_many::<u32, u64, u32, 24>(5, 1);
}
#[test]
fn restore_two() {
generic_restore_many::<u32, u64, u32, 24>(5, 2);
}
#[test]
fn restore_ten() {
generic_restore_many::<u32, u64, u32, 24>(20, 10);
}
#[test]
fn restore_twenty() {
generic_restore_many::<u32, u64, u32, 24>(19, 20);
}
#[test]
fn restore_many_u32_u64_32() {
generic_restore_many::<u32, u64, u32, 32>(1024, 1000);
}
#[test]
fn restore_many_u32_u64_24() {
generic_restore_many::<u32, u64, u32, 24>(1024, 1000);
}
#[test]
fn restore_many_u32_u64_16() {
generic_restore_many::<u32, u64, u16, 16>(1024, 1000);
}
#[test]
fn restore_many_u16_u64_16() {
generic_restore_many::<u16, u64, u16, 16>(1024, 1000);
}
#[test]
fn restore_many_u32_u64_8() {
generic_restore_many::<u32, u64, u8, 8>(1024, 1000);
}
#[test]
fn restore_many_u16_u64_8() {
generic_restore_many::<u16, u64, u8, 8>(1024, 1000);
}
#[test]
fn restore_many_u8_u64_8() {
generic_restore_many::<u8, u64, u8, 8>(1024, 1000);
}
#[test]
fn restore_many_u16_u32_16() {
generic_restore_many::<u16, u32, u16, 16>(1024, 1000);
}
#[test]
fn restore_many_u16_u32_8() {
generic_restore_many::<u16, u32, u8, 8>(1024, 1000);
}
#[test]
fn restore_many_u8_u32_8() {
generic_restore_many::<u8, u32, u8, 8>(1024, 1000);
}
fn generic_restore_many<Word, State, Probability, const PRECISION: usize>(
amt_compressed_words: usize,
amt_symbols: usize,
) where
State: BitArray + AsPrimitive<Word>,
Word: BitArray + Into<State> + AsPrimitive<Probability>,
Probability: BitArray + Into<Word> + AsPrimitive<usize> + Into<f64>,
u64: AsPrimitive<Word>,
u32: AsPrimitive<Probability>,
usize: AsPrimitive<Probability>,
f64: AsPrimitive<Probability>,
i32: AsPrimitive<Probability>,
{
#[cfg(miri)]
let (amt_compressed_words, amt_symbols) =
(amt_compressed_words.min(128), amt_symbols.min(100));
let mut rng = Xoshiro256StarStar::seed_from_u64(
(amt_compressed_words as u64).rotate_left(32) ^ amt_symbols as u64,
);
let mut compressed = (0..amt_compressed_words)
.map(|_| rng.next_u64().as_())
.collect::<Vec<_>>();
// Make the last compressed word have a random number of leading zero bits so that
// we test various filling levels.
let leading_zeros = (rng.next_u32() % (Word::BITS as u32 - 1)) as usize;
let last_word = compressed.last_mut().unwrap();
*last_word = *last_word | Word::one() << (Word::BITS - leading_zeros - 1);
*last_word = *last_word & Word::max_value() >> leading_zeros;
let distributions = (0..amt_symbols)
.map(|_| {
let mean = (200.0 / u32::MAX as f64) * rng.next_u32() as f64 - 100.0;
let std_dev = (10.0 / u32::MAX as f64) * rng.next_u32() as f64 + 0.001;
Gaussian::new(mean, std_dev)
})
.collect::<Vec<_>>();
let quantizer = LeakyQuantizer::<_, _, Probability, PRECISION>::new(-100..=100);
let mut coder =
ChainCoder::<Word, State, Vec<Word>, Vec<Word>, PRECISION>::from_compressed(
compressed.clone(),
)
.unwrap();
let symbols = coder
.decode_symbols(
distributions
.iter()
.map(|&distribution| quantizer.quantize(distribution)),
)
.collect::<Result<Vec<_>, _>>()
.unwrap();
assert!(!coder.maybe_exhausted());
let (remainders_prefix, remainders_suffix) = coder.clone().into_remainders().unwrap();
let mut remainders = remainders_prefix.clone();
remainders.extend_from_slice(&remainders_suffix);
let coder2 = ChainCoder::from_remainders(remainders).unwrap();
let coder3 = ChainCoder::from_remainders(remainders_suffix).unwrap();
for (mut coder, prefix) in [
(coder, vec![]),
(coder2, vec![]),
(coder3, remainders_prefix),
] {
coder
.encode_symbols_reverse(
symbols
.iter()
.zip(&distributions)
.map(|(&symbol, &distribution)| (symbol, quantizer.quantize(distribution))),
)
.unwrap();
let (compressed_prefix, compressed_suffix) = coder.into_compressed().unwrap();
let mut reconstructed = prefix;
reconstructed.extend(compressed_prefix);
reconstructed.extend(compressed_suffix);
assert_eq!(reconstructed, compressed);
}
}
}