Initial vendor packages

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

1 files changed, 1030 insertions, 0 deletions

diff --git a/vendor/memchr/src/memmem/searcher.rs b/vendor/memchr/src/memmem/searcher.rs
new file mode 100644
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+++ b/vendor/memchr/src/memmem/searcher.rs
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+use crate::arch::all::{
+    packedpair::{HeuristicFrequencyRank, Pair},
+    rabinkarp, twoway,
+};
+
+#[cfg(target_arch = "aarch64")]
+use crate::arch::aarch64::neon::packedpair as neon;
+#[cfg(target_arch = "wasm32")]
+use crate::arch::wasm32::simd128::packedpair as simd128;
+#[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+use crate::arch::x86_64::{
+    avx2::packedpair as avx2, sse2::packedpair as sse2,
+};
+
+/// A "meta" substring searcher.
+///
+/// To a first approximation, this chooses what it believes to be the "best"
+/// substring search implemnetation based on the needle at construction time.
+/// Then, every call to `find` will execute that particular implementation. To
+/// a second approximation, multiple substring search algorithms may be used,
+/// depending on the haystack. For example, for supremely short haystacks,
+/// Rabin-Karp is typically used.
+///
+/// See the documentation on `Prefilter` for an explanation of the dispatching
+/// mechanism. The quick summary is that an enum has too much overhead and
+/// we can't use dynamic dispatch via traits because we need to work in a
+/// core-only environment. (Dynamic dispatch works in core-only, but you
+/// need `&dyn Trait` and we really need a `Box<dyn Trait>` here. The latter
+/// requires `alloc`.) So instead, we use a union and an appropriately paired
+/// free function to read from the correct field on the union and execute the
+/// chosen substring search implementation.
+#[derive(Clone)]
+pub(crate) struct Searcher {
+    call: SearcherKindFn,
+    kind: SearcherKind,
+    rabinkarp: rabinkarp::Finder,
+}
+
+impl Searcher {
+    /// Creates a new "meta" substring searcher that attempts to choose the
+    /// best algorithm based on the needle, heuristics and what the current
+    /// target supports.
+    #[inline]
+    pub(crate) fn new<R: HeuristicFrequencyRank>(
+        prefilter: PrefilterConfig,
+        ranker: R,
+        needle: &[u8],
+    ) -> Searcher {
+        let rabinkarp = rabinkarp::Finder::new(needle);
+        if needle.len() <= 1 {
+            return if needle.is_empty() {
+                trace!("building empty substring searcher");
+                Searcher {
+                    call: searcher_kind_empty,
+                    kind: SearcherKind { empty: () },
+                    rabinkarp,
+                }
+            } else {
+                trace!("building one-byte substring searcher");
+                debug_assert_eq!(1, needle.len());
+                Searcher {
+                    call: searcher_kind_one_byte,
+                    kind: SearcherKind { one_byte: needle[0] },
+                    rabinkarp,
+                }
+            };
+        }
+        let pair = match Pair::with_ranker(needle, &ranker) {
+            Some(pair) => pair,
+            None => return Searcher::twoway(needle, rabinkarp, None),
+        };
+        debug_assert_ne!(
+            pair.index1(),
+            pair.index2(),
+            "pair offsets should not be equivalent"
+        );
+        #[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+        {
+            if let Some(pp) = avx2::Finder::with_pair(needle, pair) {
+                if do_packed_search(needle) {
+                    trace!("building x86_64 AVX2 substring searcher");
+                    let kind = SearcherKind { avx2: pp };
+                    Searcher { call: searcher_kind_avx2, kind, rabinkarp }
+                } else if prefilter.is_none() {
+                    Searcher::twoway(needle, rabinkarp, None)
+                } else {
+                    let prestrat = Prefilter::avx2(pp, needle);
+                    Searcher::twoway(needle, rabinkarp, Some(prestrat))
+                }
+            } else if let Some(pp) = sse2::Finder::with_pair(needle, pair) {
+                if do_packed_search(needle) {
+                    trace!("building x86_64 SSE2 substring searcher");
+                    let kind = SearcherKind { sse2: pp };
+                    Searcher { call: searcher_kind_sse2, kind, rabinkarp }
+                } else if prefilter.is_none() {
+                    Searcher::twoway(needle, rabinkarp, None)
+                } else {
+                    let prestrat = Prefilter::sse2(pp, needle);
+                    Searcher::twoway(needle, rabinkarp, Some(prestrat))
+                }
+            } else if prefilter.is_none() {
+                Searcher::twoway(needle, rabinkarp, None)
+            } else {
+                // We're pretty unlikely to get to this point, but it is
+                // possible to be running on x86_64 without SSE2. Namely, it's
+                // really up to the OS whether it wants to support vector
+                // registers or not.
+                let prestrat = Prefilter::fallback(ranker, pair, needle);
+                Searcher::twoway(needle, rabinkarp, prestrat)
+            }
+        }
+        #[cfg(target_arch = "wasm32")]
+        {
+            if let Some(pp) = simd128::Finder::with_pair(needle, pair) {
+                if do_packed_search(needle) {
+                    trace!("building wasm32 simd128 substring searcher");
+                    let kind = SearcherKind { simd128: pp };
+                    Searcher { call: searcher_kind_simd128, kind, rabinkarp }
+                } else if prefilter.is_none() {
+                    Searcher::twoway(needle, rabinkarp, None)
+                } else {
+                    let prestrat = Prefilter::simd128(pp, needle);
+                    Searcher::twoway(needle, rabinkarp, Some(prestrat))
+                }
+            } else if prefilter.is_none() {
+                Searcher::twoway(needle, rabinkarp, None)
+            } else {
+                let prestrat = Prefilter::fallback(ranker, pair, needle);
+                Searcher::twoway(needle, rabinkarp, prestrat)
+            }
+        }
+        #[cfg(target_arch = "aarch64")]
+        {
+            if let Some(pp) = neon::Finder::with_pair(needle, pair) {
+                if do_packed_search(needle) {
+                    trace!("building aarch64 neon substring searcher");
+                    let kind = SearcherKind { neon: pp };
+                    Searcher { call: searcher_kind_neon, kind, rabinkarp }
+                } else if prefilter.is_none() {
+                    Searcher::twoway(needle, rabinkarp, None)
+                } else {
+                    let prestrat = Prefilter::neon(pp, needle);
+                    Searcher::twoway(needle, rabinkarp, Some(prestrat))
+                }
+            } else if prefilter.is_none() {
+                Searcher::twoway(needle, rabinkarp, None)
+            } else {
+                let prestrat = Prefilter::fallback(ranker, pair, needle);
+                Searcher::twoway(needle, rabinkarp, prestrat)
+            }
+        }
+        #[cfg(not(any(
+            all(target_arch = "x86_64", target_feature = "sse2"),
+            target_arch = "wasm32",
+            target_arch = "aarch64"
+        )))]
+        {
+            if prefilter.is_none() {
+                Searcher::twoway(needle, rabinkarp, None)
+            } else {
+                let prestrat = Prefilter::fallback(ranker, pair, needle);
+                Searcher::twoway(needle, rabinkarp, prestrat)
+            }
+        }
+    }
+
+    /// Creates a new searcher that always uses the Two-Way algorithm. This is
+    /// typically used when vector algorithms are unavailable or inappropriate.
+    /// (For example, when the needle is "too long.")
+    ///
+    /// If a prefilter is given, then the searcher returned will be accelerated
+    /// by the prefilter.
+    #[inline]
+    fn twoway(
+        needle: &[u8],
+        rabinkarp: rabinkarp::Finder,
+        prestrat: Option<Prefilter>,
+    ) -> Searcher {
+        let finder = twoway::Finder::new(needle);
+        match prestrat {
+            None => {
+                trace!("building scalar two-way substring searcher");
+                let kind = SearcherKind { two_way: finder };
+                Searcher { call: searcher_kind_two_way, kind, rabinkarp }
+            }
+            Some(prestrat) => {
+                trace!(
+                    "building scalar two-way \
+                     substring searcher with a prefilter"
+                );
+                let two_way_with_prefilter =
+                    TwoWayWithPrefilter { finder, prestrat };
+                let kind = SearcherKind { two_way_with_prefilter };
+                Searcher {
+                    call: searcher_kind_two_way_with_prefilter,
+                    kind,
+                    rabinkarp,
+                }
+            }
+        }
+    }
+
+    /// Searches the given haystack for the given needle. The needle given
+    /// should be the same as the needle that this finder was initialized
+    /// with.
+    ///
+    /// Inlining this can lead to big wins for latency, and #[inline] doesn't
+    /// seem to be enough in some cases.
+    #[inline(always)]
+    pub(crate) fn find(
+        &self,
+        prestate: &mut PrefilterState,
+        haystack: &[u8],
+        needle: &[u8],
+    ) -> Option<usize> {
+        if haystack.len() < needle.len() {
+            None
+        } else {
+            // SAFETY: By construction, we've ensured that the function
+            // in `self.call` is properly paired with the union used in
+            // `self.kind`.
+            unsafe { (self.call)(self, prestate, haystack, needle) }
+        }
+    }
+}
+
+impl core::fmt::Debug for Searcher {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
+        f.debug_struct("Searcher")
+            .field("call", &"<searcher function>")
+            .field("kind", &"<searcher kind union>")
+            .field("rabinkarp", &self.rabinkarp)
+            .finish()
+    }
+}
+
+/// A union indicating one of several possible substring search implementations
+/// that are in active use.
+///
+/// This union should only be read by one of the functions prefixed with
+/// `searcher_kind_`. Namely, the correct function is meant to be paired with
+/// the union by the caller, such that the function always reads from the
+/// designated union field.
+#[derive(Clone, Copy)]
+union SearcherKind {
+    empty: (),
+    one_byte: u8,
+    two_way: twoway::Finder,
+    two_way_with_prefilter: TwoWayWithPrefilter,
+    #[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+    sse2: crate::arch::x86_64::sse2::packedpair::Finder,
+    #[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+    avx2: crate::arch::x86_64::avx2::packedpair::Finder,
+    #[cfg(target_arch = "wasm32")]
+    simd128: crate::arch::wasm32::simd128::packedpair::Finder,
+    #[cfg(target_arch = "aarch64")]
+    neon: crate::arch::aarch64::neon::packedpair::Finder,
+}
+
+/// A two-way substring searcher with a prefilter.
+#[derive(Copy, Clone, Debug)]
+struct TwoWayWithPrefilter {
+    finder: twoway::Finder,
+    prestrat: Prefilter,
+}
+
+/// The type of a substring search function.
+///
+/// # Safety
+///
+/// When using a function of this type, callers must ensure that the correct
+/// function is paired with the value populated in `SearcherKind` union.
+type SearcherKindFn = unsafe fn(
+    searcher: &Searcher,
+    prestate: &mut PrefilterState,
+    haystack: &[u8],
+    needle: &[u8],
+) -> Option<usize>;
+
+/// Reads from the `empty` field of `SearcherKind` to handle the case of
+/// searching for the empty needle. Works on all platforms.
+///
+/// # Safety
+///
+/// Callers must ensure that the `searcher.kind.empty` union field is set.
+unsafe fn searcher_kind_empty(
+    _searcher: &Searcher,
+    _prestate: &mut PrefilterState,
+    _haystack: &[u8],
+    _needle: &[u8],
+) -> Option<usize> {
+    Some(0)
+}
+
+/// Reads from the `one_byte` field of `SearcherKind` to handle the case of
+/// searching for a single byte needle. Works on all platforms.
+///
+/// # Safety
+///
+/// Callers must ensure that the `searcher.kind.one_byte` union field is set.
+unsafe fn searcher_kind_one_byte(
+    searcher: &Searcher,
+    _prestate: &mut PrefilterState,
+    haystack: &[u8],
+    _needle: &[u8],
+) -> Option<usize> {
+    let needle = searcher.kind.one_byte;
+    crate::memchr(needle, haystack)
+}
+
+/// Reads from the `two_way` field of `SearcherKind` to handle the case of
+/// searching for an arbitrary needle without prefilter acceleration. Works on
+/// all platforms.
+///
+/// # Safety
+///
+/// Callers must ensure that the `searcher.kind.two_way` union field is set.
+unsafe fn searcher_kind_two_way(
+    searcher: &Searcher,
+    _prestate: &mut PrefilterState,
+    haystack: &[u8],
+    needle: &[u8],
+) -> Option<usize> {
+    if rabinkarp::is_fast(haystack, needle) {
+        searcher.rabinkarp.find(haystack, needle)
+    } else {
+        searcher.kind.two_way.find(haystack, needle)
+    }
+}
+
+/// Reads from the `two_way_with_prefilter` field of `SearcherKind` to handle
+/// the case of searching for an arbitrary needle with prefilter acceleration.
+/// Works on all platforms.
+///
+/// # Safety
+///
+/// Callers must ensure that the `searcher.kind.two_way_with_prefilter` union
+/// field is set.
+unsafe fn searcher_kind_two_way_with_prefilter(
+    searcher: &Searcher,
+    prestate: &mut PrefilterState,
+    haystack: &[u8],
+    needle: &[u8],
+) -> Option<usize> {
+    if rabinkarp::is_fast(haystack, needle) {
+        searcher.rabinkarp.find(haystack, needle)
+    } else {
+        let TwoWayWithPrefilter { ref finder, ref prestrat } =
+            searcher.kind.two_way_with_prefilter;
+        let pre = Pre { prestate, prestrat };
+        finder.find_with_prefilter(Some(pre), haystack, needle)
+    }
+}
+
+/// Reads from the `sse2` field of `SearcherKind` to execute the x86_64 SSE2
+/// vectorized substring search implementation.
+///
+/// # Safety
+///
+/// Callers must ensure that the `searcher.kind.sse2` union field is set.
+#[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+unsafe fn searcher_kind_sse2(
+    searcher: &Searcher,
+    _prestate: &mut PrefilterState,
+    haystack: &[u8],
+    needle: &[u8],
+) -> Option<usize> {
+    let finder = &searcher.kind.sse2;
+    if haystack.len() < finder.min_haystack_len() {
+        searcher.rabinkarp.find(haystack, needle)
+    } else {
+        finder.find(haystack, needle)
+    }
+}
+
+/// Reads from the `avx2` field of `SearcherKind` to execute the x86_64 AVX2
+/// vectorized substring search implementation.
+///
+/// # Safety
+///
+/// Callers must ensure that the `searcher.kind.avx2` union field is set.
+#[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+unsafe fn searcher_kind_avx2(
+    searcher: &Searcher,
+    _prestate: &mut PrefilterState,
+    haystack: &[u8],
+    needle: &[u8],
+) -> Option<usize> {
+    let finder = &searcher.kind.avx2;
+    if haystack.len() < finder.min_haystack_len() {
+        searcher.rabinkarp.find(haystack, needle)
+    } else {
+        finder.find(haystack, needle)
+    }
+}
+
+/// Reads from the `simd128` field of `SearcherKind` to execute the wasm32
+/// simd128 vectorized substring search implementation.
+///
+/// # Safety
+///
+/// Callers must ensure that the `searcher.kind.simd128` union field is set.
+#[cfg(target_arch = "wasm32")]
+unsafe fn searcher_kind_simd128(
+    searcher: &Searcher,
+    _prestate: &mut PrefilterState,
+    haystack: &[u8],
+    needle: &[u8],
+) -> Option<usize> {
+    let finder = &searcher.kind.simd128;
+    if haystack.len() < finder.min_haystack_len() {
+        searcher.rabinkarp.find(haystack, needle)
+    } else {
+        finder.find(haystack, needle)
+    }
+}
+
+/// Reads from the `neon` field of `SearcherKind` to execute the aarch64 neon
+/// vectorized substring search implementation.
+///
+/// # Safety
+///
+/// Callers must ensure that the `searcher.kind.neon` union field is set.
+#[cfg(target_arch = "aarch64")]
+unsafe fn searcher_kind_neon(
+    searcher: &Searcher,
+    _prestate: &mut PrefilterState,
+    haystack: &[u8],
+    needle: &[u8],
+) -> Option<usize> {
+    let finder = &searcher.kind.neon;
+    if haystack.len() < finder.min_haystack_len() {
+        searcher.rabinkarp.find(haystack, needle)
+    } else {
+        finder.find(haystack, needle)
+    }
+}
+
+/// A reverse substring searcher.
+#[derive(Clone, Debug)]
+pub(crate) struct SearcherRev {
+    kind: SearcherRevKind,
+    rabinkarp: rabinkarp::FinderRev,
+}
+
+/// The kind of the reverse searcher.
+///
+/// For the reverse case, we don't do any SIMD acceleration or prefilters.
+/// There is no specific technical reason why we don't, but rather don't do it
+/// because it's not clear it's worth the extra code to do so. If you have a
+/// use case for it, please file an issue.
+///
+/// We also don't do the union trick as we do with the forward case and
+/// prefilters. Basically for the same reason we don't have prefilters or
+/// vector algorithms for reverse searching: it's not clear it's worth doing.
+/// Please file an issue if you have a compelling use case for fast reverse
+/// substring search.
+#[derive(Clone, Debug)]
+enum SearcherRevKind {
+    Empty,
+    OneByte { needle: u8 },
+    TwoWay { finder: twoway::FinderRev },
+}
+
+impl SearcherRev {
+    /// Creates a new searcher for finding occurrences of the given needle in
+    /// reverse. That is, it reports the last (instead of the first) occurrence
+    /// of a needle in a haystack.
+    #[inline]
+    pub(crate) fn new(needle: &[u8]) -> SearcherRev {
+        let kind = if needle.len() <= 1 {
+            if needle.is_empty() {
+                trace!("building empty reverse substring searcher");
+                SearcherRevKind::Empty
+            } else {
+                trace!("building one-byte reverse substring searcher");
+                debug_assert_eq!(1, needle.len());
+                SearcherRevKind::OneByte { needle: needle[0] }
+            }
+        } else {
+            trace!("building scalar two-way reverse substring searcher");
+            let finder = twoway::FinderRev::new(needle);
+            SearcherRevKind::TwoWay { finder }
+        };
+        let rabinkarp = rabinkarp::FinderRev::new(needle);
+        SearcherRev { kind, rabinkarp }
+    }
+
+    /// Searches the given haystack for the last occurrence of the given
+    /// needle. The needle given should be the same as the needle that this
+    /// finder was initialized with.
+    #[inline]
+    pub(crate) fn rfind(
+        &self,
+        haystack: &[u8],
+        needle: &[u8],
+    ) -> Option<usize> {
+        if haystack.len() < needle.len() {
+            return None;
+        }
+        match self.kind {
+            SearcherRevKind::Empty => Some(haystack.len()),
+            SearcherRevKind::OneByte { needle } => {
+                crate::memrchr(needle, haystack)
+            }
+            SearcherRevKind::TwoWay { ref finder } => {
+                if rabinkarp::is_fast(haystack, needle) {
+                    self.rabinkarp.rfind(haystack, needle)
+                } else {
+                    finder.rfind(haystack, needle)
+                }
+            }
+        }
+    }
+}
+
+/// Prefilter controls whether heuristics are used to accelerate searching.
+///
+/// A prefilter refers to the idea of detecting candidate matches very quickly,
+/// and then confirming whether those candidates are full matches. This
+/// idea can be quite effective since it's often the case that looking for
+/// candidates can be a lot faster than running a complete substring search
+/// over the entire input. Namely, looking for candidates can be done with
+/// extremely fast vectorized code.
+///
+/// The downside of a prefilter is that it assumes false positives (which are
+/// candidates generated by a prefilter that aren't matches) are somewhat rare
+/// relative to the frequency of full matches. That is, if a lot of false
+/// positives are generated, then it's possible for search time to be worse
+/// than if the prefilter wasn't enabled in the first place.
+///
+/// Another downside of a prefilter is that it can result in highly variable
+/// performance, where some cases are extraordinarily fast and others aren't.
+/// Typically, variable performance isn't a problem, but it may be for your use
+/// case.
+///
+/// The use of prefilters in this implementation does use a heuristic to detect
+/// when a prefilter might not be carrying its weight, and will dynamically
+/// disable its use. Nevertheless, this configuration option gives callers
+/// the ability to disable prefilters if you have knowledge that they won't be
+/// useful.
+#[derive(Clone, Copy, Debug)]
+#[non_exhaustive]
+pub enum PrefilterConfig {
+    /// Never used a prefilter in substring search.
+    None,
+    /// Automatically detect whether a heuristic prefilter should be used. If
+    /// it is used, then heuristics will be used to dynamically disable the
+    /// prefilter if it is believed to not be carrying its weight.
+    Auto,
+}
+
+impl Default for PrefilterConfig {
+    fn default() -> PrefilterConfig {
+        PrefilterConfig::Auto
+    }
+}
+
+impl PrefilterConfig {
+    /// Returns true when this prefilter is set to the `None` variant.
+    fn is_none(&self) -> bool {
+        matches!(*self, PrefilterConfig::None)
+    }
+}
+
+/// The implementation of a prefilter.
+///
+/// This type encapsulates dispatch to one of several possible choices for a
+/// prefilter. Generally speaking, all prefilters have the same approximate
+/// algorithm: they choose a couple of bytes from the needle that are believed
+/// to be rare, use a fast vector algorithm to look for those bytes and return
+/// positions as candidates for some substring search algorithm (currently only
+/// Two-Way) to confirm as a match or not.
+///
+/// The differences between the algorithms are actually at the vector
+/// implementation level. Namely, we need different routines based on both
+/// which target architecture we're on and what CPU features are supported.
+///
+/// The straight-forwardly obvious approach here is to use an enum, and make
+/// `Prefilter::find` do case analysis to determine which algorithm was
+/// selected and invoke it. However, I've observed that this leads to poor
+/// codegen in some cases, especially in latency sensitive benchmarks. That is,
+/// this approach comes with overhead that I wasn't able to eliminate.
+///
+/// The second obvious approach is to use dynamic dispatch with traits. Doing
+/// that in this context where `Prefilter` owns the selection generally
+/// requires heap allocation, and this code is designed to run in core-only
+/// environments.
+///
+/// So we settle on using a union (that's `PrefilterKind`) and a function
+/// pointer (that's `PrefilterKindFn`). We select the right function pointer
+/// based on which field in the union we set, and that function in turn
+/// knows which field of the union to access. The downside of this approach
+/// is that it forces us to think about safety, but the upside is that
+/// there are some nice latency improvements to benchmarks. (Especially the
+/// `memmem/sliceslice/short` benchmark.)
+///
+/// In cases where we've selected a vector algorithm and the haystack given
+/// is too short, we fallback to the scalar version of `memchr` on the
+/// `rarest_byte`. (The scalar version of `memchr` is still better than a naive
+/// byte-at-a-time loop because it will read in `usize`-sized chunks at a
+/// time.)
+#[derive(Clone, Copy)]
+struct Prefilter {
+    call: PrefilterKindFn,
+    kind: PrefilterKind,
+    rarest_byte: u8,
+    rarest_offset: u8,
+}
+
+impl Prefilter {
+    /// Return a "fallback" prefilter, but only if it is believed to be
+    /// effective.
+    #[inline]
+    fn fallback<R: HeuristicFrequencyRank>(
+        ranker: R,
+        pair: Pair,
+        needle: &[u8],
+    ) -> Option<Prefilter> {
+        /// The maximum frequency rank permitted for the fallback prefilter.
+        /// If the rarest byte in the needle has a frequency rank above this
+        /// value, then no prefilter is used if the fallback prefilter would
+        /// otherwise be selected.
+        const MAX_FALLBACK_RANK: u8 = 250;
+
+        trace!("building fallback prefilter");
+        let rarest_offset = pair.index1();
+        let rarest_byte = needle[usize::from(rarest_offset)];
+        let rarest_rank = ranker.rank(rarest_byte);
+        if rarest_rank > MAX_FALLBACK_RANK {
+            None
+        } else {
+            let finder = crate::arch::all::packedpair::Finder::with_pair(
+                needle,
+                pair.clone(),
+            )?;
+            let call = prefilter_kind_fallback;
+            let kind = PrefilterKind { fallback: finder };
+            Some(Prefilter { call, kind, rarest_byte, rarest_offset })
+        }
+    }
+
+    /// Return a prefilter using a x86_64 SSE2 vector algorithm.
+    #[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+    #[inline]
+    fn sse2(finder: sse2::Finder, needle: &[u8]) -> Prefilter {
+        trace!("building x86_64 SSE2 prefilter");
+        let rarest_offset = finder.pair().index1();
+        let rarest_byte = needle[usize::from(rarest_offset)];
+        Prefilter {
+            call: prefilter_kind_sse2,
+            kind: PrefilterKind { sse2: finder },
+            rarest_byte,
+            rarest_offset,
+        }
+    }
+
+    /// Return a prefilter using a x86_64 AVX2 vector algorithm.
+    #[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+    #[inline]
+    fn avx2(finder: avx2::Finder, needle: &[u8]) -> Prefilter {
+        trace!("building x86_64 AVX2 prefilter");
+        let rarest_offset = finder.pair().index1();
+        let rarest_byte = needle[usize::from(rarest_offset)];
+        Prefilter {
+            call: prefilter_kind_avx2,
+            kind: PrefilterKind { avx2: finder },
+            rarest_byte,
+            rarest_offset,
+        }
+    }
+
+    /// Return a prefilter using a wasm32 simd128 vector algorithm.
+    #[cfg(target_arch = "wasm32")]
+    #[inline]
+    fn simd128(finder: simd128::Finder, needle: &[u8]) -> Prefilter {
+        trace!("building wasm32 simd128 prefilter");
+        let rarest_offset = finder.pair().index1();
+        let rarest_byte = needle[usize::from(rarest_offset)];
+        Prefilter {
+            call: prefilter_kind_simd128,
+            kind: PrefilterKind { simd128: finder },
+            rarest_byte,
+            rarest_offset,
+        }
+    }
+
+    /// Return a prefilter using a aarch64 neon vector algorithm.
+    #[cfg(target_arch = "aarch64")]
+    #[inline]
+    fn neon(finder: neon::Finder, needle: &[u8]) -> Prefilter {
+        trace!("building aarch64 neon prefilter");
+        let rarest_offset = finder.pair().index1();
+        let rarest_byte = needle[usize::from(rarest_offset)];
+        Prefilter {
+            call: prefilter_kind_neon,
+            kind: PrefilterKind { neon: finder },
+            rarest_byte,
+            rarest_offset,
+        }
+    }
+
+    /// Return a *candidate* position for a match.
+    ///
+    /// When this returns an offset, it implies that a match could begin at
+    /// that offset, but it may not. That is, it is possible for a false
+    /// positive to be returned.
+    ///
+    /// When `None` is returned, then it is guaranteed that there are no
+    /// matches for the needle in the given haystack. That is, it is impossible
+    /// for a false negative to be returned.
+    ///
+    /// The purpose of this routine is to look for candidate matching positions
+    /// as quickly as possible before running a (likely) slower confirmation
+    /// step.
+    #[inline]
+    fn find(&self, haystack: &[u8]) -> Option<usize> {
+        // SAFETY: By construction, we've ensured that the function in
+        // `self.call` is properly paired with the union used in `self.kind`.
+        unsafe { (self.call)(self, haystack) }
+    }
+
+    /// A "simple" prefilter that just looks for the occurrence of the rarest
+    /// byte from the needle. This is generally only used for very small
+    /// haystacks.
+    #[inline]
+    fn find_simple(&self, haystack: &[u8]) -> Option<usize> {
+        // We don't use crate::memchr here because the haystack should be small
+        // enough that memchr won't be able to use vector routines anyway. So
+        // we just skip straight to the fallback implementation which is likely
+        // faster. (A byte-at-a-time loop is only used when the haystack is
+        // smaller than `size_of::<usize>()`.)
+        crate::arch::all::memchr::One::new(self.rarest_byte)
+            .find(haystack)
+            .map(|i| i.saturating_sub(usize::from(self.rarest_offset)))
+    }
+}
+
+impl core::fmt::Debug for Prefilter {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
+        f.debug_struct("Prefilter")
+            .field("call", &"<prefilter function>")
+            .field("kind", &"<prefilter kind union>")
+            .field("rarest_byte", &self.rarest_byte)
+            .field("rarest_offset", &self.rarest_offset)
+            .finish()
+    }
+}
+
+/// A union indicating one of several possible prefilters that are in active
+/// use.
+///
+/// This union should only be read by one of the functions prefixed with
+/// `prefilter_kind_`. Namely, the correct function is meant to be paired with
+/// the union by the caller, such that the function always reads from the
+/// designated union field.
+#[derive(Clone, Copy)]
+union PrefilterKind {
+    fallback: crate::arch::all::packedpair::Finder,
+    #[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+    sse2: crate::arch::x86_64::sse2::packedpair::Finder,
+    #[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+    avx2: crate::arch::x86_64::avx2::packedpair::Finder,
+    #[cfg(target_arch = "wasm32")]
+    simd128: crate::arch::wasm32::simd128::packedpair::Finder,
+    #[cfg(target_arch = "aarch64")]
+    neon: crate::arch::aarch64::neon::packedpair::Finder,
+}
+
+/// The type of a prefilter function.
+///
+/// # Safety
+///
+/// When using a function of this type, callers must ensure that the correct
+/// function is paired with the value populated in `PrefilterKind` union.
+type PrefilterKindFn =
+    unsafe fn(strat: &Prefilter, haystack: &[u8]) -> Option<usize>;
+
+/// Reads from the `fallback` field of `PrefilterKind` to execute the fallback
+/// prefilter. Works on all platforms.
+///
+/// # Safety
+///
+/// Callers must ensure that the `strat.kind.fallback` union field is set.
+unsafe fn prefilter_kind_fallback(
+    strat: &Prefilter,
+    haystack: &[u8],
+) -> Option<usize> {
+    strat.kind.fallback.find_prefilter(haystack)
+}
+
+/// Reads from the `sse2` field of `PrefilterKind` to execute the x86_64 SSE2
+/// prefilter.
+///
+/// # Safety
+///
+/// Callers must ensure that the `strat.kind.sse2` union field is set.
+#[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+unsafe fn prefilter_kind_sse2(
+    strat: &Prefilter,
+    haystack: &[u8],
+) -> Option<usize> {
+    let finder = &strat.kind.sse2;
+    if haystack.len() < finder.min_haystack_len() {
+        strat.find_simple(haystack)
+    } else {
+        finder.find_prefilter(haystack)
+    }
+}
+
+/// Reads from the `avx2` field of `PrefilterKind` to execute the x86_64 AVX2
+/// prefilter.
+///
+/// # Safety
+///
+/// Callers must ensure that the `strat.kind.avx2` union field is set.
+#[cfg(all(target_arch = "x86_64", target_feature = "sse2"))]
+unsafe fn prefilter_kind_avx2(
+    strat: &Prefilter,
+    haystack: &[u8],
+) -> Option<usize> {
+    let finder = &strat.kind.avx2;
+    if haystack.len() < finder.min_haystack_len() {
+        strat.find_simple(haystack)
+    } else {
+        finder.find_prefilter(haystack)
+    }
+}
+
+/// Reads from the `simd128` field of `PrefilterKind` to execute the wasm32
+/// simd128 prefilter.
+///
+/// # Safety
+///
+/// Callers must ensure that the `strat.kind.simd128` union field is set.
+#[cfg(target_arch = "wasm32")]
+unsafe fn prefilter_kind_simd128(
+    strat: &Prefilter,
+    haystack: &[u8],
+) -> Option<usize> {
+    let finder = &strat.kind.simd128;
+    if haystack.len() < finder.min_haystack_len() {
+        strat.find_simple(haystack)
+    } else {
+        finder.find_prefilter(haystack)
+    }
+}
+
+/// Reads from the `neon` field of `PrefilterKind` to execute the aarch64 neon
+/// prefilter.
+///
+/// # Safety
+///
+/// Callers must ensure that the `strat.kind.neon` union field is set.
+#[cfg(target_arch = "aarch64")]
+unsafe fn prefilter_kind_neon(
+    strat: &Prefilter,
+    haystack: &[u8],
+) -> Option<usize> {
+    let finder = &strat.kind.neon;
+    if haystack.len() < finder.min_haystack_len() {
+        strat.find_simple(haystack)
+    } else {
+        finder.find_prefilter(haystack)
+    }
+}
+
+/// PrefilterState tracks state associated with the effectiveness of a
+/// prefilter. It is used to track how many bytes, on average, are skipped by
+/// the prefilter. If this average dips below a certain threshold over time,
+/// then the state renders the prefilter inert and stops using it.
+///
+/// A prefilter state should be created for each search. (Where creating an
+/// iterator is treated as a single search.) A prefilter state should only be
+/// created from a `Freqy`. e.g., An inert `Freqy` will produce an inert
+/// `PrefilterState`.
+#[derive(Clone, Copy, Debug)]
+pub(crate) struct PrefilterState {
+    /// The number of skips that has been executed. This is always 1 greater
+    /// than the actual number of skips. The special sentinel value of 0
+    /// indicates that the prefilter is inert. This is useful to avoid
+    /// additional checks to determine whether the prefilter is still
+    /// "effective." Once a prefilter becomes inert, it should no longer be
+    /// used (according to our heuristics).
+    skips: u32,
+    /// The total number of bytes that have been skipped.
+    skipped: u32,
+}
+
+impl PrefilterState {
+    /// The minimum number of skip attempts to try before considering whether
+    /// a prefilter is effective or not.
+    const MIN_SKIPS: u32 = 50;
+
+    /// The minimum amount of bytes that skipping must average.
+    ///
+    /// This value was chosen based on varying it and checking
+    /// the microbenchmarks. In particular, this can impact the
+    /// pathological/repeated-{huge,small} benchmarks quite a bit if it's set
+    /// too low.
+    const MIN_SKIP_BYTES: u32 = 8;
+
+    /// Create a fresh prefilter state.
+    #[inline]
+    pub(crate) fn new() -> PrefilterState {
+        PrefilterState { skips: 1, skipped: 0 }
+    }
+
+    /// Update this state with the number of bytes skipped on the last
+    /// invocation of the prefilter.
+    #[inline]
+    fn update(&mut self, skipped: usize) {
+        self.skips = self.skips.saturating_add(1);
+        // We need to do this dance since it's technically possible for
+        // `skipped` to overflow a `u32`. (And we use a `u32` to reduce the
+        // size of a prefilter state.)
+        self.skipped = match u32::try_from(skipped) {
+            Err(_) => core::u32::MAX,
+            Ok(skipped) => self.skipped.saturating_add(skipped),
+        };
+    }
+
+    /// Return true if and only if this state indicates that a prefilter is
+    /// still effective.
+    #[inline]
+    fn is_effective(&mut self) -> bool {
+        if self.is_inert() {
+            return false;
+        }
+        if self.skips() < PrefilterState::MIN_SKIPS {
+            return true;
+        }
+        if self.skipped >= PrefilterState::MIN_SKIP_BYTES * self.skips() {
+            return true;
+        }
+
+        // We're inert.
+        self.skips = 0;
+        false
+    }
+
+    /// Returns true if the prefilter this state represents should no longer
+    /// be used.
+    #[inline]
+    fn is_inert(&self) -> bool {
+        self.skips == 0
+    }
+
+    /// Returns the total number of times the prefilter has been used.
+    #[inline]
+    fn skips(&self) -> u32 {
+        // Remember, `0` is a sentinel value indicating inertness, so we
+        // always need to subtract `1` to get our actual number of skips.
+        self.skips.saturating_sub(1)
+    }
+}
+
+/// A combination of prefilter effectiveness state and the prefilter itself.
+#[derive(Debug)]
+pub(crate) struct Pre<'a> {
+    /// State that tracks the effectiveness of a prefilter.
+    prestate: &'a mut PrefilterState,
+    /// The actual prefilter.
+    prestrat: &'a Prefilter,
+}
+
+impl<'a> Pre<'a> {
+    /// Call this prefilter on the given haystack with the given needle.
+    #[inline]
+    pub(crate) fn find(&mut self, haystack: &[u8]) -> Option<usize> {
+        let result = self.prestrat.find(haystack);
+        self.prestate.update(result.unwrap_or(haystack.len()));
+        result
+    }
+
+    /// Return true if and only if this prefilter should be used.
+    #[inline]
+    pub(crate) fn is_effective(&mut self) -> bool {
+        self.prestate.is_effective()
+    }
+}
+
+/// Returns true if the needle has the right characteristics for a vector
+/// algorithm to handle the entirety of substring search.
+///
+/// Vector algorithms can be used for prefilters for other substring search
+/// algorithms (like Two-Way), but they can also be used for substring search
+/// on their own. When used for substring search, vector algorithms will
+/// quickly identify candidate match positions (just like in the prefilter
+/// case), but instead of returning the candidate position they will try to
+/// confirm the match themselves. Confirmation happens via `memcmp`. This
+/// works well for short needles, but can break down when many false candidate
+/// positions are generated for large needles. Thus, we only permit vector
+/// algorithms to own substring search when the needle is of a certain length.
+#[inline]
+fn do_packed_search(needle: &[u8]) -> bool {
+    /// The minimum length of a needle required for this algorithm. The minimum
+    /// is 2 since a length of 1 should just use memchr and a length of 0 isn't
+    /// a case handled by this searcher.
+    const MIN_LEN: usize = 2;
+
+    /// The maximum length of a needle required for this algorithm.
+    ///
+    /// In reality, there is no hard max here. The code below can handle any
+    /// length needle. (Perhaps that suggests there are missing optimizations.)
+    /// Instead, this is a heuristic and a bound guaranteeing our linear time
+    /// complexity.
+    ///
+    /// It is a heuristic because when a candidate match is found, memcmp is
+    /// run. For very large needles with lots of false positives, memcmp can
+    /// make the code run quite slow.
+    ///
+    /// It is a bound because the worst case behavior with memcmp is
+    /// multiplicative in the size of the needle and haystack, and we want
+    /// to keep that additive. This bound ensures we still meet that bound
+    /// theoretically, since it's just a constant. We aren't acting in bad
+    /// faith here, memcmp on tiny needles is so fast that even in pathological
+    /// cases (see pathological vector benchmarks), this is still just as fast
+    /// or faster in practice.
+    ///
+    /// This specific number was chosen by tweaking a bit and running
+    /// benchmarks. The rare-medium-needle, for example, gets about 5% faster
+    /// by using this algorithm instead of a prefilter-accelerated Two-Way.
+    /// There's also a theoretical desire to keep this number reasonably
+    /// low, to mitigate the impact of pathological cases. I did try 64, and
+    /// some benchmarks got a little better, and others (particularly the
+    /// pathological ones), got a lot worse. So... 32 it is?
+    const MAX_LEN: usize = 32;
+    MIN_LEN <= needle.len() && needle.len() <= MAX_LEN
+}