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// Original implementation taken from rust-memchr.
// Copyright 2015 Andrew Gallant, bluss and Nicolas Koch
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]
#[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]
#[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]
#[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_cmp)]
#[rustc_allow_const_fn_unstable(const_slice_index)]
#[rustc_allow_const_fn_unstable(const_align_offset)]
#[rustc_const_stable(feature = "const_memchr", since = "1.65.0")]
const fn memchr_aligned(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 {
// FIXME(const-hack, fee1-dead): replace with min
offset = if offset < len { offset } else { len };
// FIXME(const-hack, fee1-dead): replace with range slicing
// SAFETY: offset is within bounds
let slice = unsafe { super::from_raw_parts(text.as_ptr(), 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
// SAFETY: offset is within bounds
let slice = 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)
}
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