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diff --git a/src/arch/x86_64/avx2/packedpair.rs b/src/arch/x86_64/avx2/packedpair.rs
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+/*!
+A 256-bit vector implementation of the "packed pair" SIMD 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.
+
+[generic SIMD]: http://0x80.pl/articles/simd-strfind.html#first-and-last
+*/
+
+use core::arch::x86_64::{__m128i, __m256i};
+
+use crate::arch::{all::packedpair::Pair, generic::packedpair};
+
+/// A "packed pair" finder that uses 256-bit vector operations.
+///
+/// This finder picks two bytes that it believes have high predictive power
+/// for indicating an overall match of a needle. Depending on whether
+/// `Finder::find` or `Finder::find_prefilter` is used, it reports offsets
+/// where the needle matches or could match. In the prefilter case, candidates
+/// are reported whenever the [`Pair`] of bytes given matches.
+#[derive(Clone, Copy, Debug)]
+pub struct Finder {
+    sse2: packedpair::Finder<__m128i>,
+    avx2: packedpair::Finder<__m256i>,
+}
+
+impl Finder {
+    /// Create a new pair searcher. The searcher returned can either report
+    /// exact matches of `needle` or act as a prefilter and report candidate
+    /// positions of `needle`.
+    ///
+    /// If AVX2 is unavailable in the current environment or if a [`Pair`]
+    /// could not be constructed from the needle given, then `None` is
+    /// returned.
+    #[inline]
+    pub fn new(needle: &[u8]) -> Option<Finder> {
+        Finder::with_pair(needle, Pair::new(needle)?)
+    }
+
+    /// Create a new "packed pair" finder using the pair of bytes given.
+    ///
+    /// This constructor permits callers to control precisely which pair of
+    /// bytes is used as a predicate.
+    ///
+    /// If AVX2 is unavailable in the current environment, then `None` is
+    /// returned.
+    #[inline]
+    pub fn with_pair(needle: &[u8], pair: Pair) -> Option<Finder> {
+        if Finder::is_available() {
+            // SAFETY: we check that sse2/avx2 is available above. We are also
+            // guaranteed to have needle.len() > 1 because we have a valid
+            // Pair.
+            unsafe { Some(Finder::with_pair_impl(needle, pair)) }
+        } else {
+            None
+        }
+    }
+
+    /// Create a new `Finder` specific to SSE2 vectors and routines.
+    ///
+    /// # Safety
+    ///
+    /// Same as the safety for `packedpair::Finder::new`, and callers must also
+    /// ensure that both SSE2 and AVX2 are available.
+    #[target_feature(enable = "sse2", enable = "avx2")]
+    #[inline]
+    unsafe fn with_pair_impl(needle: &[u8], pair: Pair) -> Finder {
+        let sse2 = packedpair::Finder::<__m128i>::new(needle, pair);
+        let avx2 = packedpair::Finder::<__m256i>::new(needle, pair);
+        Finder { sse2, avx2 }
+    }
+
+    /// Returns true when this implementation is available in the current
+    /// environment.
+    ///
+    /// When this is true, it is guaranteed that [`Finder::with_pair`] will
+    /// return a `Some` value. Similarly, when it is false, it is guaranteed
+    /// that `Finder::with_pair` will return a `None` value. Notice that this
+    /// does not guarantee that [`Finder::new`] will return a `Finder`. Namely,
+    /// even when `Finder::is_available` is true, it is not guaranteed that a
+    /// valid [`Pair`] can be found from the needle given.
+    ///
+    /// Note also that for the lifetime of a single program, if this returns
+    /// true then it will always return true.
+    #[inline]
+    pub fn is_available() -> bool {
+        #[cfg(not(target_feature = "sse2"))]
+        {
+            false
+        }
+        #[cfg(target_feature = "sse2")]
+        {
+            #[cfg(target_feature = "avx2")]
+            {
+                true
+            }
+            #[cfg(not(target_feature = "avx2"))]
+            {
+                #[cfg(feature = "std")]
+                {
+                    std::is_x86_feature_detected!("avx2")
+                }
+                #[cfg(not(feature = "std"))]
+                {
+                    false
+                }
+            }
+        }
+    }
+
+    /// Execute a search using AVX2 vectors and routines.
+    ///
+    /// # Panics
+    ///
+    /// When `haystack.len()` is less than [`Finder::min_haystack_len`].
+    #[inline]
+    pub fn find(&self, haystack: &[u8], needle: &[u8]) -> Option<usize> {
+        // SAFETY: Building a `Finder` means it's safe to call 'sse2' routines.
+        unsafe { self.find_impl(haystack, needle) }
+    }
+
+    /// 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.
+    ///
+    /// # Panics
+    ///
+    /// When `haystack.len()` is less than [`Finder::min_haystack_len`].
+    #[inline]
+    pub fn find_prefilter(&self, haystack: &[u8]) -> Option<usize> {
+        // SAFETY: Building a `Finder` means it's safe to call 'sse2' routines.
+        unsafe { self.find_prefilter_impl(haystack) }
+    }
+
+    /// Execute a search using AVX2 vectors and routines.
+    ///
+    /// # Panics
+    ///
+    /// When `haystack.len()` is less than [`Finder::min_haystack_len`].
+    ///
+    /// # Safety
+    ///
+    /// (The target feature safety obligation is automatically fulfilled by
+    /// virtue of being a method on `Finder`, which can only be constructed
+    /// when it is safe to call `sse2` and `avx2` routines.)
+    #[target_feature(enable = "sse2", enable = "avx2")]
+    #[inline]
+    unsafe fn find_impl(
+        &self,
+        haystack: &[u8],
+        needle: &[u8],
+    ) -> Option<usize> {
+        if haystack.len() < self.avx2.min_haystack_len() {
+            self.sse2.find(haystack, needle)
+        } else {
+            self.avx2.find(haystack, needle)
+        }
+    }
+
+    /// Execute a prefilter search using AVX2 vectors and routines.
+    ///
+    /// # Panics
+    ///
+    /// When `haystack.len()` is less than [`Finder::min_haystack_len`].
+    ///
+    /// # Safety
+    ///
+    /// (The target feature safety obligation is automatically fulfilled by
+    /// virtue of being a method on `Finder`, which can only be constructed
+    /// when it is safe to call `sse2` and `avx2` routines.)
+    #[target_feature(enable = "sse2", enable = "avx2")]
+    #[inline]
+    unsafe fn find_prefilter_impl(&self, haystack: &[u8]) -> Option<usize> {
+        if haystack.len() < self.avx2.min_haystack_len() {
+            self.sse2.find_prefilter(haystack)
+        } else {
+            self.avx2.find_prefilter(haystack)
+        }
+    }
+
+    /// 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.avx2.pair()
+    }
+
+    /// Returns the minimum haystack length that this `Finder` can search.
+    ///
+    /// Using a haystack with length smaller than this in a search will result
+    /// in a panic. The reason for this restriction is that this finder is
+    /// meant to be a low-level component that is part of a larger substring
+    /// strategy. In that sense, it avoids trying to handle all cases and
+    /// instead only handles the cases that it can handle very well.
+    #[inline]
+    pub fn min_haystack_len(&self) -> usize {
+        // The caller doesn't need to care about AVX2's min_haystack_len
+        // since this implementation will automatically switch to the SSE2
+        // implementation if the haystack is too short for AVX2. Therefore, the
+        // caller only needs to care about SSE2's min_haystack_len.
+        //
+        // This does assume that SSE2's min_haystack_len is less than or
+        // equal to AVX2's min_haystack_len. In practice, this is true and
+        // there is no way it could be false based on how this Finder is
+        // implemented. Namely, both SSE2 and AVX2 use the same `Pair`. If
+        // they used different pairs, then it's possible (although perhaps
+        // pathological) for SSE2's min_haystack_len to be bigger than AVX2's.
+        self.sse2.min_haystack_len()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn find(haystack: &[u8], needle: &[u8]) -> Option<Option<usize>> {
+        let f = Finder::new(needle)?;
+        if haystack.len() < f.min_haystack_len() {
+            return None;
+        }
+        Some(f.find(haystack, needle))
+    }
+
+    define_substring_forward_quickcheck!(find);
+
+    #[test]
+    fn forward_substring() {
+        crate::tests::substring::Runner::new().fwd(find).run()
+    }
+
+    #[test]
+    fn forward_packedpair() {
+        fn find(
+            haystack: &[u8],
+            needle: &[u8],
+            index1: u8,
+            index2: u8,
+        ) -> Option<Option<usize>> {
+            let pair = Pair::with_indices(needle, index1, index2)?;
+            let f = Finder::with_pair(needle, pair)?;
+            if haystack.len() < f.min_haystack_len() {
+                return None;
+            }
+            Some(f.find(haystack, needle))
+        }
+        crate::tests::packedpair::Runner::new().fwd(find).run()
+    }
+
+    #[test]
+    fn forward_packedpair_prefilter() {
+        fn find(
+            haystack: &[u8],
+            needle: &[u8],
+            index1: u8,
+            index2: u8,
+        ) -> Option<Option<usize>> {
+            if !cfg!(target_feature = "sse2") {
+                return None;
+            }
+            let pair = Pair::with_indices(needle, index1, index2)?;
+            let f = Finder::with_pair(needle, pair)?;
+            if haystack.len() < f.min_haystack_len() {
+                return None;
+            }
+            Some(f.find_prefilter(haystack))
+        }
+        crate::tests::packedpair::Runner::new().fwd(find).run()
+    }
+}