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Diffstat (limited to 'vendor/memchr/src/arch/all/rabinkarp.rs')
| -rw-r--r-- | vendor/memchr/src/arch/all/rabinkarp.rs | 390 |
1 files changed, 0 insertions, 390 deletions
diff --git a/vendor/memchr/src/arch/all/rabinkarp.rs b/vendor/memchr/src/arch/all/rabinkarp.rs deleted file mode 100644 index e0bafba..0000000 --- a/vendor/memchr/src/arch/all/rabinkarp.rs +++ /dev/null @@ -1,390 +0,0 @@ -/*! -An implementation of the [Rabin-Karp substring search algorithm][rabinkarp]. - -Rabin-Karp works by creating a hash of the needle provided and then computing -a rolling hash for each needle sized window in the haystack. When the rolling -hash matches the hash of the needle, a byte-wise comparison is done to check -if a match exists. The worst case time complexity of Rabin-Karp is `O(m * -n)` where `m ~ len(needle)` and `n ~ len(haystack)`. Its worst case space -complexity is constant. - -The main utility of Rabin-Karp is that the searcher can be constructed very -quickly with very little memory. This makes it especially useful when searching -for small needles in small haystacks, as it might finish its search before a -beefier algorithm (like Two-Way) even starts. - -[rabinkarp]: https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm -*/ - -/* -(This was the comment I wrote for this module originally when it was not -exposed. The comment still looks useful, but it's a bit in the weeds, so it's -not public itself.) - -This module implements the classical Rabin-Karp substring search algorithm, -with no extra frills. While its use would seem to break our time complexity -guarantee of O(m+n) (RK's time complexity is O(mn)), we are careful to only -ever use RK on a constant subset of haystacks. The main point here is that -RK has good latency properties for small needles/haystacks. It's very quick -to compute a needle hash and zip through the haystack when compared to -initializing Two-Way, for example. And this is especially useful for cases -where the haystack is just too short for vector instructions to do much good. - -The hashing function used here is the same one recommended by ESMAJ. - -Another choice instead of Rabin-Karp would be Shift-Or. But its latency -isn't quite as good since its preprocessing time is a bit more expensive -(both in practice and in theory). However, perhaps Shift-Or has a place -somewhere else for short patterns. I think the main problem is that it -requires space proportional to the alphabet and the needle. If we, for -example, supported needles up to length 16, then the total table size would be -len(alphabet)*size_of::<u16>()==512 bytes. Which isn't exactly small, and it's -probably bad to put that on the stack. So ideally, we'd throw it on the heap, -but we'd really like to write as much code without using alloc/std as possible. -But maybe it's worth the special casing. It's a TODO to benchmark. - -Wikipedia has a decent explanation, if a bit heavy on the theory: -https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm - -But ESMAJ provides something a bit more concrete: -http://www-igm.univ-mlv.fr/~lecroq/string/node5.html - -Finally, aho-corasick uses Rabin-Karp for multiple pattern match in some cases: -https://github.com/BurntSushi/aho-corasick/blob/3852632f10587db0ff72ef29e88d58bf305a0946/src/packed/rabinkarp.rs -*/ - -use crate::ext::Pointer; - -/// A forward substring searcher using the Rabin-Karp algorithm. -/// -/// Note that, as a lower level API, a `Finder` does not have access to the -/// needle it was constructed with. For this reason, executing a search -/// with a `Finder` requires passing both the needle and the haystack, -/// where the needle is exactly equivalent to the one given to the `Finder` -/// at construction time. This design was chosen so that callers can have -/// more precise control over where and how many times a needle is stored. -/// For example, in cases where Rabin-Karp is just one of several possible -/// substring search algorithms. -#[derive(Clone, Debug)] -pub struct Finder { - /// The actual hash. - hash: Hash, - /// The factor needed to multiply a byte by in order to subtract it from - /// the hash. It is defined to be 2^(n-1) (using wrapping exponentiation), - /// where n is the length of the needle. This is how we "remove" a byte - /// from the hash once the hash window rolls past it. - hash_2pow: u32, -} - -impl Finder { - /// Create a new Rabin-Karp forward searcher for the given `needle`. - /// - /// The needle may be empty. The empty needle matches at every byte offset. - /// - /// Note that callers must pass the same needle to all search calls using - /// this `Finder`. - #[inline] - pub fn new(needle: &[u8]) -> Finder { - let mut s = Finder { hash: Hash::new(), hash_2pow: 1 }; - let first_byte = match needle.get(0) { - None => return s, - Some(&first_byte) => first_byte, - }; - s.hash.add(first_byte); - for b in needle.iter().copied().skip(1) { - s.hash.add(b); - s.hash_2pow = s.hash_2pow.wrapping_shl(1); - } - s - } - - /// Return the first occurrence of the `needle` in the `haystack` - /// given. If no such occurrence exists, then `None` is returned. - /// - /// The `needle` provided must match the needle given to this finder at - /// construction time. - /// - /// The maximum value this can return is `haystack.len()`, which can only - /// occur when the needle and haystack both have length zero. Otherwise, - /// for non-empty haystacks, the maximum value is `haystack.len() - 1`. - #[inline] - pub fn find(&self, haystack: &[u8], needle: &[u8]) -> Option<usize> { - unsafe { - let hstart = haystack.as_ptr(); - let hend = hstart.add(haystack.len()); - let nstart = needle.as_ptr(); - let nend = nstart.add(needle.len()); - let found = self.find_raw(hstart, hend, nstart, nend)?; - Some(found.distance(hstart)) - } - } - - /// Like `find`, but accepts and returns raw pointers. - /// - /// When a match is found, the pointer returned is guaranteed to be - /// `>= start` and `<= end`. The pointer returned is only ever equivalent - /// to `end` when both the needle and haystack are empty. (That is, the - /// empty string matches the empty string.) - /// - /// This routine is useful if you're already using raw pointers and would - /// like to avoid converting back to a slice before executing a search. - /// - /// # Safety - /// - /// Note that `start` and `end` below refer to both pairs of pointers given - /// to this routine. That is, the conditions apply to both `hstart`/`hend` - /// and `nstart`/`nend`. - /// - /// * Both `start` and `end` must be valid for reads. - /// * Both `start` and `end` must point to an initialized value. - /// * Both `start` and `end` must point to the same allocated object and - /// must either be in bounds or at most one byte past the end of the - /// allocated object. - /// * Both `start` and `end` must be _derived from_ a pointer to the same - /// object. - /// * The distance between `start` and `end` must not overflow `isize`. - /// * The distance being in bounds must not rely on "wrapping around" the - /// address space. - /// * It must be the case that `start <= end`. - #[inline] - pub unsafe fn find_raw( - &self, - hstart: *const u8, - hend: *const u8, - nstart: *const u8, - nend: *const u8, - ) -> Option<*const u8> { - let hlen = hend.distance(hstart); - let nlen = nend.distance(nstart); - if nlen > hlen { - return None; - } - let mut cur = hstart; - let end = hend.sub(nlen); - let mut hash = Hash::forward(cur, cur.add(nlen)); - loop { - if self.hash == hash && is_equal_raw(cur, nstart, nlen) { - return Some(cur); - } - if cur >= end { - return None; - } - hash.roll(self, cur.read(), cur.add(nlen).read()); - cur = cur.add(1); - } - } -} - -/// A reverse substring searcher using the Rabin-Karp algorithm. -#[derive(Clone, Debug)] -pub struct FinderRev(Finder); - -impl FinderRev { - /// Create a new Rabin-Karp reverse searcher for the given `needle`. - #[inline] - pub fn new(needle: &[u8]) -> FinderRev { - let mut s = FinderRev(Finder { hash: Hash::new(), hash_2pow: 1 }); - let last_byte = match needle.last() { - None => return s, - Some(&last_byte) => last_byte, - }; - s.0.hash.add(last_byte); - for b in needle.iter().rev().copied().skip(1) { - s.0.hash.add(b); - s.0.hash_2pow = s.0.hash_2pow.wrapping_shl(1); - } - s - } - - /// Return the last occurrence of the `needle` in the `haystack` - /// given. If no such occurrence exists, then `None` is returned. - /// - /// The `needle` provided must match the needle given to this finder at - /// construction time. - /// - /// The maximum value this can return is `haystack.len()`, which can only - /// occur when the needle and haystack both have length zero. Otherwise, - /// for non-empty haystacks, the maximum value is `haystack.len() - 1`. - #[inline] - pub fn rfind(&self, haystack: &[u8], needle: &[u8]) -> Option<usize> { - unsafe { - let hstart = haystack.as_ptr(); - let hend = hstart.add(haystack.len()); - let nstart = needle.as_ptr(); - let nend = nstart.add(needle.len()); - let found = self.rfind_raw(hstart, hend, nstart, nend)?; - Some(found.distance(hstart)) - } - } - - /// Like `rfind`, but accepts and returns raw pointers. - /// - /// When a match is found, the pointer returned is guaranteed to be - /// `>= start` and `<= end`. The pointer returned is only ever equivalent - /// to `end` when both the needle and haystack are empty. (That is, the - /// empty string matches the empty string.) - /// - /// This routine is useful if you're already using raw pointers and would - /// like to avoid converting back to a slice before executing a search. - /// - /// # Safety - /// - /// Note that `start` and `end` below refer to both pairs of pointers given - /// to this routine. That is, the conditions apply to both `hstart`/`hend` - /// and `nstart`/`nend`. - /// - /// * Both `start` and `end` must be valid for reads. - /// * Both `start` and `end` must point to an initialized value. - /// * Both `start` and `end` must point to the same allocated object and - /// must either be in bounds or at most one byte past the end of the - /// allocated object. - /// * Both `start` and `end` must be _derived from_ a pointer to the same - /// object. - /// * The distance between `start` and `end` must not overflow `isize`. - /// * The distance being in bounds must not rely on "wrapping around" the - /// address space. - /// * It must be the case that `start <= end`. - #[inline] - pub unsafe fn rfind_raw( - &self, - hstart: *const u8, - hend: *const u8, - nstart: *const u8, - nend: *const u8, - ) -> Option<*const u8> { - let hlen = hend.distance(hstart); - let nlen = nend.distance(nstart); - if nlen > hlen { - return None; - } - let mut cur = hend.sub(nlen); - let start = hstart; - let mut hash = Hash::reverse(cur, cur.add(nlen)); - loop { - if self.0.hash == hash && is_equal_raw(cur, nstart, nlen) { - return Some(cur); - } - if cur <= start { - return None; - } - cur = cur.sub(1); - hash.roll(&self.0, cur.add(nlen).read(), cur.read()); - } - } -} - -/// Whether RK is believed to be very fast for the given needle/haystack. -#[inline] -pub(crate) fn is_fast(haystack: &[u8], _needle: &[u8]) -> bool { - haystack.len() < 16 -} - -/// A Rabin-Karp hash. This might represent the hash of a needle, or the hash -/// of a rolling window in the haystack. -#[derive(Clone, Copy, Debug, Default, Eq, PartialEq)] -struct Hash(u32); - -impl Hash { - /// Create a new hash that represents the empty string. - #[inline(always)] - fn new() -> Hash { - Hash(0) - } - - /// Create a new hash from the bytes given for use in forward searches. - /// - /// # Safety - /// - /// The given pointers must be valid to read from within their range. - #[inline(always)] - unsafe fn forward(mut start: *const u8, end: *const u8) -> Hash { - let mut hash = Hash::new(); - while start < end { - hash.add(start.read()); - start = start.add(1); - } - hash - } - - /// Create a new hash from the bytes given for use in reverse searches. - /// - /// # Safety - /// - /// The given pointers must be valid to read from within their range. - #[inline(always)] - unsafe fn reverse(start: *const u8, mut end: *const u8) -> Hash { - let mut hash = Hash::new(); - while start < end { - end = end.sub(1); - hash.add(end.read()); - } - hash - } - - /// Add 'new' and remove 'old' from this hash. The given needle hash should - /// correspond to the hash computed for the needle being searched for. - /// - /// This is meant to be used when the rolling window of the haystack is - /// advanced. - #[inline(always)] - fn roll(&mut self, finder: &Finder, old: u8, new: u8) { - self.del(finder, old); - self.add(new); - } - - /// Add a byte to this hash. - #[inline(always)] - fn add(&mut self, byte: u8) { - self.0 = self.0.wrapping_shl(1).wrapping_add(u32::from(byte)); - } - - /// Remove a byte from this hash. The given needle hash should correspond - /// to the hash computed for the needle being searched for. - #[inline(always)] - fn del(&mut self, finder: &Finder, byte: u8) { - let factor = finder.hash_2pow; - self.0 = self.0.wrapping_sub(u32::from(byte).wrapping_mul(factor)); - } -} - -/// Returns true when `x[i] == y[i]` for all `0 <= i < n`. -/// -/// We forcefully don't inline this to hint at the compiler that it is unlikely -/// to be called. This causes the inner rabinkarp loop above to be a bit -/// tighter and leads to some performance improvement. See the -/// memmem/krate/prebuilt/sliceslice-words/words benchmark. -/// -/// # Safety -/// -/// Same as `crate::arch::all::is_equal_raw`. -#[cold] -#[inline(never)] -unsafe fn is_equal_raw(x: *const u8, y: *const u8, n: usize) -> bool { - crate::arch::all::is_equal_raw(x, y, n) -} - -#[cfg(test)] -mod tests { - use super::*; - - define_substring_forward_quickcheck!(|h, n| Some( - Finder::new(n).find(h, n) - )); - define_substring_reverse_quickcheck!(|h, n| Some( - FinderRev::new(n).rfind(h, n) - )); - - #[test] - fn forward() { - crate::tests::substring::Runner::new() - .fwd(|h, n| Some(Finder::new(n).find(h, n))) - .run(); - } - - #[test] - fn reverse() { - crate::tests::substring::Runner::new() - .rev(|h, n| Some(FinderRev::new(n).rfind(h, n))) - .run(); - } -} |