1 /*! 2 This library provides heavily optimized routines for string search primitives. 3 4 # Overview 5 6 This section gives a brief high level overview of what this crate offers. 7 8 * The top-level module provides routines for searching for 1, 2 or 3 bytes 9 in the forward or reverse direction. When searching for more than one byte, 10 positions are considered a match if the byte at that position matches any 11 of the bytes. 12 * The [`memmem`] sub-module provides forward and reverse substring search 13 routines. 14 15 In all such cases, routines operate on `&[u8]` without regard to encoding. This 16 is exactly what you want when searching either UTF-8 or arbitrary bytes. 17 18 # Example: using `memchr` 19 20 This example shows how to use `memchr` to find the first occurrence of `z` in 21 a haystack: 22 23 ``` 24 use memchr::memchr; 25 26 let haystack = b"foo bar baz quuz"; 27 assert_eq!(Some(10), memchr(b'z', haystack)); 28 ``` 29 30 # Example: matching one of three possible bytes 31 32 This examples shows how to use `memrchr3` to find occurrences of `a`, `b` or 33 `c`, starting at the end of the haystack. 34 35 ``` 36 use memchr::memchr3_iter; 37 38 let haystack = b"xyzaxyzbxyzc"; 39 40 let mut it = memchr3_iter(b'a', b'b', b'c', haystack).rev(); 41 assert_eq!(Some(11), it.next()); 42 assert_eq!(Some(7), it.next()); 43 assert_eq!(Some(3), it.next()); 44 assert_eq!(None, it.next()); 45 ``` 46 47 # Example: iterating over substring matches 48 49 This example shows how to use the [`memmem`] sub-module to find occurrences of 50 a substring in a haystack. 51 52 ``` 53 use memchr::memmem; 54 55 let haystack = b"foo bar foo baz foo"; 56 57 let mut it = memmem::find_iter(haystack, "foo"); 58 assert_eq!(Some(0), it.next()); 59 assert_eq!(Some(8), it.next()); 60 assert_eq!(Some(16), it.next()); 61 assert_eq!(None, it.next()); 62 ``` 63 64 # Example: repeating a search for the same needle 65 66 It may be possible for the overhead of constructing a substring searcher to be 67 measurable in some workloads. In cases where the same needle is used to search 68 many haystacks, it is possible to do construction once and thus to avoid it for 69 subsequent searches. This can be done with a [`memmem::Finder`]: 70 71 ``` 72 use memchr::memmem; 73 74 let finder = memmem::Finder::new("foo"); 75 76 assert_eq!(Some(4), finder.find(b"baz foo quux")); 77 assert_eq!(None, finder.find(b"quux baz bar")); 78 ``` 79 80 # Why use this crate? 81 82 At first glance, the APIs provided by this crate might seem weird. Why provide 83 a dedicated routine like `memchr` for something that could be implemented 84 clearly and trivially in one line: 85 86 ``` 87 fn memchr(needle: u8, haystack: &[u8]) -> Option<usize> { 88 haystack.iter().position(|&b| b == needle) 89 } 90 ``` 91 92 Or similarly, why does this crate provide substring search routines when Rust's 93 core library already provides them? 94 95 ``` 96 fn search(haystack: &str, needle: &str) -> Option<usize> { 97 haystack.find(needle) 98 } 99 ``` 100 101 The primary reason for both of them to exist is performance. When it comes to 102 performance, at a high level at least, there are two primary ways to look at 103 it: 104 105 * **Throughput**: For this, think about it as, "given some very large haystack 106 and a byte that never occurs in that haystack, how long does it take to 107 search through it and determine that it, in fact, does not occur?" 108 * **Latency**: For this, think about it as, "given a tiny haystack---just a 109 few bytes---how long does it take to determine if a byte is in it?" 110 111 The `memchr` routine in this crate has _slightly_ worse latency than the 112 solution presented above, however, its throughput can easily be over an 113 order of magnitude faster. This is a good general purpose trade off to make. 114 You rarely lose, but often gain big. 115 116 **NOTE:** The name `memchr` comes from the corresponding routine in libc. A key 117 advantage of using this library is that its performance is not tied to its 118 quality of implementation in the libc you happen to be using, which can vary 119 greatly from platform to platform. 120 121 But what about substring search? This one is a bit more complicated. The 122 primary reason for its existence is still indeed performance, but it's also 123 useful because Rust's core library doesn't actually expose any substring 124 search routine on arbitrary bytes. The only substring search routine that 125 exists works exclusively on valid UTF-8. 126 127 So if you have valid UTF-8, is there a reason to use this over the standard 128 library substring search routine? Yes. This routine is faster on almost every 129 metric, including latency. The natural question then, is why isn't this 130 implementation in the standard library, even if only for searching on UTF-8? 131 The reason is that the implementation details for using SIMD in the standard 132 library haven't quite been worked out yet. 133 134 **NOTE:** Currently, only `x86_64` targets have highly accelerated 135 implementations of substring search. For `memchr`, all targets have 136 somewhat-accelerated implementations, while only `x86_64` targets have highly 137 accelerated implementations. This limitation is expected to be lifted once the 138 standard library exposes a platform independent SIMD API. 139 140 # Crate features 141 142 * **std** - When enabled (the default), this will permit this crate to use 143 features specific to the standard library. Currently, the only thing used 144 from the standard library is runtime SIMD CPU feature detection. This means 145 that this feature must be enabled to get AVX accelerated routines. When 146 `std` is not enabled, this crate will still attempt to use SSE2 accelerated 147 routines on `x86_64`. 148 * **libc** - When enabled (**not** the default), this library will use your 149 platform's libc implementation of `memchr` (and `memrchr` on Linux). This 150 can be useful on non-`x86_64` targets where the fallback implementation in 151 this crate is not as good as the one found in your libc. All other routines 152 (e.g., `memchr[23]` and substring search) unconditionally use the 153 implementation in this crate. 154 */ 155 156 #![deny(missing_docs)] 157 #![cfg_attr(not(feature = "std"), no_std)] 158 // It's not worth trying to gate all code on just miri, so turn off relevant 159 // dead code warnings. 160 #![cfg_attr(miri, allow(dead_code, unused_macros))] 161 162 // Supporting 8-bit (or others) would be fine. If you need it, please submit a 163 // bug report at https://github.com/BurntSushi/rust-memchr 164 #[cfg(not(any( 165 target_pointer_width = "16", 166 target_pointer_width = "32", 167 target_pointer_width = "64" 168 )))] 169 compile_error!("memchr currently not supported on non-{16,32,64}"); 170 171 pub use crate::memchr::{ 172 memchr, memchr2, memchr2_iter, memchr3, memchr3_iter, memchr_iter, 173 memrchr, memrchr2, memrchr2_iter, memrchr3, memrchr3_iter, memrchr_iter, 174 Memchr, Memchr2, Memchr3, 175 }; 176 177 mod cow; 178 mod memchr; 179 pub mod memmem; 180 #[cfg(test)] 181 mod tests; 182