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108 lines
4.4 KiB
Markdown
108 lines
4.4 KiB
Markdown
memchr
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======
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This library provides heavily optimized routines for string search primitives.
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[](https://github.com/BurntSushi/memchr/actions)
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[](https://crates.io/crates/memchr)
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Dual-licensed under MIT or the [UNLICENSE](https://unlicense.org/).
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### Documentation
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[https://docs.rs/memchr](https://docs.rs/memchr)
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### Overview
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* The top-level module provides routines for searching for 1, 2 or 3 bytes
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in the forward or reverse direction. When searching for more than one byte,
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positions are considered a match if the byte at that position matches any
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of the bytes.
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* The `memmem` sub-module provides forward and reverse substring search
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routines.
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In all such cases, routines operate on `&[u8]` without regard to encoding. This
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is exactly what you want when searching either UTF-8 or arbitrary bytes.
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### Compiling without the standard library
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memchr links to the standard library by default, but you can disable the
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`std` feature if you want to use it in a `#![no_std]` crate:
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```toml
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[dependencies]
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memchr = { version = "2", default-features = false }
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```
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On x86 platforms, when the `std` feature is disabled, the SSE2 accelerated
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implementations will be used. When `std` is enabled, AVX accelerated
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implementations will be used if the CPU is determined to support it at runtime.
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### Using libc
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`memchr` is a routine that is part of libc, although this crate does not use
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libc by default. Instead, it uses its own routines, which are either vectorized
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or generic fallback routines. In general, these should be competitive with
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what's in libc, although this has not been tested for all architectures. If
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using `memchr` from libc is desirable and a vectorized routine is not otherwise
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available in this crate, then enabling the `libc` feature will use libc's
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version of `memchr`.
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The rest of the functions in this crate, e.g., `memchr2` or `memrchr3` and the
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substring search routines, will always use the implementations in this crate.
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One exception to this is `memrchr`, which is an extension in `libc` found on
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Linux. On Linux, `memrchr` is used in precisely the same scenario as `memchr`,
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as described above.
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### Minimum Rust version policy
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This crate's minimum supported `rustc` version is `1.41.1`.
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The current policy is that the minimum Rust version required to use this crate
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can be increased in minor version updates. For example, if `crate 1.0` requires
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Rust 1.20.0, then `crate 1.0.z` for all values of `z` will also require Rust
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1.20.0 or newer. However, `crate 1.y` for `y > 0` may require a newer minimum
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version of Rust.
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In general, this crate will be conservative with respect to the minimum
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supported version of Rust.
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### Testing strategy
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Given the complexity of the code in this crate, along with the pervasive use
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of `unsafe`, this crate has an extensive testing strategy. It combines multiple
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approaches:
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* Hand-written tests.
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* Exhaustive-style testing meant to exercise all possible branching and offset
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calculations.
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* Property based testing through [`quickcheck`](https://github.com/BurntSushi/quickcheck).
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* Fuzz testing through [`cargo fuzz`](https://github.com/rust-fuzz/cargo-fuzz).
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* A huge suite of benchmarks that are also run as tests. Benchmarks always
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confirm that the expected result occurs.
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Improvements to the testing infrastructure are very welcome.
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### Algorithms used
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At time of writing, this crate's implementation of substring search actually
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has a few different algorithms to choose from depending on the situation.
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* For very small haystacks,
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[Rabin-Karp](https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm)
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is used to reduce latency. Rabin-Karp has very small overhead and can often
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complete before other searchers have even been constructed.
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* For small needles, a variant of the
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["Generic SIMD"](http://0x80.pl/articles/simd-strfind.html#algorithm-1-generic-simd)
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algorithm is used. Instead of using the first and last bytes, a heuristic is
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used to select bytes based on a background distribution of byte frequencies.
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* In all other cases,
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[Two-Way](https://en.wikipedia.org/wiki/Two-way_string-matching_algorithm)
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is used. If possible, a prefilter based on the "Generic SIMD" algorithm
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linked above is used to find candidates quickly. A dynamic heuristic is used
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to detect if the prefilter is ineffective, and if so, disables it.
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