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-memchr
-======
-This library provides heavily optimized routines for string search primitives.
-
-[![Build status](https://github.com/BurntSushi/memchr/workflows/ci/badge.svg)](https://github.com/BurntSushi/memchr/actions)
-[![Crates.io](https://img.shields.io/crates/v/memchr.svg)](https://crates.io/crates/memchr)
-
-Dual-licensed under MIT or the [UNLICENSE](https://unlicense.org/).
-
-
-### Documentation
-
-[https://docs.rs/memchr](https://docs.rs/memchr)
-
-
-### Overview
-
-* The top-level module provides routines for searching for 1, 2 or 3 bytes
- in the forward or reverse direction. When searching for more than one byte,
- positions are considered a match if the byte at that position matches any
- of the bytes.
-* The `memmem` sub-module provides forward and reverse substring search
- routines.
-
-In all such cases, routines operate on `&[u8]` without regard to encoding. This
-is exactly what you want when searching either UTF-8 or arbitrary bytes.
-
-### Compiling without the standard library
-
-memchr links to the standard library by default, but you can disable the
-`std` feature if you want to use it in a `#![no_std]` crate:
-
-```toml
-[dependencies]
-memchr = { version = "2", default-features = false }
-```
-
-On `x86_64` platforms, when the `std` feature is disabled, the SSE2 accelerated
-implementations will be used. When `std` is enabled, AVX2 accelerated
-implementations will be used if the CPU is determined to support it at runtime.
-
-SIMD accelerated routines are also available on the `wasm32` and `aarch64`
-targets. The `std` feature is not required to use them.
-
-When a SIMD version is not available, then this crate falls back to
-[SWAR](https://en.wikipedia.org/wiki/SWAR) techniques.
-
-### Minimum Rust version policy
-
-This crate's minimum supported `rustc` version is `1.61.0`.
-
-The current policy is that the minimum Rust version required to use this crate
-can be increased in minor version updates. For example, if `crate 1.0` requires
-Rust 1.20.0, then `crate 1.0.z` for all values of `z` will also require Rust
-1.20.0 or newer. However, `crate 1.y` for `y > 0` may require a newer minimum
-version of Rust.
-
-In general, this crate will be conservative with respect to the minimum
-supported version of Rust.
-
-
-### Testing strategy
-
-Given the complexity of the code in this crate, along with the pervasive use
-of `unsafe`, this crate has an extensive testing strategy. It combines multiple
-approaches:
-
-* Hand-written tests.
-* Exhaustive-style testing meant to exercise all possible branching and offset
- calculations.
-* Property based testing through [`quickcheck`](https://github.com/BurntSushi/quickcheck).
-* Fuzz testing through [`cargo fuzz`](https://github.com/rust-fuzz/cargo-fuzz).
-* A huge suite of benchmarks that are also run as tests. Benchmarks always
- confirm that the expected result occurs.
-
-Improvements to the testing infrastructure are very welcome.
-
-
-### Algorithms used
-
-At time of writing, this crate's implementation of substring search actually
-has a few different algorithms to choose from depending on the situation.
-
-* For very small haystacks,
- [Rabin-Karp](https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm)
- is used to reduce latency. Rabin-Karp has very small overhead and can often
- complete before other searchers have even been constructed.
-* For small needles, a variant of the
- ["Generic SIMD"](http://0x80.pl/articles/simd-strfind.html#algorithm-1-generic-simd)
- algorithm is used. Instead of using the first and last bytes, a heuristic is
- used to select bytes based on a background distribution of byte frequencies.
-* In all other cases,
- [Two-Way](https://en.wikipedia.org/wiki/Two-way_string-matching_algorithm)
- is used. If possible, a prefilter based on the "Generic SIMD" algorithm
- linked above is used to find candidates quickly. A dynamic heuristic is used
- to detect if the prefilter is ineffective, and if so, disables it.
-
-
-### Why is the standard library's substring search so much slower?
-
-We'll start by establishing what the difference in performance actually
-is. There are two relevant benchmark classes to consider: `prebuilt` and
-`oneshot`. The `prebuilt` benchmarks are designed to measure---to the extent
-possible---search time only. That is, the benchmark first starts by building a
-searcher and then only tracking the time for _using_ the searcher:
-
-```
-$ rebar rank benchmarks/record/x86_64/2023-08-26.csv --intersection -e memchr/memmem/prebuilt -e std/memmem/prebuilt
-Engine Version Geometric mean of speed ratios Benchmark count
------- ------- ------------------------------ ---------------
-rust/memchr/memmem/prebuilt 2.5.0 1.03 53
-rust/std/memmem/prebuilt 1.73.0-nightly 180dffba1 6.50 53
-```
-
-Conversely, the `oneshot` benchmark class measures the time it takes to both
-build the searcher _and_ use it:
-
-```
-$ rebar rank benchmarks/record/x86_64/2023-08-26.csv --intersection -e memchr/memmem/oneshot -e std/memmem/oneshot
-Engine Version Geometric mean of speed ratios Benchmark count
------- ------- ------------------------------ ---------------
-rust/memchr/memmem/oneshot 2.5.0 1.04 53
-rust/std/memmem/oneshot 1.73.0-nightly 180dffba1 5.26 53
-```
-
-**NOTE:** Replace `rebar rank` with `rebar cmp` in the above commands to
-explore the specific benchmarks and their differences.
-
-So in both cases, this crate is quite a bit faster over a broad sampling of
-benchmarks regardless of whether you measure only search time or search time
-plus construction time. The difference is a little smaller when you include
-construction time in your measurements.
-
-These two different types of benchmark classes make for a nice segue into
-one reason why the standard library's substring search can be slower: API
-design. In the standard library, the only APIs available to you require
-one to re-construct the searcher for every search. While you can benefit
-from building a searcher once and iterating over all matches in a single
-string, you cannot reuse that searcher to search other strings. This might
-come up when, for example, searching a file one line at a time. You'll need
-to re-build the searcher for every line searched, and this can [really
-matter][burntsushi-bstr-blog].
-
-**NOTE:** The `prebuilt` benchmark for the standard library can't actually
-avoid measuring searcher construction at some level, because there is no API
-for it. Instead, the benchmark consists of building the searcher once and then
-finding all matches in a single string via an iterator. This tends to
-approximate a benchmark where searcher construction isn't measured, but it
-isn't perfect. While this means the comparison is not strictly
-apples-to-apples, it does reflect what is maximally possible with the standard
-library, and thus reflects the best that one could do in a real world scenario.
-
-While there is more to the story than just API design here, it's important to
-point out that even if the standard library's substring search were a precise
-clone of this crate internally, it would still be at a disadvantage in some
-workloads because of its API. (The same also applies to C's standard library
-`memmem` function. There is no way to amortize construction of the searcher.
-You need to pay for it on every call.)
-
-The other reason for the difference in performance is that
-the standard library has trouble using SIMD. In particular, substring search
-is implemented in the `core` library, where platform specific code generally
-can't exist. That's an issue because in order to utilize SIMD beyond SSE2
-while maintaining portable binaries, one needs to use [dynamic CPU feature
-detection][dynamic-cpu], and that in turn requires platform specific code.
-While there is [an RFC for enabling target feature detection in
-`core`][core-feature], it doesn't yet exist.
-
-The bottom line here is that `core`'s substring search implementation is
-limited to making use of SSE2, but not AVX.
-
-Still though, this crate does accelerate substring search even when only SSE2
-is available. The standard library could therefore adopt the techniques in this
-crate just for SSE2. The reason why that hasn't happened yet isn't totally
-clear to me. It likely needs a champion to push it through. The standard
-library tends to be more conservative in these things. With that said, the
-standard library does use some [SSE2 acceleration on `x86-64`][std-sse2] added
-in [this PR][std-sse2-pr]. However, at the time of writing, it is only used
-for short needles and doesn't use the frequency based heuristics found in this
-crate.
-
-**NOTE:** Another thing worth mentioning is that the standard library's
-substring search routine requires that both the needle and haystack have type
-`&str`. Unless you can assume that your data is valid UTF-8, building a `&str`
-will come with the overhead of UTF-8 validation. This may in turn result in
-overall slower searching depending on your workload. In contrast, the `memchr`
-crate permits both the needle and the haystack to have type `&[u8]`, where
-`&[u8]` can be created from a `&str` with zero cost. Therefore, the substring
-search in this crate is strictly more flexible than what the standard library
-provides.
-
-[burntsushi-bstr-blog]: https://blog.burntsushi.net/bstr/#motivation-based-on-performance
-[dynamic-cpu]: https://doc.rust-lang.org/std/arch/index.html#dynamic-cpu-feature-detection
-[core-feature]: https://github.com/rust-lang/rfcs/pull/3469
-[std-sse2]: https://github.com/rust-lang/rust/blob/bf9229a2e366b4c311f059014a4aa08af16de5d8/library/core/src/str/pattern.rs#L1719-L1857
-[std-sse2-pr]: https://github.com/rust-lang/rust/pull/103779