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/// A heuristic frequency based detection of rare bytes for substring search.
///
/// This detector attempts to pick out two bytes in a needle that are predicted
/// to occur least frequently. The purpose is to use these bytes to implement
/// fast candidate search using vectorized code.
///
/// A set of offsets is only computed for needles of length 2 or greater.
/// Smaller needles should be special cased by the substring search algorithm
/// in use. (e.g., Use memchr for single byte needles.)
///
/// Note that we use `u8` to represent the offsets of the rare bytes in a
/// needle to reduce space usage. This means that rare byte occurring after the
/// first 255 bytes in a needle will never be used.
#[derive(Clone, Copy, Debug, Default)]
pub(crate) struct RareNeedleBytes {
/// The leftmost offset of the rarest byte in the needle, according to
/// pre-computed frequency analysis. The "leftmost offset" means that
/// rare1i <= i for all i where needle[i] == needle[rare1i].
rare1i: u8,
/// The leftmost offset of the second rarest byte in the needle, according
/// to pre-computed frequency analysis. The "leftmost offset" means that
/// rare2i <= i for all i where needle[i] == needle[rare2i].
///
/// The second rarest byte is used as a type of guard for quickly detecting
/// a mismatch if the first byte matches. This is a hedge against
/// pathological cases where the pre-computed frequency analysis may be
/// off. (But of course, does not prevent *all* pathological cases.)
///
/// In general, rare1i != rare2i by construction, although there is no hard
/// requirement that they be different. However, since the case of a single
/// byte needle is handled specially by memchr itself, rare2i generally
/// always should be different from rare1i since it would otherwise be
/// ineffective as a guard.
rare2i: u8,
}
impl RareNeedleBytes {
/// Create a new pair of rare needle bytes with the given offsets. This is
/// only used in tests for generating input data.
#[cfg(all(test, feature = "std"))]
pub(crate) fn new(rare1i: u8, rare2i: u8) -> RareNeedleBytes {
RareNeedleBytes { rare1i, rare2i }
}
/// Detect the leftmost offsets of the two rarest bytes in the given
/// needle.
pub(crate) fn forward(needle: &[u8]) -> RareNeedleBytes {
if needle.len() <= 1 || needle.len() > core::u8::MAX as usize {
// For needles bigger than u8::MAX, our offsets aren't big enough.
// (We make our offsets small to reduce stack copying.)
// If you have a use case for it, please file an issue. In that
// case, we should probably just adjust the routine below to pick
// some rare bytes from the first 255 bytes of the needle.
//
// Also note that for needles of size 0 or 1, they are special
// cased in Two-Way.
//
// TODO: Benchmar this.
return RareNeedleBytes { rare1i: 0, rare2i: 0 };
}
// Find the rarest two bytes. We make them distinct by construction.
let (mut rare1, mut rare1i) = (needle[0], 0);
let (mut rare2, mut rare2i) = (needle[1], 1);
if rank(rare2) < rank(rare1) {
core::mem::swap(&mut rare1, &mut rare2);
core::mem::swap(&mut rare1i, &mut rare2i);
}
for (i, &b) in needle.iter().enumerate().skip(2) {
if rank(b) < rank(rare1) {
rare2 = rare1;
rare2i = rare1i;
rare1 = b;
rare1i = i as u8;
} else if b != rare1 && rank(b) < rank(rare2) {
rare2 = b;
rare2i = i as u8;
}
}
// While not strictly required, we really don't want these to be
// equivalent. If they were, it would reduce the effectiveness of
// candidate searching using these rare bytes by increasing the rate of
// false positives.
assert_ne!(rare1i, rare2i);
RareNeedleBytes { rare1i, rare2i }
}
/// Return the rare bytes in the given needle in the forward direction.
/// The needle given must be the same one given to the RareNeedleBytes
/// constructor.
pub(crate) fn as_rare_bytes(&self, needle: &[u8]) -> (u8, u8) {
(needle[self.rare1i as usize], needle[self.rare2i as usize])
}
/// Return the rare offsets such that the first offset is always <= to the
/// second offset. This is useful when the caller doesn't care whether
/// rare1 is rarer than rare2, but just wants to ensure that they are
/// ordered with respect to one another.
#[cfg(memchr_runtime_simd)]
pub(crate) fn as_rare_ordered_usize(&self) -> (usize, usize) {
let (rare1i, rare2i) = self.as_rare_ordered_u8();
(rare1i as usize, rare2i as usize)
}
/// Like as_rare_ordered_usize, but returns the offsets as their native
/// u8 values.
#[cfg(memchr_runtime_simd)]
pub(crate) fn as_rare_ordered_u8(&self) -> (u8, u8) {
if self.rare1i <= self.rare2i {
(self.rare1i, self.rare2i)
} else {
(self.rare2i, self.rare1i)
}
}
/// Return the rare offsets as usize values in the order in which they were
/// constructed. rare1, for example, is constructed as the "rarer" byte,
/// and thus, callers may want to treat it differently from rare2.
pub(crate) fn as_rare_usize(&self) -> (usize, usize) {
(self.rare1i as usize, self.rare2i as usize)
}
/// Return the byte frequency rank of each byte. The higher the rank, the
/// more frequency the byte is predicted to be. The needle given must be
/// the same one given to the RareNeedleBytes constructor.
pub(crate) fn as_ranks(&self, needle: &[u8]) -> (usize, usize) {
let (b1, b2) = self.as_rare_bytes(needle);
(rank(b1), rank(b2))
}
}
/// Return the heuristical frequency rank of the given byte. A lower rank
/// means the byte is believed to occur less frequently.
fn rank(b: u8) -> usize {
crate::memmem::byte_frequencies::BYTE_FREQUENCIES[b as usize] as usize
}
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