Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

1 files changed, 359 insertions, 0 deletions

diff --git a/vendor/memchr/src/arch/all/packedpair/mod.rs b/vendor/memchr/src/arch/all/packedpair/mod.rs
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+/*!
+Provides an architecture independent implementation of the "packed pair"
+algorithm.
+
+The "packed pair" algorithm is based on the [generic SIMD] algorithm. The main
+difference is that it (by default) uses a background distribution of byte
+frequencies to heuristically select the pair of bytes to search for. Note that
+this module provides an architecture independent version that doesn't do as
+good of a job keeping the search for candidates inside a SIMD hot path. It
+however can be good enough in many circumstances.
+
+[generic SIMD]: http://0x80.pl/articles/simd-strfind.html#first-and-last
+*/
+
+use crate::memchr;
+
+mod default_rank;
+
+/// An architecture independent "packed pair" finder.
+///
+/// This finder picks two bytes that it believes have high predictive power for
+/// indicating an overall match of a needle. At search time, it reports offsets
+/// where the needle could match based on whether the pair of bytes it chose
+/// match.
+///
+/// This is architecture independent because it utilizes `memchr` to find the
+/// occurrence of one of the bytes in the pair, and then checks whether the
+/// second byte matches. If it does, in the case of [`Finder::find_prefilter`],
+/// the location at which the needle could match is returned.
+///
+/// It is generally preferred to use architecture specific routines for a
+/// "packed pair" prefilter, but this can be a useful fallback when the
+/// architecture independent routines are unavailable.
+#[derive(Clone, Copy, Debug)]
+pub struct Finder {
+    pair: Pair,
+    byte1: u8,
+    byte2: u8,
+}
+
+impl Finder {
+    /// Create a new prefilter that reports possible locations where the given
+    /// needle matches.
+    #[inline]
+    pub fn new(needle: &[u8]) -> Option<Finder> {
+        Finder::with_pair(needle, Pair::new(needle)?)
+    }
+
+    /// Create a new prefilter using the pair given.
+    ///
+    /// If the prefilter could not be constructed, then `None` is returned.
+    ///
+    /// This constructor permits callers to control precisely which pair of
+    /// bytes is used as a predicate.
+    #[inline]
+    pub fn with_pair(needle: &[u8], pair: Pair) -> Option<Finder> {
+        let byte1 = needle[usize::from(pair.index1())];
+        let byte2 = needle[usize::from(pair.index2())];
+        // Currently this can never fail so we could just return a Finder,
+        // but it's conceivable this could change.
+        Some(Finder { pair, byte1, byte2 })
+    }
+
+    /// Run this finder on the given haystack as a prefilter.
+    ///
+    /// If a candidate match is found, then an offset where the needle *could*
+    /// begin in the haystack is returned.
+    #[inline]
+    pub fn find_prefilter(&self, haystack: &[u8]) -> Option<usize> {
+        let mut i = 0;
+        let index1 = usize::from(self.pair.index1());
+        let index2 = usize::from(self.pair.index2());
+        loop {
+            // Use a fast vectorized implementation to skip to the next
+            // occurrence of the rarest byte (heuristically chosen) in the
+            // needle.
+            i += memchr(self.byte1, &haystack[i..])?;
+            let found = i;
+            i += 1;
+
+            // If we can't align our first byte match with the haystack, then a
+            // match is impossible.
+            let aligned1 = match found.checked_sub(index1) {
+                None => continue,
+                Some(aligned1) => aligned1,
+            };
+
+            // Now align the second byte match with the haystack. A mismatch
+            // means that a match is impossible.
+            let aligned2 = match aligned1.checked_add(index2) {
+                None => continue,
+                Some(aligned_index2) => aligned_index2,
+            };
+            if haystack.get(aligned2).map_or(true, |&b| b != self.byte2) {
+                continue;
+            }
+
+            // We've done what we can. There might be a match here.
+            return Some(aligned1);
+        }
+    }
+
+    /// Returns the pair of offsets (into the needle) used to check as a
+    /// predicate before confirming whether a needle exists at a particular
+    /// position.
+    #[inline]
+    pub fn pair(&self) -> &Pair {
+        &self.pair
+    }
+}
+
+/// A pair of byte offsets into a needle to use as a predicate.
+///
+/// This pair is used as a predicate to quickly filter out positions in a
+/// haystack in which a needle cannot match. In some cases, this pair can even
+/// be used in vector algorithms such that the vector algorithm only switches
+/// over to scalar code once this pair has been found.
+///
+/// A pair of offsets can be used in both substring search implementations and
+/// in prefilters. The former will report matches of a needle in a haystack
+/// where as the latter will only report possible matches of a needle.
+///
+/// The offsets are limited each to a maximum of 255 to keep memory usage low.
+/// Moreover, it's rarely advantageous to create a predicate using offsets
+/// greater than 255 anyway.
+///
+/// The only guarantee enforced on the pair of offsets is that they are not
+/// equivalent. It is not necessarily the case that `index1 < index2` for
+/// example. By convention, `index1` corresponds to the byte in the needle
+/// that is believed to be most the predictive. Note also that because of the
+/// requirement that the indices be both valid for the needle used to build
+/// the pair and not equal, it follows that a pair can only be constructed for
+/// needles with length at least 2.
+#[derive(Clone, Copy, Debug)]
+pub struct Pair {
+    index1: u8,
+    index2: u8,
+}
+
+impl Pair {
+    /// Create a new pair of offsets from the given needle.
+    ///
+    /// If a pair could not be created (for example, if the needle is too
+    /// short), then `None` is returned.
+    ///
+    /// This chooses the pair in the needle that is believed to be as
+    /// predictive of an overall match of the needle as possible.
+    #[inline]
+    pub fn new(needle: &[u8]) -> Option<Pair> {
+        Pair::with_ranker(needle, DefaultFrequencyRank)
+    }
+
+    /// Create a new pair of offsets from the given needle and ranker.
+    ///
+    /// This permits the caller to choose a background frequency distribution
+    /// with which bytes are selected. The idea is to select a pair of bytes
+    /// that is believed to strongly predict a match in the haystack. This
+    /// usually means selecting bytes that occur rarely in a haystack.
+    ///
+    /// If a pair could not be created (for example, if the needle is too
+    /// short), then `None` is returned.
+    #[inline]
+    pub fn with_ranker<R: HeuristicFrequencyRank>(
+        needle: &[u8],
+        ranker: R,
+    ) -> Option<Pair> {
+        if needle.len() <= 1 {
+            return None;
+        }
+        // Find the rarest two bytes. We make them distinct indices by
+        // construction. (The actual byte value may be the same in degenerate
+        // cases, but that's OK.)
+        let (mut rare1, mut index1) = (needle[0], 0);
+        let (mut rare2, mut index2) = (needle[1], 1);
+        if ranker.rank(rare2) < ranker.rank(rare1) {
+            core::mem::swap(&mut rare1, &mut rare2);
+            core::mem::swap(&mut index1, &mut index2);
+        }
+        let max = usize::from(core::u8::MAX);
+        for (i, &b) in needle.iter().enumerate().take(max).skip(2) {
+            if ranker.rank(b) < ranker.rank(rare1) {
+                rare2 = rare1;
+                index2 = index1;
+                rare1 = b;
+                index1 = u8::try_from(i).unwrap();
+            } else if b != rare1 && ranker.rank(b) < ranker.rank(rare2) {
+                rare2 = b;
+                index2 = u8::try_from(i).unwrap();
+            }
+        }
+        // While not strictly required for how a Pair is normally used, 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!(index1, index2);
+        Some(Pair { index1, index2 })
+    }
+
+    /// Create a new pair using the offsets given for the needle given.
+    ///
+    /// This bypasses any sort of heuristic process for choosing the offsets
+    /// and permits the caller to choose the offsets themselves.
+    ///
+    /// Indices are limited to valid `u8` values so that a `Pair` uses less
+    /// memory. It is not possible to create a `Pair` with offsets bigger than
+    /// `u8::MAX`. It's likely that such a thing is not needed, but if it is,
+    /// it's suggested to build your own bespoke algorithm because you're
+    /// likely working on a very niche case. (File an issue if this suggestion
+    /// does not make sense to you.)
+    ///
+    /// If a pair could not be created (for example, if the needle is too
+    /// short), then `None` is returned.
+    #[inline]
+    pub fn with_indices(
+        needle: &[u8],
+        index1: u8,
+        index2: u8,
+    ) -> Option<Pair> {
+        // While not strictly required for how a Pair is normally used, 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.
+        if index1 == index2 {
+            return None;
+        }
+        // Similarly, invalid indices means the Pair is invalid too.
+        if usize::from(index1) >= needle.len() {
+            return None;
+        }
+        if usize::from(index2) >= needle.len() {
+            return None;
+        }
+        Some(Pair { index1, index2 })
+    }
+
+    /// Returns the first offset of the pair.
+    #[inline]
+    pub fn index1(&self) -> u8 {
+        self.index1
+    }
+
+    /// Returns the second offset of the pair.
+    #[inline]
+    pub fn index2(&self) -> u8 {
+        self.index2
+    }
+}
+
+/// This trait allows the user to customize the heuristic used to determine the
+/// relative frequency of a given byte in the dataset being searched.
+///
+/// The use of this trait can have a dramatic impact on performance depending
+/// on the type of data being searched. The details of why are explained in the
+/// docs of [`crate::memmem::Prefilter`]. To summarize, the core algorithm uses
+/// a prefilter to quickly identify candidate matches that are later verified
+/// more slowly. This prefilter is implemented in terms of trying to find
+/// `rare` bytes at specific offsets that will occur less frequently in the
+/// dataset. While the concept of a `rare` byte is similar for most datasets,
+/// there are some specific datasets (like binary executables) that have
+/// dramatically different byte distributions. For these datasets customizing
+/// the byte frequency heuristic can have a massive impact on performance, and
+/// might even need to be done at runtime.
+///
+/// The default implementation of `HeuristicFrequencyRank` reads from the
+/// static frequency table defined in `src/memmem/byte_frequencies.rs`. This
+/// is optimal for most inputs, so if you are unsure of the impact of using a
+/// custom `HeuristicFrequencyRank` you should probably just use the default.
+///
+/// # Example
+///
+/// ```
+/// use memchr::{
+///     arch::all::packedpair::HeuristicFrequencyRank,
+///     memmem::FinderBuilder,
+/// };
+///
+/// /// A byte-frequency table that is good for scanning binary executables.
+/// struct Binary;
+///
+/// impl HeuristicFrequencyRank for Binary {
+///     fn rank(&self, byte: u8) -> u8 {
+///         const TABLE: [u8; 256] = [
+///             255, 128, 61, 43, 50, 41, 27, 28, 57, 15, 21, 13, 24, 17, 17,
+///             89, 58, 16, 11, 7, 14, 23, 7, 6, 24, 9, 6, 5, 9, 4, 7, 16,
+///             68, 11, 9, 6, 88, 7, 4, 4, 23, 9, 4, 8, 8, 5, 10, 4, 30, 11,
+///             9, 24, 11, 5, 5, 5, 19, 11, 6, 17, 9, 9, 6, 8,
+///             48, 58, 11, 14, 53, 40, 9, 9, 254, 35, 3, 6, 52, 23, 6, 6, 27,
+///             4, 7, 11, 14, 13, 10, 11, 11, 5, 2, 10, 16, 12, 6, 19,
+///             19, 20, 5, 14, 16, 31, 19, 7, 14, 20, 4, 4, 19, 8, 18, 20, 24,
+///             1, 25, 19, 58, 29, 10, 5, 15, 20, 2, 2, 9, 4, 3, 5,
+///             51, 11, 4, 53, 23, 39, 6, 4, 13, 81, 4, 186, 5, 67, 3, 2, 15,
+///             0, 0, 1, 3, 2, 0, 0, 5, 0, 0, 0, 2, 0, 0, 0,
+///             12, 2, 1, 1, 3, 1, 1, 1, 6, 1, 2, 1, 3, 1, 1, 2, 9, 1, 1, 0,
+///             2, 2, 4, 4, 11, 6, 7, 3, 6, 9, 4, 5,
+///             46, 18, 8, 18, 17, 3, 8, 20, 16, 10, 3, 7, 175, 4, 6, 7, 13,
+///             3, 7, 3, 3, 1, 3, 3, 10, 3, 1, 5, 2, 0, 1, 2,
+///             16, 3, 5, 1, 6, 1, 1, 2, 58, 20, 3, 14, 12, 2, 1, 3, 16, 3, 5,
+///             8, 3, 1, 8, 6, 17, 6, 5, 3, 8, 6, 13, 175,
+///         ];
+///         TABLE[byte as usize]
+///     }
+/// }
+/// // Create a new finder with the custom heuristic.
+/// let finder = FinderBuilder::new()
+///     .build_forward_with_ranker(Binary, b"\x00\x00\xdd\xdd");
+/// // Find needle with custom heuristic.
+/// assert!(finder.find(b"\x00\x00\x00\xdd\xdd").is_some());
+/// ```
+pub trait HeuristicFrequencyRank {
+    /// Return the heuristic frequency rank of the given byte. A lower rank
+    /// means the byte is believed to occur less frequently in the haystack.
+    ///
+    /// Some uses of this heuristic may treat arbitrary absolute rank values as
+    /// significant. For example, an implementation detail in this crate may
+    /// determine that heuristic prefilters are inappropriate if every byte in
+    /// the needle has a "high" rank.
+    fn rank(&self, byte: u8) -> u8;
+}
+
+/// The default byte frequency heuristic that is good for most haystacks.
+pub(crate) struct DefaultFrequencyRank;
+
+impl HeuristicFrequencyRank for DefaultFrequencyRank {
+    fn rank(&self, byte: u8) -> u8 {
+        self::default_rank::RANK[usize::from(byte)]
+    }
+}
+
+/// This permits passing any implementation of `HeuristicFrequencyRank` as a
+/// borrowed version of itself.
+impl<'a, R> HeuristicFrequencyRank for &'a R
+where
+    R: HeuristicFrequencyRank,
+{
+    fn rank(&self, byte: u8) -> u8 {
+        (**self).rank(byte)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn forward_packedpair() {
+        fn find(
+            haystack: &[u8],
+            needle: &[u8],
+            _index1: u8,
+            _index2: u8,
+        ) -> Option<Option<usize>> {
+            // We ignore the index positions requested since it winds up making
+            // this test too slow overall.
+            let f = Finder::new(needle)?;
+            Some(f.find_prefilter(haystack))
+        }
+        crate::tests::packedpair::Runner::new().fwd(find).run()
+    }
+}