1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764
//! Entropy Coding Primitives for Research and Production
//!
//! The `constriction` crate provides a set of composable entropy coding algorithms with a
//! focus on correctness, versatility, ease of use, compression performance, and
//! computational efficiency. The goals of `constriction` are to three-fold:
//!
//! 1. **to facilitate research on novel lossless and lossy compression methods** by
//! providing a *composable* set of entropy coding primitives rather than a rigid
//! implementation of a single preconfigured method;
//! 2. **to simplify the transition from research code to production software** by exposing
//! the exact same functionality via both a Python API (for rapid prototyping on research
//! code) and a Rust API (for turning successful prototypes into production); and
//! 3. **to serve as a teaching resource** by providing a wide range of entropy coding
//! algorithms within a single consistent framework, thus making the various algorithms
//! easily discoverable and comparable on example models and data. [Additional teaching
//! material](https://robamler.github.io/teaching/compress21/) is being made publicly
//! available as a by-product of an ongoing university course on data compression with
//! deep probabilistic models.
//!
//! For an example of a compression codec that started as research code in Python and was
//! then deployed as a fast and dependency-free WebAssembly module using `constriction`'s
//! Rust API, have a look at [The Linguistic Flux
//! Capacitor](https://robamler.github.io/linguistic-flux-capacitor).
//!
//! # Project Status
//!
//! We currently provide implementations of the following entropy coding algorithms:
//!
//! - **Asymmetric Numeral Systems (ANS):** a fast modern entropy coder with near-optimal
//! compression effectiveness that supports advanced use cases like bits-back coding.
//! - **Range Coding:** a computationally efficient variant of Arithmetic Coding, that has
//! essentially the same compression effectiveness as ANS Coding but operates as a queue
//! ("first in first out"), which makes it preferable for autoregressive models.
//! - **Chain Coding:** an experimental new entropy coder that combines the (net)
//! effectiveness of stream codes with the locality of symbol codes; it is meant for
//! experimental new compression approaches that perform joint inference, quantization,
//! and bits-back coding in an end-to-end optimization. This experimental coder is mainly
//! provided to prove to ourselves that the API for encoding and decoding, which is shared
//! across all stream coders, is flexible enough to express complex novel tasks.
//! - **Huffman Coding:** a well-known symbol code, mainly provided here for teaching
//! purpose; you'll usually want to use a stream code like ANS or Range Coding instead
//! since symbol codes can have a considerable overhead on the bitrate, especially in the
//! regime of low entropy per symbol, which is common in machine-learning based
//! compression methods.
//!
//! Further, `constriction` provides implementations of common probability distributions in
//! fixed-point arithmetic, which can be used as entropy models in either of the above
//! stream codes. The crate also provides adapters for turning custom probability
//! distributions into exactly invertible fixed-point arithmetic.
//!
//! The provided implementations of entropy coding algorithms and probability distributions
//! are extensively tested and should be considered reliable (except for the still
//! experimental Chain Coder). However, their APIs may change in future versions of
//! `constriction` if more user experience reveals any shortcomings of the current APIs in
//! terms of ergonomics. Please [file an
//! issue](https://github.com/bamler-lab/constriction/issues) if you run into a scenario
//! where the current APIs are suboptimal.
//!
//! # Quick Start With the Rust API
//!
//! You are currently reading the documentation of `constriction`'s Rust API. If Rust is not
//! your language of choice then head over to the [Python API
//! Documentation](https://bamler-lab.github.io/constriction/apidoc/python/). The Rust API
//! provides efficient and composable entropy coding primitives that can be adjusted to a
//! fine degree of detail using type parameters and const generics (type aliases with sane
//! defaults for all generic parameters are provided as a guidance). The Python API exposes
//! the most common use cases of these entropy coding primitives to an environment that
//! feels more natural to many data scientists.
//!
//! ## Setup
//!
//! To use `constriction` in your Rust project, just add the following line to the
//! `[dependencies]` section of your `Cargo.toml`:
//!
//! ```toml
//! [dependencies]
//! constriction = "0.3.5"
//! ```
//!
//! ## System Requirements
//!
//! `constriction` requires Rust version 1.51 or later for its use of the
//! `min_const_generics` feature. If you have an older version of Rust, update to the latest
//! version by running `rustup update stable`.
//!
//! ## Encoding Example
//!
//! In this example, we'll encode some symbols using a quantized Gaussian distribution as
//! entropy model. Each symbol will be modeled by a quantized Gaussian with a different
//! mean and standard deviation (so that the example is not too simplistic). We'll use the
//! `probability` crate for the Gaussian distributions, so also add the following dependency
//! to your `Cargo.toml`:
//!
//! ```toml
//! probability = "0.17"
//! ```
//!
//! Now, let's encode (i.e., compress) some symbols. We'll use an Asymmetric Numeral Systems
//! (ANS) Coder here for its speed and compression performance. We'll discuss how you could
//! replace the ANS Coder with a Range Coder or a symbol code like Huffman Coding
//! [below](#exercise).
//!
//! ```
//! use constriction::stream::{stack::DefaultAnsCoder, model::DefaultLeakyQuantizer};
//! use probability::distribution::Gaussian;
//!
//! fn encode_sample_data() -> Vec<u32> {
//! // Create an empty ANS Coder with default word and state size:
//! let mut coder = DefaultAnsCoder::new();
//!
//! // Some made up data and entropy models for demonstration purpose:
//! let symbols = [23i32, -15, 78, 43, -69];
//! let means = [35.2, -1.7, 30.1, 71.2, -75.1];
//! let stds = [10.1, 25.3, 23.8, 35.4, 3.9];
//!
//! // Create an adapter that integrates 1-d probability density functions over bins
//! // `[n - 0.5, n + 0.5)` for all integers `n` from `-100` to `100` using fixed point
//! // arithmetic with default precision, guaranteeing a nonzero probability for each bin:
//! let quantizer = DefaultLeakyQuantizer::new(-100..=100);
//!
//! // Encode the data (in reverse order, since ANS Coding operates as a stack):
//! coder.encode_symbols_reverse(
//! symbols.iter().zip(&means).zip(&stds).map(
//! |((&sym, &mean), &std)| (sym, quantizer.quantize(Gaussian::new(mean, std)))
//! )).unwrap();
//!
//! // Retrieve the compressed representation (filling it up to full words with zero bits).
//! coder.into_compressed().unwrap()
//! }
//!
//! assert_eq!(encode_sample_data(), [0x421C_7EC3, 0x000B_8ED1]);
//! ```
//!
//! ## Decoding Example
//!
//! Now, let's reconstruct the sample data from its compressed representation.
//!
//! ```
//! use constriction::stream::{stack::DefaultAnsCoder, model::DefaultLeakyQuantizer, Decode};
//! use probability::distribution::Gaussian;
//!
//! fn decode_sample_data(compressed: Vec<u32>) -> Vec<i32> {
//! // Create an ANS Coder with default word and state size from the compressed data:
//! // (ANS uses the same type for encoding and decoding, which makes the method very flexible
//! // and allows interleaving small encoding and decoding chunks, e.g., for bits-back coding.)
//! let mut coder = DefaultAnsCoder::from_compressed(compressed).unwrap();
//!
//! // Same entropy models and quantizer we used for encoding:
//! let means = [35.2, -1.7, 30.1, 71.2, -75.1];
//! let stds = [10.1, 25.3, 23.8, 35.4, 3.9];
//! let quantizer = DefaultLeakyQuantizer::new(-100..=100);
//!
//! // Decode the data:
//! coder.decode_symbols(
//! means.iter().zip(&stds).map(
//! |(&mean, &std)| quantizer.quantize(Gaussian::new(mean, std))
//! )).collect::<Result<Vec<_>, _>>().unwrap()
//! }
//!
//! assert_eq!(decode_sample_data(vec![0x421C_7EC3, 0x000B_8ED1]), [23, -15, 78, 43, -69]);
//! ```
//!
//! ## Exercise
//!
//! Try out the above examples and verify that decoding reconstructs the original data. Then
//! see how easy `constriction` makes it to replace the ANS Coder with a Range Coder by
//! making the following substitutions:
//!
//! **In the encoder,**
//!
//! - replace `constriction::stream::stack::DefaultAnsCoder` with
//! `constriction::stream::queue::DefaultRangeEncoder`; and
//! - replace `coder.encode_symbols_reverse` with `coder.encode_symbols` (you no longer need
//! to encode symbols in reverse order since Range Coding operates as a queue, i.e.,
//! first-in-first-out). You'll also have to add the line
//! `use constriction::stream::Encode;` to the top of the file to bring the trait method
//! `encode_symbols` into scope.
//!
//! **In the decoder,**
//!
//! - replace `constriction::stream::stack::DefaultAnsCoder` with
//! `constriction::stream::queue::DefaultRangeDecoder` (note that Range Coding
//! distinguishes between an encoder and a decoder type since the encoder writes to the
//! back while the decoder reads from the front; by contrast, ANS Coding is a stack, i.e.,
//! it reads and writes at the same position and allows interleaving reads and writes).
//!
//! *Remark:* You could also use a symbol code like Huffman Coding (see module [`symbol`])
//! but that would have considerably worse compression performance, especially on large
//! files, since symbol codes always emit an integer number of bits per compressed symbol,
//! even if the information content of the symbol is a fractional number (stream codes like
//! ANS and Range Coding *effectively* emit a fractional number of bits per symbol since
//! they amortize over several symbols).
//!
//! The above replacements should lead you to something like this:
//!
//! ```
//! use constriction::stream::{
//! model::DefaultLeakyQuantizer,
//! queue::{DefaultRangeEncoder, DefaultRangeDecoder},
//! Encode, Decode,
//! };
//! use probability::distribution::Gaussian;
//!
//! fn encode_sample_data() -> Vec<u32> {
//! // Create an empty Range Encoder with default word and state size:
//! let mut encoder = DefaultRangeEncoder::new();
//!
//! // Same made up data, entropy models, and quantizer as in the ANS Coding example above:
//! let symbols = [23i32, -15, 78, 43, -69];
//! let means = [35.2, -1.7, 30.1, 71.2, -75.1];
//! let stds = [10.1, 25.3, 23.8, 35.4, 3.9];
//! let quantizer = DefaultLeakyQuantizer::new(-100..=100);
//!
//! // Encode the data (this time in normal order, since Range Coding is a queue):
//! encoder.encode_symbols(
//! symbols.iter().zip(&means).zip(&stds).map(
//! |((&sym, &mean), &std)| (sym, quantizer.quantize(Gaussian::new(mean, std)))
//! )).unwrap();
//!
//! // Retrieve the (sealed up) compressed representation.
//! encoder.into_compressed().unwrap()
//! }
//!
//! fn decode_sample_data(compressed: Vec<u32>) -> Vec<i32> {
//! // Create a Range Decoder with default word and state size from the compressed data:
//! let mut decoder = DefaultRangeDecoder::from_compressed(compressed).unwrap();
//!
//! // Same entropy models and quantizer we used for encoding:
//! let means = [35.2, -1.7, 30.1, 71.2, -75.1];
//! let stds = [10.1, 25.3, 23.8, 35.4, 3.9];
//! let quantizer = DefaultLeakyQuantizer::new(-100..=100);
//!
//! // Decode the data:
//! decoder.decode_symbols(
//! means.iter().zip(&stds).map(
//! |(&mean, &std)| quantizer.quantize(Gaussian::new(mean, std))
//! )).collect::<Result<Vec<_>, _>>().unwrap()
//! }
//!
//! let compressed = encode_sample_data();
//!
//! // We'll get a different compressed representation than in the ANS Coding
//! // example because we're using a different entropy coding algorithm ...
//! assert_eq!(compressed, [0x1C31EFEB, 0x87B430DA]);
//!
//! // ... but as long as we decode with the matching algorithm we can still reconstruct the data:
//! assert_eq!(decode_sample_data(compressed), [23, -15, 78, 43, -69]);
//! ```
//!
//! # Where to Go Next?
//!
//! If you already have an entropy model and you just want to encode and decode some
//! sequence of symbols then you can probably start by adjusting the above
//! [examples](#encoding-example) to your needs. Or have a closer look at the [`stream`]
//! module.
//!
//! If you're still new to the concept of entropy coding then check out the [teaching
//! material](https://robamler.github.io/teaching/compress21/).
#![no_std]
#![warn(rust_2018_idioms, missing_debug_implementations)]
extern crate alloc;
#[cfg(feature = "std")]
extern crate std;
#[cfg(feature = "pybindings")]
mod pybindings;
pub mod backends;
pub mod stream;
pub mod symbol;
use core::{
convert::Infallible,
fmt::{Binary, Debug, Display, LowerHex, UpperHex},
hash::Hash,
num::{NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU8, NonZeroUsize},
};
use num_traits::{AsPrimitive, PrimInt, Unsigned, WrappingAdd, WrappingMul, WrappingSub};
// READ WRITE SEMANTICS =======================================================
/// A trait for marking how reading and writing order relate to each other.
///
/// This is currently only used in the [`backends`] module. Future versions of
/// `constriction` may expand its use to frontends.
pub trait Semantics: Default {}
/// Zero sized marker struct for last-in-first-out read/write [`Semantics`]
///
/// This type typically only comes up in advanced use cases that are generic over read/write
/// semantics. If you are looking for an entropy coder that operates as a stack, check out
/// the module [`stream::stack`].
#[derive(Debug, Default)]
pub struct Stack {}
impl Semantics for Stack {}
/// Zero sized marker struct for first-in-first-out read/write [`Semantics`]
///
/// This type typically only comes up in advanced use cases that are generic over read/write
/// semantics. If you are looking for an entropy coder that operates as a queue, check out
/// the module [`stream::queue`].
#[derive(Debug, Default)]
pub struct Queue {}
impl Semantics for Queue {}
// GENERIC ERROR TYPES ========================================================
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum CoderError<FrontendError, BackendError> {
Frontend(FrontendError),
Backend(BackendError),
}
impl<FrontendError, BackendError> CoderError<FrontendError, BackendError> {
pub fn map_frontend<E>(self, f: impl Fn(FrontendError) -> E) -> CoderError<E, BackendError> {
match self {
Self::Frontend(err) => CoderError::Frontend(f(err)),
Self::Backend(err) => CoderError::Backend(err),
}
}
pub fn map_backend<E>(self, f: impl Fn(BackendError) -> E) -> CoderError<FrontendError, E> {
match self {
Self::Backend(err) => CoderError::Backend(f(err)),
Self::Frontend(err) => CoderError::Frontend(err),
}
}
}
impl<BackendError: Display, FrontendError: Display> Display
for CoderError<FrontendError, BackendError>
{
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
match self {
Self::Frontend(err) => write!(f, "Invalid compressed data: {err}"),
Self::Backend(err) => write!(f, "Error while reading compressed data: {err}"),
}
}
}
#[cfg(feature = "std")]
impl<FrontendError: std::error::Error + 'static, BackendError: std::error::Error + 'static>
std::error::Error for CoderError<FrontendError, BackendError>
{
fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
match self {
Self::Frontend(source) => Some(source),
Self::Backend(source) => Some(source),
}
}
}
impl<FrontendError, BackendError> From<BackendError> for CoderError<FrontendError, BackendError> {
fn from(read_error: BackendError) -> Self {
Self::Backend(read_error)
}
}
impl<FrontendError> CoderError<FrontendError, Infallible> {
fn into_frontend_error(self) -> FrontendError {
match self {
CoderError::Frontend(frontend_error) => frontend_error,
CoderError::Backend(infallible) => match infallible {},
}
}
}
type DefaultEncoderError<BackendError> = CoderError<DefaultEncoderFrontendError, BackendError>;
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum DefaultEncoderFrontendError {
/// Tried to encode a symbol with zero probability under the used entropy model.
///
/// This error can usually be avoided by using a "leaky" distribution, as the
/// entropy model, i.e., a distribution that assigns a nonzero probability to all
/// symbols within a finite domain. Leaky distributions can be constructed with,
/// e.g., a [`LeakyQuantizer`](models/struct.LeakyQuantizer.html) or with
/// [`LeakyCategorical::from_floating_point_probabilities`](
/// models/struct.LeakyCategorical.html#method.from_floating_point_probabilities).
ImpossibleSymbol,
}
impl Display for DefaultEncoderFrontendError {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
match self {
Self::ImpossibleSymbol => write!(
f,
"Tried to encode symbol that has zero probability under the used entropy model."
),
}
}
}
#[cfg(feature = "std")]
impl std::error::Error for DefaultEncoderFrontendError {}
impl DefaultEncoderFrontendError {
#[inline(always)]
const fn into_coder_error<BackendError>(self) -> DefaultEncoderError<BackendError> {
DefaultEncoderError::Frontend(self)
}
}
/// Trait for coders or backends that *might* implement [`Pos`] and/or [`Seek`]
///
/// If a type implements `PosSeek` then that doesn't necessarily mean that it also
/// implements [`Pos`] or [`Seek`]. Implementing `PosSeek` only fixes the common `Position`
/// type that is used *if* the type implements `Pos` and/or `Seek`.
pub trait PosSeek {
type Position: Clone;
}
/// A trait for entropy coders that keep track of their current position within the
/// compressed data.
///
/// This is the counterpart of [`Seek`]. Call [`Pos::pos`] to record
/// "snapshots" of an entropy coder, and then call [`Seek::seek`] at a later time
/// to jump back to these snapshots. See examples in the documentations of [`Seek`]
/// and [`Seek::seek`].
pub trait Pos: PosSeek {
/// Returns the position in the compressed data, in units of `Word`s.
///
/// It is up to the entropy coder to define what constitutes the beginning and end
/// positions within the compressed data (for example, a [`AnsCoder`] begins encoding
/// at position zero but it begins decoding at position `ans.buf().len()`).
///
/// [`AnsCoder`]: stream::stack::AnsCoder
fn pos(&self) -> Self::Position;
}
/// A trait for entropy coders that support random access.
///
/// This is the counterpart of [`Pos`]. While [`Pos::pos`] can be used to
/// record "snapshots" of an entropy coder, [`Seek::seek`] can be used to jump to these
/// recorded snapshots.
///
/// Not all entropy coders that implement `Pos` also implement `Seek`. For example,
/// [`DefaultAnsCoder`] implements `Pos` but it doesn't implement `Seek` because it
/// supports both encoding and decoding and therefore always operates at the head. In
/// such a case one can usually obtain a seekable entropy coder in return for
/// surrendering some other property. For example, `DefaultAnsCoder` provides the methods
/// [`as_seekable_decoder`] and [`into_seekable_decoder`] that return a decoder which
/// implements `Seek` but which can no longer be used for encoding (i.e., it doesn't
/// implement [`Encode`]).
///
/// # Example
///
/// ```
/// use constriction::stream::{
/// model::DefaultContiguousCategoricalEntropyModel, stack::DefaultAnsCoder, Decode
/// };
/// use constriction::{Pos, Seek};
///
/// // Create a `AnsCoder` encoder and an entropy model:
/// let mut ans = DefaultAnsCoder::new();
/// let probabilities = vec![0.03, 0.07, 0.1, 0.1, 0.2, 0.2, 0.1, 0.15, 0.05];
/// let entropy_model = DefaultContiguousCategoricalEntropyModel
/// ::from_floating_point_probabilities(&probabilities).unwrap();
///
/// // Encode some symbols in two chunks and take a snapshot after each chunk.
/// let symbols1 = vec![8, 2, 0, 7];
/// ans.encode_iid_symbols_reverse(&symbols1, &entropy_model).unwrap();
/// let snapshot1 = ans.pos();
///
/// let symbols2 = vec![3, 1, 5];
/// ans.encode_iid_symbols_reverse(&symbols2, &entropy_model).unwrap();
/// let snapshot2 = ans.pos();
///
/// // As discussed above, `DefaultAnsCoder` doesn't impl `Seek` but we can get a decoder that does:
/// let mut seekable_decoder = ans.as_seekable_decoder();
///
/// // `seekable_decoder` is still a `AnsCoder`, so decoding would start with the items we encoded
/// // last. But since it implements `Seek` we can jump ahead to our first snapshot:
/// seekable_decoder.seek(snapshot1);
/// let decoded1 = seekable_decoder
/// .decode_iid_symbols(4, &entropy_model)
/// .collect::<Result<Vec<_>, _>>()
/// .unwrap();
/// assert_eq!(decoded1, symbols1);
///
/// // We've reached the end of the compressed data ...
/// assert!(seekable_decoder.is_empty());
///
/// // ... but we can still jump to somewhere else and continue decoding from there:
/// seekable_decoder.seek(snapshot2);
///
/// // Creating snapshots didn't mutate the coder, so we can just decode through `snapshot1`:
/// let decoded_both = seekable_decoder.decode_iid_symbols(7, &entropy_model).map(Result::unwrap);
/// assert!(decoded_both.eq(symbols2.into_iter().chain(symbols1)));
/// assert!(seekable_decoder.is_empty()); // <-- We've reached the end again.
/// ```
///
/// [`Encode`]: stream::Encode
/// [`DefaultAnsCoder`]: stream::stack::DefaultAnsCoder
/// [`as_seekable_decoder`]: stream::stack::AnsCoder::as_seekable_decoder
/// [`into_seekable_decoder`]: stream::stack::AnsCoder::into_seekable_decoder
pub trait Seek: PosSeek {
/// Jumps to a given position in the compressed data.
///
/// The argument `pos` is the same pair of values returned by
/// [`Pos::pos`], i.e., it is a tuple of the position in the compressed
/// data and the `State` to which the entropy coder should be restored. Both values
/// are absolute (i.e., seeking happens independently of the current state or
/// position of the entropy coder). The position is measured in units of
/// `Word`s (see second example below where we manipulate a position
/// obtained from `Pos::pos` in order to reflect a manual reordering of
/// the `Word`s in the compressed data).
///
/// # Examples
///
/// The method takes the position and state as a tuple rather than as independent
/// method arguments so that one can simply pass in the tuple obtained from
/// [`Pos::pos`] as sketched below:
///
/// ```
/// // Step 1: Obtain an encoder and encode some data (omitted for brevity) ...
/// # use constriction::{stream::stack::DefaultAnsCoder, Pos, Seek};
/// # let encoder = DefaultAnsCoder::new();
///
/// // Step 2: Take a snapshot by calling `Pos::pos`:
/// let snapshot = encoder.pos(); // <-- Returns a tuple `(pos, state)`.
///
/// // Step 3: Encode some more data and then obtain a decoder (omitted for brevity) ...
/// # let mut decoder = encoder.as_seekable_decoder();
///
/// // Step 4: Jump to snapshot by calling `Seek::seek`:
/// decoder.seek(snapshot); // <-- No need to deconstruct `snapshot` into `(pos, state)`.
/// ```
///
/// For more fine-grained control, one may want to assemble the tuple
/// `pos` manually. For example, a [`DefaultAnsCoder`] encodes data from
/// front to back and then decodes the data in the reverse direction from back to
/// front. Decoding from back to front may be inconvenient in some use cases, so one
/// might prefer to instead reverse the order of the `Word`s once encoding
/// is finished, and then decode them in the more natural direction from front to
/// back. Reversing the compressed data changes the position of each
/// `Word`, and so any positions obtained from `Pos` need to be adjusted
/// accordingly before they may be passed to `seek`, as in the following example:
///
/// ```
/// use constriction::{
/// stream::{model::LeakyQuantizer, stack::{DefaultAnsCoder, AnsCoder}, Decode},
/// Pos, Seek
/// };
///
/// // Construct a `DefaultAnsCoder` for encoding and an entropy model:
/// let mut encoder = DefaultAnsCoder::new();
/// let quantizer = LeakyQuantizer::<_, _, u32, 24>::new(-100..=100);
/// let entropy_model = quantizer.quantize(probability::distribution::Gaussian::new(0.0, 10.0));
///
/// // Encode two chunks of symbols and take a snapshot in-between:
/// encoder.encode_iid_symbols_reverse(-100..40, &entropy_model).unwrap();
/// let (mut snapshot_pos, snapshot_state) = encoder.pos();
/// encoder.encode_iid_symbols_reverse(50..101, &entropy_model).unwrap();
///
/// // Obtain compressed data, reverse it, and create a decoder that reads it from front to back:
/// let mut compressed = encoder.into_compressed().unwrap();
/// compressed.reverse();
/// snapshot_pos = compressed.len() - snapshot_pos; // <-- Adjusts the snapshot position.
/// let mut decoder = AnsCoder::from_reversed_compressed(compressed).unwrap();
///
/// // Since we chose to encode onto a stack, decoding yields the last encoded chunk first:
/// assert_eq!(decoder.decode_symbol(entropy_model).unwrap(), 50);
/// assert_eq!(decoder.decode_symbol(entropy_model).unwrap(), 51);
///
/// // To jump to our snapshot, we have to use the adjusted `snapshot_pos`:
/// decoder.seek((snapshot_pos, snapshot_state));
/// assert!(decoder.decode_iid_symbols(140, &entropy_model).map(Result::unwrap).eq(-100..40));
/// assert!(decoder.is_empty()); // <-- We've reached the end of the compressed data.
/// ```
///
/// [`DefaultAnsCoder`]: stream::stack::DefaultAnsCoder
#[allow(clippy::result_unit_err)]
fn seek(&mut self, pos: Self::Position) -> Result<(), ()>;
}
/// A trait for bit strings of fixed (and usually small) length.
///
/// Short fixed-length bit strings are fundamental building blocks of efficient entropy
/// coding algorithms. They are currently used for the following purposes:
/// - to represent the smallest unit of compressed data (see [`stream::Code::Word`]);
/// - to represent probabilities in fixed point arithmetic (see
/// [`stream::model::EntropyModel::Probability`]); and
/// - the internal state of entropy coders (see [`stream::Code::State`]) is typically comprised of
/// one or more `BitArray`s, although this is not a requirement.
///
/// This trait is implemented on all primitive unsigned integer types. It is not recommended
/// to implement this trait for custom types since coders will assume, for performance
/// considerations, that `BitArray`s can be represented and manipulated efficiently in
/// hardware.
///
/// # Safety
///
/// This trait is marked `unsafe` so that entropy coders may rely on the assumption that all
/// `BitArray`s have precisely the same behavior as builtin unsigned integer types, and that
/// [`BitArray::BITS`] has the correct value.
pub unsafe trait BitArray:
PrimInt
+ Unsigned
+ WrappingAdd
+ WrappingSub
+ WrappingMul
+ LowerHex
+ UpperHex
+ Binary
+ Default
+ Copy
+ Display
+ Debug
+ Eq
+ Hash
+ 'static
{
/// The (fixed) length of the `BitArray` in bits.
///
/// Defaults to `8 * core::mem::size_of::<Self>()`, which is suitable for all
/// primitive unsigned integers.
///
/// This could arguably be called `LEN` instead, but that may be confusing since
/// "lengths" are typically not measured in bits in the Rust ecosystem.
const BITS: usize = 8 * core::mem::size_of::<Self>();
type NonZero: NonZeroBitArray<Base = Self>;
#[inline(always)]
fn into_nonzero(self) -> Option<Self::NonZero> {
Self::NonZero::new(self)
}
/// # Safety
///
/// The provided value must be nonzero.
#[inline(always)]
unsafe fn into_nonzero_unchecked(self) -> Self::NonZero {
Self::NonZero::new_unchecked(self)
}
}
#[inline(always)]
fn wrapping_pow2<T: BitArray>(exponent: usize) -> T {
if exponent >= T::BITS {
T::zero()
} else {
T::one() << exponent
}
}
/// A trait for bit strings like [`BitArray`] but with guaranteed nonzero values
///
/// # Safety
///
/// Must guarantee that the value is indeed nonzero. Failing to do so could, e.g., cause a
/// division by zero in entropy coders, which is undefined behavior.
pub unsafe trait NonZeroBitArray: Copy + Display + Debug + Eq + Hash + 'static {
type Base: BitArray<NonZero = Self>;
fn new(n: Self::Base) -> Option<Self>;
/// # Safety
///
/// The provided value `n` must be nonzero.
unsafe fn new_unchecked(n: Self::Base) -> Self;
fn get(self) -> Self::Base;
}
/// Iterates from most significant to least significant bits in chunks but skips any
/// initial zero chunks.
fn bit_array_to_chunks_truncated<Data, Chunk>(
data: Data,
) -> impl Iterator<Item = Chunk> + ExactSizeIterator + DoubleEndedIterator
where
Data: BitArray + AsPrimitive<Chunk>,
Chunk: BitArray,
{
(0..(Data::BITS - data.leading_zeros() as usize))
.step_by(Chunk::BITS)
.rev()
.map(move |shift| (data >> shift).as_())
}
macro_rules! unsafe_impl_bit_array {
($(($base:ty, $non_zero:ty)),+ $(,)?) => {
$(
unsafe impl BitArray for $base {
type NonZero = $non_zero;
}
unsafe impl NonZeroBitArray for $non_zero {
type Base = $base;
#[inline(always)]
fn new(n: Self::Base) -> Option<Self> {
Self::new(n)
}
#[inline(always)]
unsafe fn new_unchecked(n: Self::Base) -> Self {
Self::new_unchecked(n)
}
#[inline(always)]
fn get(self) -> Self::Base {
let non_zero = self.get();
unsafe {
// SAFETY: This is trivially safe because `non_zero` came from a
// `NonZero` type. We really shouldn't have to give the compiler
// this hint it turns out the compiler would otherwise emit an
// unnecessary check for div by zero. The unnecessary check used to
// have a significant impact on performance, but it doesn't seem to
// anymore as of rust version 1.58.0 (although the check itself is
// still there).
if non_zero == num_traits::zero::<Self::Base>() {
core::hint::unreachable_unchecked();
} else {
non_zero
}
}
}
}
)+
};
}
unsafe_impl_bit_array!(
(u8, NonZeroU8),
(u16, NonZeroU16),
(u32, NonZeroU32),
(u64, NonZeroU64),
(usize, NonZeroUsize),
);
#[cfg(feature = "std")]
unsafe_impl_bit_array!((u128, core::num::NonZeroU128),);
pub trait UnwrapInfallible<T> {
fn unwrap_infallible(self) -> T;
}
impl<T> UnwrapInfallible<T> for Result<T, Infallible> {
#[inline(always)]
fn unwrap_infallible(self) -> T {
match self {
Ok(x) => x,
Err(infallible) => match infallible {},
}
}
}
impl<T> UnwrapInfallible<T> for Result<T, CoderError<Infallible, Infallible>> {
fn unwrap_infallible(self) -> T {
match self {
Ok(x) => x,
Err(infallible) => match infallible {
CoderError::Backend(infallible) => match infallible {},
CoderError::Frontend(infallible) => match infallible {},
},
}
}
}