Initial vendor packages

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

1 files changed, 269 insertions, 0 deletions

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+//! [![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&ndash;10&times;
+//! 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`** | 10.1 K | 0.96 ns | 0.95 ns | 1.09 ns | 1.55 ns |
+//! | **`unicode-xid`** | 11.5 K | 1.88 ns | 2.14 ns | 3.48 ns | 15.63 ns |
+//! | **`ucd-trie`** | 10.2 K | 1.29 ns | 1.28 ns | 1.36 ns | 2.15 ns |
+//! | **`fst`** | 139 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 &ndash; 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&times; 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&times; 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]
+#![doc(html_root_url = "https://docs.rs/unicode-ident/1.0.12")]
+#![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
+}