core/slice/memchr.rs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168
// Original implementation taken from rust-memchr.
// Copyright 2015 Andrew Gallant, bluss and Nicolas Koch
use crate::intrinsics::const_eval_select;
use crate::mem;
const LO_USIZE: usize = usize::repeat_u8(0x01);
const HI_USIZE: usize = usize::repeat_u8(0x80);
const USIZE_BYTES: usize = mem::size_of::<usize>();
/// Returns `true` if `x` contains any zero byte.
///
/// From *Matters Computational*, J. Arndt:
///
/// "The idea is to subtract one from each of the bytes and then look for
/// bytes where the borrow propagated all the way to the most significant
/// bit."
#[inline]
#[cfg_attr(bootstrap, rustc_const_stable(feature = "const_memchr", since = "1.65.0"))]
const fn contains_zero_byte(x: usize) -> bool {
x.wrapping_sub(LO_USIZE) & !x & HI_USIZE != 0
}
/// Returns the first index matching the byte `x` in `text`.
#[inline]
#[must_use]
#[cfg_attr(bootstrap, rustc_const_stable(feature = "const_memchr", since = "1.65.0"))]
pub const fn memchr(x: u8, text: &[u8]) -> Option<usize> {
// Fast path for small slices.
if text.len() < 2 * USIZE_BYTES {
return memchr_naive(x, text);
}
memchr_aligned(x, text)
}
#[inline]
#[cfg_attr(bootstrap, rustc_const_stable(feature = "const_memchr", since = "1.65.0"))]
const fn memchr_naive(x: u8, text: &[u8]) -> Option<usize> {
let mut i = 0;
// FIXME(const-hack): Replace with `text.iter().pos(|c| *c == x)`.
while i < text.len() {
if text[i] == x {
return Some(i);
}
i += 1;
}
None
}
#[rustc_allow_const_fn_unstable(const_eval_select)] // fallback impl has same behavior
#[cfg_attr(bootstrap, rustc_const_stable(feature = "const_memchr", since = "1.65.0"))]
const fn memchr_aligned(x: u8, text: &[u8]) -> Option<usize> {
// The runtime version behaves the same as the compiletime version, it's
// just more optimized.
return const_eval_select((x, text), compiletime, runtime);
const fn compiletime(x: u8, text: &[u8]) -> Option<usize> {
memchr_naive(x, text)
}
#[inline]
fn runtime(x: u8, text: &[u8]) -> Option<usize> {
// Scan for a single byte value by reading two `usize` words at a time.
//
// Split `text` in three parts
// - unaligned initial part, before the first word aligned address in text
// - body, scan by 2 words at a time
// - the last remaining part, < 2 word size
// search up to an aligned boundary
let len = text.len();
let ptr = text.as_ptr();
let mut offset = ptr.align_offset(USIZE_BYTES);
if offset > 0 {
offset = offset.min(len);
let slice = &text[..offset];
if let Some(index) = memchr_naive(x, slice) {
return Some(index);
}
}
// search the body of the text
let repeated_x = usize::repeat_u8(x);
while offset <= len - 2 * USIZE_BYTES {
// SAFETY: the while's predicate guarantees a distance of at least 2 * usize_bytes
// between the offset and the end of the slice.
unsafe {
let u = *(ptr.add(offset) as *const usize);
let v = *(ptr.add(offset + USIZE_BYTES) as *const usize);
// break if there is a matching byte
let zu = contains_zero_byte(u ^ repeated_x);
let zv = contains_zero_byte(v ^ repeated_x);
if zu || zv {
break;
}
}
offset += USIZE_BYTES * 2;
}
// Find the byte after the point the body loop stopped.
// FIXME(const-hack): Use `?` instead.
// FIXME(const-hack, fee1-dead): use range slicing
let slice =
// SAFETY: offset is within bounds
unsafe { super::from_raw_parts(text.as_ptr().add(offset), text.len() - offset) };
if let Some(i) = memchr_naive(x, slice) { Some(offset + i) } else { None }
}
}
/// Returns the last index matching the byte `x` in `text`.
#[must_use]
pub fn memrchr(x: u8, text: &[u8]) -> Option<usize> {
// Scan for a single byte value by reading two `usize` words at a time.
//
// Split `text` in three parts:
// - unaligned tail, after the last word aligned address in text,
// - body, scanned by 2 words at a time,
// - the first remaining bytes, < 2 word size.
let len = text.len();
let ptr = text.as_ptr();
type Chunk = usize;
let (min_aligned_offset, max_aligned_offset) = {
// We call this just to obtain the length of the prefix and suffix.
// In the middle we always process two chunks at once.
// SAFETY: transmuting `[u8]` to `[usize]` is safe except for size differences
// which are handled by `align_to`.
let (prefix, _, suffix) = unsafe { text.align_to::<(Chunk, Chunk)>() };
(prefix.len(), len - suffix.len())
};
let mut offset = max_aligned_offset;
if let Some(index) = text[offset..].iter().rposition(|elt| *elt == x) {
return Some(offset + index);
}
// Search the body of the text, make sure we don't cross min_aligned_offset.
// offset is always aligned, so just testing `>` is sufficient and avoids possible
// overflow.
let repeated_x = usize::repeat_u8(x);
let chunk_bytes = mem::size_of::<Chunk>();
while offset > min_aligned_offset {
// SAFETY: offset starts at len - suffix.len(), as long as it is greater than
// min_aligned_offset (prefix.len()) the remaining distance is at least 2 * chunk_bytes.
unsafe {
let u = *(ptr.add(offset - 2 * chunk_bytes) as *const Chunk);
let v = *(ptr.add(offset - chunk_bytes) as *const Chunk);
// Break if there is a matching byte.
let zu = contains_zero_byte(u ^ repeated_x);
let zv = contains_zero_byte(v ^ repeated_x);
if zu || zv {
break;
}
}
offset -= 2 * chunk_bytes;
}
// Find the byte before the point the body loop stopped.
text[..offset].iter().rposition(|elt| *elt == x)
}