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Diffstat (limited to 'third_party/rust/unicode-ident/src/lib.rs')
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diff --git a/third_party/rust/unicode-ident/src/lib.rs b/third_party/rust/unicode-ident/src/lib.rs new file mode 100644 index 0000000000..42126dd2b5 --- /dev/null +++ b/third_party/rust/unicode-ident/src/lib.rs @@ -0,0 +1,268 @@ +//! [![github]](https://github.com/dtolnay/unicode-ident) [![crates-io]](https://crates.io/crates/unicode-ident) [![docs-rs]](https://docs.rs/unicode-ident) +//! +//! [github]: https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github +//! [crates-io]: https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust +//! [docs-rs]: https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs +//! +//! <br> +//! +//! Implementation of [Unicode Standard Annex #31][tr31] for determining which +//! `char` values are valid in programming language identifiers. +//! +//! [tr31]: https://www.unicode.org/reports/tr31/ +//! +//! This crate is a better optimized implementation of the older `unicode-xid` +//! crate. This crate uses less static storage, and is able to classify both +//! ASCII and non-ASCII codepoints with better performance, 2–10× +//! faster than `unicode-xid`. +//! +//! <br> +//! +//! ## Comparison of performance +//! +//! The following table shows a comparison between five Unicode identifier +//! implementations. +//! +//! - `unicode-ident` is this crate; +//! - [`unicode-xid`] is a widely used crate run by the "unicode-rs" org; +//! - `ucd-trie` and `fst` are two data structures supported by the +//! [`ucd-generate`] tool; +//! - [`roaring`] is a Rust implementation of Roaring bitmap. +//! +//! The *static storage* column shows the total size of `static` tables that the +//! crate bakes into your binary, measured in 1000s of bytes. +//! +//! The remaining columns show the **cost per call** to evaluate whether a +//! single `char` has the XID\_Start or XID\_Continue Unicode property, +//! comparing across different ratios of ASCII to non-ASCII codepoints in the +//! input data. +//! +//! [`unicode-xid`]: https://github.com/unicode-rs/unicode-xid +//! [`ucd-generate`]: https://github.com/BurntSushi/ucd-generate +//! [`roaring`]: https://github.com/RoaringBitmap/roaring-rs +//! +//! | | static storage | 0% nonascii | 1% | 10% | 100% nonascii | +//! |---|---|---|---|---|---| +//! | **`unicode-ident`** | 9.75 K | 0.96 ns | 0.95 ns | 1.09 ns | 1.55 ns | +//! | **`unicode-xid`** | 11.34 K | 1.88 ns | 2.14 ns | 3.48 ns | 15.63 ns | +//! | **`ucd-trie`** | 9.95 K | 1.29 ns | 1.28 ns | 1.36 ns | 2.15 ns | +//! | **`fst`** | 133 K | 55.1 ns | 54.9 ns | 53.2 ns | 28.5 ns | +//! | **`roaring`** | 66.1 K | 2.78 ns | 3.09 ns | 3.37 ns | 4.70 ns | +//! +//! Source code for the benchmark is provided in the *bench* directory of this +//! repo and may be repeated by running `cargo criterion`. +//! +//! <br> +//! +//! ## Comparison of data structures +//! +//! #### unicode-xid +//! +//! They use a sorted array of character ranges, and do a binary search to look +//! up whether a given character lands inside one of those ranges. +//! +//! ```rust +//! # const _: &str = stringify! { +//! static XID_Continue_table: [(char, char); 763] = [ +//! ('\u{30}', '\u{39}'), // 0-9 +//! ('\u{41}', '\u{5a}'), // A-Z +//! # " +//! … +//! # " +//! ('\u{e0100}', '\u{e01ef}'), +//! ]; +//! # }; +//! ``` +//! +//! The static storage used by this data structure scales with the number of +//! contiguous ranges of identifier codepoints in Unicode. Every table entry +//! consumes 8 bytes, because it consists of a pair of 32-bit `char` values. +//! +//! In some ranges of the Unicode codepoint space, this is quite a sparse +//! representation – there are some ranges where tens of thousands of +//! adjacent codepoints are all valid identifier characters. In other places, +//! the representation is quite inefficient. A characater like `µ` (U+00B5) +//! which is surrounded by non-identifier codepoints consumes 64 bits in the +//! table, while it would be just 1 bit in a dense bitmap. +//! +//! On a system with 64-byte cache lines, binary searching the table touches 7 +//! cache lines on average. Each cache line fits only 8 table entries. +//! Additionally, the branching performed during the binary search is probably +//! mostly unpredictable to the branch predictor. +//! +//! Overall, the crate ends up being about 10× slower on non-ASCII input +//! compared to the fastest crate. +//! +//! A potential improvement would be to pack the table entries more compactly. +//! Rust's `char` type is a 21-bit integer padded to 32 bits, which means every +//! table entry is holding 22 bits of wasted space, adding up to 3.9 K. They +//! could instead fit every table entry into 6 bytes, leaving out some of the +//! padding, for a 25% improvement in space used. With some cleverness it may be +//! possible to fit in 5 bytes or even 4 bytes by storing a low char and an +//! extent, instead of low char and high char. I don't expect that performance +//! would improve much but this could be the most efficient for space across all +//! the libraries, needing only about 7 K to store. +//! +//! #### ucd-trie +//! +//! Their data structure is a compressed trie set specifically tailored for +//! Unicode codepoints. The design is credited to Raph Levien in +//! [rust-lang/rust#33098]. +//! +//! [rust-lang/rust#33098]: https://github.com/rust-lang/rust/pull/33098 +//! +//! ```rust +//! pub struct TrieSet { +//! tree1_level1: &'static [u64; 32], +//! tree2_level1: &'static [u8; 992], +//! tree2_level2: &'static [u64], +//! tree3_level1: &'static [u8; 256], +//! tree3_level2: &'static [u8], +//! tree3_level3: &'static [u64], +//! } +//! ``` +//! +//! It represents codepoint sets using a trie to achieve prefix compression. The +//! final states of the trie are embedded in leaves or "chunks", where each +//! chunk is a 64-bit integer. Each bit position of the integer corresponds to +//! whether a particular codepoint is in the set or not. These chunks are not +//! just a compact representation of the final states of the trie, but are also +//! a form of suffix compression. In particular, if multiple ranges of 64 +//! contiguous codepoints have the same Unicode properties, then they all map to +//! the same chunk in the final level of the trie. +//! +//! Being tailored for Unicode codepoints, this trie is partitioned into three +//! disjoint sets: tree1, tree2, tree3. The first set corresponds to codepoints +//! \[0, 0x800), the second \[0x800, 0x10000) and the third \[0x10000, +//! 0x110000). These partitions conveniently correspond to the space of 1 or 2 +//! byte UTF-8 encoded codepoints, 3 byte UTF-8 encoded codepoints and 4 byte +//! UTF-8 encoded codepoints, respectively. +//! +//! Lookups in this data structure are significantly more efficient than binary +//! search. A lookup touches either 1, 2, or 3 cache lines based on which of the +//! trie partitions is being accessed. +//! +//! One possible performance improvement would be for this crate to expose a way +//! to query based on a UTF-8 encoded string, returning the Unicode property +//! corresponding to the first character in the string. Without such an API, the +//! caller is required to tokenize their UTF-8 encoded input data into `char`, +//! hand the `char` into `ucd-trie`, only for `ucd-trie` to undo that work by +//! converting back into the variable-length representation for trie traversal. +//! +//! #### fst +//! +//! Uses a [finite state transducer][fst]. This representation is built into +//! [ucd-generate] but I am not aware of any advantage over the `ucd-trie` +//! representation. In particular `ucd-trie` is optimized for storing Unicode +//! properties while `fst` is not. +//! +//! [fst]: https://github.com/BurntSushi/fst +//! [ucd-generate]: https://github.com/BurntSushi/ucd-generate +//! +//! As far as I can tell, the main thing that causes `fst` to have large size +//! and slow lookups for this use case relative to `ucd-trie` is that it does +//! not specialize for the fact that only 21 of the 32 bits in a `char` are +//! meaningful. There are some dense arrays in the structure with large ranges +//! that could never possibly be used. +//! +//! #### roaring +//! +//! This crate is a pure-Rust implementation of [Roaring Bitmap], a data +//! structure designed for storing sets of 32-bit unsigned integers. +//! +//! [Roaring Bitmap]: https://roaringbitmap.org/about/ +//! +//! Roaring bitmaps are compressed bitmaps which tend to outperform conventional +//! compressed bitmaps such as WAH, EWAH or Concise. In some instances, they can +//! be hundreds of times faster and they often offer significantly better +//! compression. +//! +//! In this use case the performance was reasonably competitive but still +//! substantially slower than the Unicode-optimized crates. Meanwhile the +//! compression was significantly worse, requiring 6× as much storage for +//! the data structure. +//! +//! I also benchmarked the [`croaring`] crate which is an FFI wrapper around the +//! C reference implementation of Roaring Bitmap. This crate was consistently +//! about 15% slower than pure-Rust `roaring`, which could just be FFI overhead. +//! I did not investigate further. +//! +//! [`croaring`]: https://crates.io/crates/croaring +//! +//! #### unicode-ident +//! +//! This crate is most similar to the `ucd-trie` library, in that it's based on +//! bitmaps stored in the leafs of a trie representation, achieving both prefix +//! compression and suffix compression. +//! +//! The key differences are: +//! +//! - Uses a single 2-level trie, rather than 3 disjoint partitions of different +//! depth each. +//! - Uses significantly larger chunks: 512 bits rather than 64 bits. +//! - Compresses the XID\_Start and XID\_Continue properties together +//! simultaneously, rather than duplicating identical trie leaf chunks across +//! the two. +//! +//! The following diagram show the XID\_Start and XID\_Continue Unicode boolean +//! properties in uncompressed form, in row-major order: +//! +//! <table> +//! <tr><th>XID_Start</th><th>XID_Continue</th></tr> +//! <tr> +//! <td><img alt="XID_Start bitmap" width="256" src="https://user-images.githubusercontent.com/1940490/168647353-c6eeb922-afec-49b2-9ef5-c03e9d1e0760.png"></td> +//! <td><img alt="XID_Continue bitmap" width="256" src="https://user-images.githubusercontent.com/1940490/168647367-f447cca7-2362-4d7d-8cd7-d21c011d329b.png"></td> +//! </tr> +//! </table> +//! +//! Uncompressed, these would take 140 K to store, which is beyond what would be +//! reasonable. However, as you can see there is a large degree of similarity +//! between the two bitmaps and across the rows, which lends well to +//! compression. +//! +//! This crate stores one 512-bit "row" of the above bitmaps in the leaf level +//! of a trie, and a single additional level to index into the leafs. It turns +//! out there are 124 unique 512-bit chunks across the two bitmaps so 7 bits are +//! sufficient to index them. +//! +//! The chunk size of 512 bits is selected as the size that minimizes the total +//! size of the data structure. A smaller chunk, like 256 or 128 bits, would +//! achieve better deduplication but require a larger index. A larger chunk +//! would increase redundancy in the leaf bitmaps. 512 bit chunks are the +//! optimum for total size of the index plus leaf bitmaps. +//! +//! In fact since there are only 124 unique chunks, we can use an 8-bit index +//! with a spare bit to index at the half-chunk level. This achieves an +//! additional 8.5% compression by eliminating redundancies between the second +//! half of any chunk and the first half of any other chunk. Note that this is +//! not the same as using chunks which are half the size, because it does not +//! necessitate raising the size of the trie's first level. +//! +//! In contrast to binary search or the `ucd-trie` crate, performing lookups in +//! this data structure is straight-line code with no need for branching. + +#![no_std] +#![allow(clippy::doc_markdown, clippy::must_use_candidate)] + +#[rustfmt::skip] +mod tables; + +use crate::tables::{ASCII_CONTINUE, ASCII_START, CHUNK, LEAF, TRIE_CONTINUE, TRIE_START}; + +pub fn is_xid_start(ch: char) -> bool { + if ch.is_ascii() { + return ASCII_START.0[ch as usize]; + } + let chunk = *TRIE_START.0.get(ch as usize / 8 / CHUNK).unwrap_or(&0); + let offset = chunk as usize * CHUNK / 2 + ch as usize / 8 % CHUNK; + unsafe { LEAF.0.get_unchecked(offset) }.wrapping_shr(ch as u32 % 8) & 1 != 0 +} + +pub fn is_xid_continue(ch: char) -> bool { + if ch.is_ascii() { + return ASCII_CONTINUE.0[ch as usize]; + } + let chunk = *TRIE_CONTINUE.0.get(ch as usize / 8 / CHUNK).unwrap_or(&0); + let offset = chunk as usize * CHUNK / 2 + ch as usize / 8 % CHUNK; + unsafe { LEAF.0.get_unchecked(offset) }.wrapping_shr(ch as u32 % 8) & 1 != 0 +} |