Crate memchr

Expand description

This library provides heavily optimized routines for string search primitives.

Overview

This section gives a brief high level overview of what this crate offers.

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.

Example: using `memchr`

This example shows how to use memchr to find the first occurrence of z in a haystack:

use memchr::memchr;

let haystack = b"foo bar baz quuz";
assert_eq!(Some(10), memchr(b'z', haystack));

Example: matching one of three possible bytes

This examples shows how to use memrchr3 to find occurrences of a, b or c, starting at the end of the haystack.

use memchr::memchr3_iter;

let haystack = b"xyzaxyzbxyzc";

let mut it = memchr3_iter(b'a', b'b', b'c', haystack).rev();
assert_eq!(Some(11), it.next());
assert_eq!(Some(7), it.next());
assert_eq!(Some(3), it.next());
assert_eq!(None, it.next());

Example: iterating over substring matches

This example shows how to use the memmem sub-module to find occurrences of a substring in a haystack.

use memchr::memmem;

let haystack = b"foo bar foo baz foo";

let mut it = memmem::find_iter(haystack, "foo");
assert_eq!(Some(0), it.next());
assert_eq!(Some(8), it.next());
assert_eq!(Some(16), it.next());
assert_eq!(None, it.next());

Example: repeating a search for the same needle

It may be possible for the overhead of constructing a substring searcher to be measurable in some workloads. In cases where the same needle is used to search many haystacks, it is possible to do construction once and thus to avoid it for subsequent searches. This can be done with a memmem::Finder:

use memchr::memmem;

let finder = memmem::Finder::new("foo");

assert_eq!(Some(4), finder.find(b"baz foo quux"));
assert_eq!(None, finder.find(b"quux baz bar"));

Why use this crate?

At first glance, the APIs provided by this crate might seem weird. Why provide a dedicated routine like memchr for something that could be implemented clearly and trivially in one line:

fn memchr(needle: u8, haystack: &[u8]) -> Option<usize> {
    haystack.iter().position(|&b| b == needle)
}

Or similarly, why does this crate provide substring search routines when Rust’s core library already provides them?

fn search(haystack: &str, needle: &str) -> Option<usize> {
    haystack.find(needle)
}

The primary reason for both of them to exist is performance. When it comes to performance, at a high level at least, there are two primary ways to look at it:

Throughput: For this, think about it as, “given some very large haystack and a byte that never occurs in that haystack, how long does it take to search through it and determine that it, in fact, does not occur?”
Latency: For this, think about it as, “given a tiny haystack—just a few bytes—how long does it take to determine if a byte is in it?”

The memchr routine in this crate has slightly worse latency than the solution presented above, however, its throughput can easily be over an order of magnitude faster. This is a good general purpose trade off to make. You rarely lose, but often gain big.

NOTE: The name memchr comes from the corresponding routine in libc. A key advantage of using this library is that its performance is not tied to its quality of implementation in the libc you happen to be using, which can vary greatly from platform to platform.

But what about substring search? This one is a bit more complicated. The primary reason for its existence is still indeed performance, but it’s also useful because Rust’s core library doesn’t actually expose any substring search routine on arbitrary bytes. The only substring search routine that exists works exclusively on valid UTF-8.

So if you have valid UTF-8, is there a reason to use this over the standard library substring search routine? Yes. This routine is faster on almost every metric, including latency. The natural question then, is why isn’t this implementation in the standard library, even if only for searching on UTF-8? The reason is that the implementation details for using SIMD in the standard library haven’t quite been worked out yet.

NOTE: Currently, only x86_64 targets have highly accelerated implementations of substring search. For memchr, all targets have somewhat-accelerated implementations, while only x86_64 targets have highly accelerated implementations. This limitation is expected to be lifted once the standard library exposes a platform independent SIMD API.

Crate features

std - When enabled (the default), this will permit this crate to use features specific to the standard library. Currently, the only thing used from the standard library is runtime SIMD CPU feature detection. This means that this feature must be enabled to get AVX accelerated routines. When std is not enabled, this crate will still attempt to use SSE2 accelerated routines on x86_64.
libc - When enabled (not the default), this library will use your platform’s libc implementation of memchr (and memrchr on Linux). This can be useful on non-x86_64 targets where the fallback implementation in this crate is not as good as the one found in your libc. All other routines (e.g., memchr[23] and substring search) unconditionally use the implementation in this crate.