// 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 } #[inline] #[cfg(target_pointer_width = "16")] #[rustc_const_stable(feature = "const_memchr", since = "1.65.0")] const fn repeat_byte(b: u8) -> usize { (b as usize) << 8 | b as usize } #[inline] #[cfg(not(target_pointer_width = "16"))] #[rustc_const_stable(feature = "const_memchr", since = "1.65.0")] const fn repeat_byte(b: u8) -> usize { (b as usize) * (usize::MAX / 255) } /// 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 = repeat_byte(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 = repeat_byte(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) }