Reorganize everything

4 files changed, 948 insertions, 0 deletions

diff --git a/asl/hashing/BUILD.bazel b/asl/hashing/BUILD.bazel
new file mode 100644
index 0000000..6b9b410
--- /dev/null
+++ b/asl/hashing/BUILD.bazel
@@ -0,0 +1,33 @@
+cc_library(
+    name = "hashing",
+    hdrs = [
+        "hash.hpp",
+    ],
+    srcs = [
+        "hash_cityhash.cpp",
+    ],
+    deps = [
+        "//asl/base",
+        "//asl/types:span",
+    ],
+    visibility = ["//visibility:public"],
+)
+
+cc_test(
+    name = "tests",
+    srcs = [
+        "hash_tests.cpp",
+    ],
+    deps = [
+        "//asl/base",
+        "//asl/tests:utils",
+        "//asl/testing",
+        ":hashing",
+        "//asl/strings:string_view",
+        "//asl/strings:string",
+        "//asl/containers:buffer",
+        "//asl/types:box",
+        "//asl/types:option",
+        "//asl/types:status",
+    ],
+)
diff --git a/asl/hashing/hash.hpp b/asl/hashing/hash.hpp
new file mode 100644
index 0000000..60550d0
--- /dev/null
+++ b/asl/hashing/hash.hpp
@@ -0,0 +1,138 @@
+#pragma once
+
+#include "asl/base/integers.hpp"
+#include "asl/base/meta.hpp"
+#include "asl/types/span.hpp"
+#include "asl/base/utility.hpp"
+
+namespace asl::city_hash
+{
+
+// Hash function for a byte array.
+uint64_t CityHash64(const char *s, size_t len);
+
+// Hash function for a byte array.  For convenience, a 64-bit seed is also
+// hashed into the result.
+uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t seed);
+
+// Hash function for a byte array.  For convenience, two seeds are also
+// hashed into the result.
+uint64_t CityHash64WithSeeds(const char *s, size_t len,
+                           uint64_t seed0, uint64_t seed1);
+
+// Hash function for a byte array.
+uint128_t CityHash128(const char *s, size_t len);
+
+// Hash function for a byte array.  For convenience, a 128-bit seed is also
+// hashed into the result.
+uint128_t CityHash128WithSeed(const char *s, size_t len, uint128_t seed);
+
+// Hash function for a byte array.  Most useful in 32-bit binaries.
+uint32_t CityHash32(const char *s, size_t len);
+
+// Hash 128 input bits down to 64 bits of output.
+// This is intended to be a reasonably good hash function.
+constexpr uint64_t Hash128to64(uint64_t high, uint64_t low)
+{
+  // Murmur-inspired hashing.
+  const uint64_t kMul = 0x9ddfea08eb382d69ULL;
+  uint64_t a = (low ^ high) * kMul;
+  a ^= (a >> 47);
+  uint64_t b = (high ^ a) * kMul;
+  b ^= (b >> 47);
+  b *= kMul;
+  return b;
+}
+
+// Hash 128 input bits down to 64 bits of output.
+// This is intended to be a reasonably good hash function.
+constexpr uint64_t Hash128to64(const uint128_t& x)
+{
+  return Hash128to64(x.high, x.low);
+}
+
+} // namespace asl::city_hash
+
+namespace asl
+{
+
+template<typename T, typename H>
+concept hashable_generic = requires(const T& value, H h)
+{
+    { AslHashValue(h, value) } -> same_as<H>;
+};
+
+struct HashState
+{
+    uint128_t state{};
+
+    constexpr HashState() = default;
+    explicit constexpr HashState(uint128_t s) : state{s} {}
+
+    template<typename T>
+    static HashState combine_contiguous(HashState h, span<const T> s)
+    {
+        if constexpr (uniquely_represented<T>)
+        {
+            auto bytes = as_bytes(s);
+            auto hashed = city_hash::CityHash128WithSeed(
+                reinterpret_cast<const char*>(bytes.data()),
+                static_cast<size_t>(bytes.size()),
+                h.state);
+            return HashState{hashed};
+        }
+        else
+        {
+            for (const auto& value: s)
+            {
+                h = AslHashValue(ASL_MOVE(h), value);
+            }
+            return h;
+        }
+    }
+
+    static constexpr HashState combine(HashState h)
+    {
+        return h;
+    }
+
+    template<hashable_generic<HashState> Arg, hashable_generic<HashState>... Remaining>
+    static constexpr HashState combine(HashState h, const Arg& arg, const Remaining&... remaining)
+    {
+        return combine(AslHashValue(ASL_MOVE(h), arg), remaining...);
+    }
+};
+
+template<typename T>
+concept hashable = hashable_generic<T, HashState>;
+
+template<typename H, uniquely_represented T>
+constexpr H AslHashValue(H h, const T& value)
+{
+    return H::combine_contiguous(ASL_MOVE(h), span<const T>{&value, 1});
+}
+
+template<typename H>
+constexpr H AslHashValue(H h, bool value)
+{
+    return AslHashValue(ASL_MOVE(h), value ? 1 : 0);
+}
+
+template<typename H, typename T>
+constexpr void AslHashValue(H h, T*); // Don't hash pointers
+
+template<typename H, hashable T>
+constexpr H AslHashValue(H h, const span<T>& s)
+{
+    return H::combine_contiguous(ASL_MOVE(h), span<const T>{s.data(), s.size()});
+}
+
+template<hashable T>
+constexpr uint64_t hash_value(const T& value)
+{
+    auto result = AslHashValue(HashState{}, value).state;
+    return city_hash::Hash128to64(result);
+}
+
+} // namespace asl
+
diff --git a/asl/hashing/hash_cityhash.cpp b/asl/hashing/hash_cityhash.cpp
new file mode 100644
index 0000000..9748344
--- /dev/null
+++ b/asl/hashing/hash_cityhash.cpp
@@ -0,0 +1,517 @@
+// Copyright (c) 2011 Google, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+// CityHash, by Geoff Pike and Jyrki Alakuijala
+//
+// This file provides CityHash64() and related functions.
+//
+// It's probably possible to create even faster hash functions by
+// writing a program that systematically explores some of the space of
+// possible hash functions, by using SIMD instructions, or by
+// compromising on hash quality.
+
+#include "asl/hashing/hash.hpp"
+#include "asl/memory/memory.hpp"
+
+using uint8 = uint8_t;
+using uint32 = uint32_t;
+using uint64 = uint64_t;
+using uint128 = uint128_t;
+
+// NOLINTBEGIN
+
+constexpr uint64 UNALIGNED_LOAD64(const char *p) {
+  uint64 result;
+  asl::memcpy(&result, p, sizeof(result));
+  return result;
+}
+
+constexpr uint32 UNALIGNED_LOAD32(const char *p) {
+  uint32 result;
+  asl::memcpy(&result, p, sizeof(result));
+  return result;
+}
+
+#ifdef _MSC_VER
+
+#include <stdlib.h>
+#define bswap_32(x) _byteswap_ulong(x)
+#define bswap_64(x) _byteswap_uint64(x)
+
+#elif defined(__APPLE__)
+
+// Mac OS X / Darwin features
+#include <libkern/OSByteOrder.h>
+#define bswap_32(x) OSSwapInt32(x)
+#define bswap_64(x) OSSwapInt64(x)
+
+#elif defined(__sun) || defined(sun)
+
+#include <sys/byteorder.h>
+#define bswap_32(x) BSWAP_32(x)
+#define bswap_64(x) BSWAP_64(x)
+
+#elif defined(__FreeBSD__)
+
+#include <sys/endian.h>
+#define bswap_32(x) bswap32(x)
+#define bswap_64(x) bswap64(x)
+
+#elif defined(__OpenBSD__)
+
+#include <sys/types.h>
+#define bswap_32(x) swap32(x)
+#define bswap_64(x) swap64(x)
+
+#elif defined(__NetBSD__)
+
+#include <sys/types.h>
+#include <machine/bswap.h>
+#if defined(__BSWAP_RENAME) && !defined(__bswap_32)
+#define bswap_32(x) bswap32(x)
+#define bswap_64(x) bswap64(x)
+#endif
+
+#else
+
+#include <byteswap.h>
+
+#endif
+
+#ifdef WORDS_BIGENDIAN
+#define uint32_in_expected_order(x) (bswap_32(x))
+#define uint64_in_expected_order(x) (bswap_64(x))
+#else
+#define uint32_in_expected_order(x) (x)
+#define uint64_in_expected_order(x) (x)
+#endif
+
+#if !defined(LIKELY)
+#if __has_builtin(__builtin_expect)
+#define LIKELY(x) (__builtin_expect(!!(x), 1))
+#else
+#define LIKELY(x) (x)
+#endif
+#endif
+
+static uint64 Fetch64(const char *p) {
+  return uint64_in_expected_order(UNALIGNED_LOAD64(p));
+}
+
+static uint32 Fetch32(const char *p) {
+  return uint32_in_expected_order(UNALIGNED_LOAD32(p));
+}
+
+// Some primes between 2^63 and 2^64 for various uses.
+static const uint64 k0 = 0xc3a5c85c97cb3127ULL;
+static const uint64 k1 = 0xb492b66fbe98f273ULL;
+static const uint64 k2 = 0x9ae16a3b2f90404fULL;
+
+// Magic numbers for 32-bit hashing.  Copied from Murmur3.
+static const uint32 c1 = 0xcc9e2d51;
+static const uint32 c2 = 0x1b873593;
+
+// A 32-bit to 32-bit integer hash copied from Murmur3.
+static uint32 fmix(uint32 h)
+{
+  h ^= h >> 16;
+  h *= 0x85ebca6b;
+  h ^= h >> 13;
+  h *= 0xc2b2ae35;
+  h ^= h >> 16;
+  return h;
+}
+
+static uint32 Rotate32(uint32 val, int shift) {
+  // Avoid shifting by 32: doing so yields an undefined result.
+  return shift == 0 ? val : ((val >> shift) | (val << (32 - shift)));
+}
+
+#undef PERMUTE3
+#define PERMUTE3(a, b, c) do { asl::swap(a, b); asl::swap(a, c); } while (0)
+
+static uint32 Mur(uint32 a, uint32 h) {
+  // Helper from Murmur3 for combining two 32-bit values.
+  a *= c1;
+  a = Rotate32(a, 17);
+  a *= c2;
+  h ^= a;
+  h = Rotate32(h, 19);
+  return h * 5 + 0xe6546b64;
+}
+
+static uint32 Hash32Len13to24(const char *s, size_t len) {
+  uint32 a = Fetch32(s - 4 + (len >> 1));
+  uint32 b = Fetch32(s + 4);
+  uint32 c = Fetch32(s + len - 8);
+  uint32 d = Fetch32(s + (len >> 1));
+  uint32 e = Fetch32(s);
+  uint32 f = Fetch32(s + len - 4);
+  uint32 h = static_cast<uint32>(len);
+
+  return fmix(Mur(f, Mur(e, Mur(d, Mur(c, Mur(b, Mur(a, h)))))));
+}
+
+static uint32 Hash32Len0to4(const char *s, size_t len) {
+  uint32 b = 0;
+  uint32 c = 9;
+  for (size_t i = 0; i < len; i++) {
+    signed char v = static_cast<signed char>(s[i]);
+    b = b * c1 + static_cast<uint32>(v);
+    c ^= b;
+  }
+  return fmix(Mur(b, Mur(static_cast<uint32>(len), c)));
+}
+
+static uint32 Hash32Len5to12(const char *s, size_t len) {
+  uint32 a = static_cast<uint32>(len), b = a * 5, c = 9, d = b;
+  a += Fetch32(s);
+  b += Fetch32(s + len - 4);
+  c += Fetch32(s + ((len >> 1) & 4));
+  return fmix(Mur(c, Mur(b, Mur(a, d))));
+}
+
+uint32 asl::city_hash::CityHash32(const char *s, size_t len) {
+  if (len <= 24) {
+    return len <= 12 ?
+        (len <= 4 ? Hash32Len0to4(s, len) : Hash32Len5to12(s, len)) :
+        Hash32Len13to24(s, len);
+  }
+
+  // len > 24
+  uint32 h = static_cast<uint32>(len), g = c1 * h, f = g;
+  uint32 a0 = Rotate32(Fetch32(s + len - 4) * c1, 17) * c2;
+  uint32 a1 = Rotate32(Fetch32(s + len - 8) * c1, 17) * c2;
+  uint32 a2 = Rotate32(Fetch32(s + len - 16) * c1, 17) * c2;
+  uint32 a3 = Rotate32(Fetch32(s + len - 12) * c1, 17) * c2;
+  uint32 a4 = Rotate32(Fetch32(s + len - 20) * c1, 17) * c2;
+  h ^= a0;
+  h = Rotate32(h, 19);
+  h = h * 5 + 0xe6546b64;
+  h ^= a2;
+  h = Rotate32(h, 19);
+  h = h * 5 + 0xe6546b64;
+  g ^= a1;
+  g = Rotate32(g, 19);
+  g = g * 5 + 0xe6546b64;
+  g ^= a3;
+  g = Rotate32(g, 19);
+  g = g * 5 + 0xe6546b64;
+  f += a4;
+  f = Rotate32(f, 19);
+  f = f * 5 + 0xe6546b64;
+  size_t iters = (len - 1) / 20;
+  do {
+    uint32 a0_ = Rotate32(Fetch32(s) * c1, 17) * c2;
+    uint32 a1_ = Fetch32(s + 4);
+    uint32 a2_ = Rotate32(Fetch32(s + 8) * c1, 17) * c2;
+    uint32 a3_ = Rotate32(Fetch32(s + 12) * c1, 17) * c2;
+    uint32 a4_ = Fetch32(s + 16);
+    h ^= a0_;
+    h = Rotate32(h, 18);
+    h = h * 5 + 0xe6546b64;
+    f += a1_;
+    f = Rotate32(f, 19);
+    f = f * c1;
+    g += a2_;
+    g = Rotate32(g, 18);
+    g = g * 5 + 0xe6546b64;
+    h ^= a3_ + a1_;
+    h = Rotate32(h, 19);
+    h = h * 5 + 0xe6546b64;
+    g ^= a4_;
+    g = bswap_32(g) * 5;
+    h += a4_ * 5;
+    h = bswap_32(h);
+    f += a0_;
+    PERMUTE3(f, h, g);
+    s += 20;
+  } while (--iters != 0);
+  g = Rotate32(g, 11) * c1;
+  g = Rotate32(g, 17) * c1;
+  f = Rotate32(f, 11) * c1;
+  f = Rotate32(f, 17) * c1;
+  h = Rotate32(h + g, 19);
+  h = h * 5 + 0xe6546b64;
+  h = Rotate32(h, 17) * c1;
+  h = Rotate32(h + f, 19);
+  h = h * 5 + 0xe6546b64;
+  h = Rotate32(h, 17) * c1;
+  return h;
+}
+
+// Bitwise right rotate.  Normally this will compile to a single
+// instruction, especially if the shift is a manifest constant.
+static uint64 Rotate(uint64 val, int shift) {
+  // Avoid shifting by 64: doing so yields an undefined result.
+  return shift == 0 ? val : ((val >> shift) | (val << (64 - shift)));
+}
+
+static uint64 ShiftMix(uint64 val) {
+  return val ^ (val >> 47);
+}
+
+static uint64 HashLen16(uint64 u, uint64 v) {
+  return asl::city_hash::Hash128to64(uint128{u, v});
+}
+
+static uint64 HashLen16(uint64 u, uint64 v, uint64 mul) {
+  // Murmur-inspired hashing.
+  uint64 a = (u ^ v) * mul;
+  a ^= (a >> 47);
+  uint64 b = (v ^ a) * mul;
+  b ^= (b >> 47);
+  b *= mul;
+  return b;
+}
+
+static uint64 HashLen0to16(const char *s, size_t len) {
+  if (len >= 8) {
+    uint64 mul = k2 + len * 2;
+    uint64 a = Fetch64(s) + k2;
+    uint64 b = Fetch64(s + len - 8);
+    uint64 c = Rotate(b, 37) * mul + a;
+    uint64 d = (Rotate(a, 25) + b) * mul;
+    return HashLen16(c, d, mul);
+  }
+  if (len >= 4) {
+    uint64 mul = k2 + len * 2;
+    uint64 a = Fetch32(s);
+    return HashLen16(len + (a << 3), Fetch32(s + len - 4), mul);
+  }
+  if (len > 0) {
+    uint8 a = static_cast<uint8>(s[0]);
+    uint8 b = static_cast<uint8>(s[len >> 1]);
+    uint8 c = static_cast<uint8>(s[len - 1]);
+    uint32 y = static_cast<uint32>(a) + (static_cast<uint32>(b) << 8);
+    uint32 z = static_cast<uint32>(len) + (static_cast<uint32>(c) << 2);
+    return ShiftMix(y * k2 ^ z * k0) * k2;
+  }
+  return k2;
+}
+
+// This probably works well for 16-byte strings as well, but it may be overkill
+// in that case.
+static uint64 HashLen17to32(const char *s, size_t len) {
+  uint64 mul = k2 + len * 2;
+  uint64 a = Fetch64(s) * k1;
+  uint64 b = Fetch64(s + 8);
+  uint64 c = Fetch64(s + len - 8) * mul;
+  uint64 d = Fetch64(s + len - 16) * k2;
+  return HashLen16(Rotate(a + b, 43) + Rotate(c, 30) + d,
+                   a + Rotate(b + k2, 18) + c, mul);
+}
+
+// Return a 16-byte hash for 48 bytes.  Quick and dirty.
+// Callers do best to use "random-looking" values for a and b.
+static uint128 WeakHashLen32WithSeeds(
+    uint64 w, uint64 x, uint64 y, uint64 z, uint64 a, uint64 b) {
+  a += w;
+  b = Rotate(b + a + z, 21);
+  uint64 c = a;
+  a += x;
+  a += y;
+  b += Rotate(a, 44);
+  return {a + z, b + c};
+}
+
+// Return a 16-byte hash for s[0] ... s[31], a, and b.  Quick and dirty.
+static uint128 WeakHashLen32WithSeeds(
+    const char* s, uint64 a, uint64 b) {
+  return WeakHashLen32WithSeeds(Fetch64(s),
+                                Fetch64(s + 8),
+                                Fetch64(s + 16),
+                                Fetch64(s + 24),
+                                a,
+                                b);
+}
+
+// Return an 8-byte hash for 33 to 64 bytes.
+static uint64 HashLen33to64(const char *s, size_t len) {
+  uint64 mul = k2 + len * 2;
+  uint64 a = Fetch64(s) * k2;
+  uint64 b = Fetch64(s + 8);
+  uint64 c = Fetch64(s + len - 24);
+  uint64 d = Fetch64(s + len - 32);
+  uint64 e = Fetch64(s + 16) * k2;
+  uint64 f = Fetch64(s + 24) * 9;
+  uint64 g = Fetch64(s + len - 8);
+  uint64 h = Fetch64(s + len - 16) * mul;
+  uint64 u = Rotate(a + g, 43) + (Rotate(b, 30) + c) * 9;
+  uint64 v = ((a + g) ^ d) + f + 1;
+  uint64 w = bswap_64((u + v) * mul) + h;
+  uint64 x = Rotate(e + f, 42) + c;
+  uint64 y = (bswap_64((v + w) * mul) + g) * mul;
+  uint64 z = e + f + c;
+  a = bswap_64((x + z) * mul + y) + b;
+  b = ShiftMix((z + a) * mul + d + h) * mul;
+  return b + x;
+}
+
+uint64 asl::city_hash::CityHash64(const char *s, size_t len) {
+  if (len <= 32) {
+    if (len <= 16) {
+      return HashLen0to16(s, len);
+    } else {
+      return HashLen17to32(s, len);
+    }
+  } else if (len <= 64) {
+    return HashLen33to64(s, len);
+  }
+
+  // For strings over 64 bytes we hash the end first, and then as we
+  // loop we keep 56 bytes of state: v, w, x, y, and z.
+  uint64 x = Fetch64(s + len - 40);
+  uint64 y = Fetch64(s + len - 16) + Fetch64(s + len - 56);
+  uint64 z = HashLen16(Fetch64(s + len - 48) + len, Fetch64(s + len - 24));
+  uint128 v = WeakHashLen32WithSeeds(s + len - 64, len, z);
+  uint128 w = WeakHashLen32WithSeeds(s + len - 32, y + k1, x);
+  x = x * k1 + Fetch64(s);
+
+  // Decrease len to the nearest multiple of 64, and operate on 64-byte chunks.
+  len = (len - 1) & ~static_cast<size_t>(63);
+  do {
+    x = Rotate(x + y + v.high + Fetch64(s + 8), 37) * k1;
+    y = Rotate(y + v.low + Fetch64(s + 48), 42) * k1;
+    x ^= w.low;
+    y += v.high + Fetch64(s + 40);
+    z = Rotate(z + w.high, 33) * k1;
+    v = WeakHashLen32WithSeeds(s, v.low * k1, x + w.high);
+    w = WeakHashLen32WithSeeds(s + 32, z + w.low, y + Fetch64(s + 16));
+    asl::swap(z, x);
+    s += 64;
+    len -= 64;
+  } while (len != 0);
+  return HashLen16(HashLen16(v.high, w.high) + ShiftMix(y) * k1 + z,
+                   HashLen16(v.low, w.low) + x);
+}
+
+uint64 asl::city_hash::CityHash64WithSeed(const char *s, size_t len, uint64 seed) {
+  return CityHash64WithSeeds(s, len, k2, seed);
+}
+
+uint64 asl::city_hash::CityHash64WithSeeds(const char *s, size_t len,
+                           uint64 seed0, uint64 seed1) {
+  return HashLen16(CityHash64(s, len) - seed0, seed1);
+}
+
+// A subroutine for CityHash128().  Returns a decent 128-bit hash for strings
+// of any length representable in signed long.  Based on City and Murmur.
+static uint128 CityMurmur(const char *s, size_t len, uint128 seed) {
+  uint64 a = seed.low;
+  uint64 b = seed.high;
+  uint64 c = 0;
+  uint64 d = 0;
+  if (len <= 16) {
+    a = ShiftMix(a * k1) * k1;
+    c = b * k1 + HashLen0to16(s, len);
+    d = ShiftMix(a + (len >= 8 ? Fetch64(s) : c));
+  } else {
+    c = HashLen16(Fetch64(s + len - 8) + k1, a);
+    d = HashLen16(b + len, c + Fetch64(s + len - 16));
+    a += d;
+    // len > 16 here, so do...while is safe
+    do {
+      a ^= ShiftMix(Fetch64(s) * k1) * k1;
+      a *= k1;
+      b ^= a;
+      c ^= ShiftMix(Fetch64(s + 8) * k1) * k1;
+      c *= k1;
+      d ^= c;
+      s += 16;
+      len -= 16;
+    } while (len > 16);
+  }
+  a = HashLen16(a, c);
+  b = HashLen16(d, b);
+  return uint128{a ^ b, HashLen16(b, a)};
+}
+
+uint128 asl::city_hash::CityHash128WithSeed(const char *s, size_t len, uint128 seed) {
+  if (len < 128) {
+    return CityMurmur(s, len, seed);
+  }
+
+  // We expect len >= 128 to be the common case.  Keep 56 bytes of state:
+  // v, w, x, y, and z.
+  uint128 v, w;
+  uint64 x = seed.low;
+  uint64 y = seed.high;
+  uint64 z = len * k1;
+  v.high = Rotate(y ^ k1, 49) * k1 + Fetch64(s);
+  v.low = Rotate(v.high, 42) * k1 + Fetch64(s + 8);
+  w.high = Rotate(y + z, 35) * k1 + x;
+  w.low = Rotate(x + Fetch64(s + 88), 53) * k1;
+
+  // This is the same inner loop as CityHash64(), manually unrolled.
+  do {
+    x = Rotate(x + y + v.high + Fetch64(s + 8), 37) * k1;
+    y = Rotate(y + v.low + Fetch64(s + 48), 42) * k1;
+    x ^= w.low;
+    y += v.high + Fetch64(s + 40);
+    z = Rotate(z + w.high, 33) * k1;
+    v = WeakHashLen32WithSeeds(s, v.low * k1, x + w.high);
+    w = WeakHashLen32WithSeeds(s + 32, z + w.low, y + Fetch64(s + 16));
+    asl::swap(z, x);
+    s += 64;
+    x = Rotate(x + y + v.high + Fetch64(s + 8), 37) * k1;
+    y = Rotate(y + v.low + Fetch64(s + 48), 42) * k1;
+    x ^= w.low;
+    y += v.high + Fetch64(s + 40);
+    z = Rotate(z + w.high, 33) * k1;
+    v = WeakHashLen32WithSeeds(s, v.low * k1, x + w.high);
+    w = WeakHashLen32WithSeeds(s + 32, z + w.low, y + Fetch64(s + 16));
+    asl::swap(z, x);
+    s += 64;
+    len -= 128;
+  } while (LIKELY(len >= 128));
+  x += Rotate(v.high + z, 49) * k0;
+  y = y * k0 + Rotate(w.low, 37);
+  z = z * k0 + Rotate(w.high, 27);
+  w.high *= 9;
+  v.high *= k0;
+  // If 0 < len < 128, hash up to 4 chunks of 32 bytes each from the end of s.
+  for (size_t tail_done = 0; tail_done < len; ) {
+    tail_done += 32;
+    y = Rotate(x + y, 42) * k0 + v.low;
+    w.high += Fetch64(s + len - tail_done + 16);
+    x = x * k0 + w.high;
+    z += w.low + Fetch64(s + len - tail_done);
+    w.low += v.high;
+    v = WeakHashLen32WithSeeds(s + len - tail_done, v.high + z, v.low);
+    v.high *= k0;
+  }
+  // At this point our 56 bytes of state should contain more than
+  // enough information for a strong 128-bit hash.  We use two
+  // different 56-byte-to-8-byte hashes to get a 16-byte final result.
+  x = HashLen16(x, v.high);
+  y = HashLen16(y + z, w.high);
+  return uint128{HashLen16(x + v.low, w.low) + y,
+                 HashLen16(x + w.low, y + v.low)};
+}
+
+uint128 asl::city_hash::CityHash128(const char *s, size_t len) {
+  return len >= 16 ?
+      asl::city_hash::CityHash128WithSeed(s + 16, len - 16,
+                          uint128{Fetch64(s), Fetch64(s + 8) + k0}) :
+      asl::city_hash::CityHash128WithSeed(s, len, uint128{k0, k1});
+}
+
+// NOLINTEND
diff --git a/asl/hashing/hash_tests.cpp b/asl/hashing/hash_tests.cpp
new file mode 100644
index 0000000..9efb497
--- /dev/null
+++ b/asl/hashing/hash_tests.cpp
@@ -0,0 +1,260 @@
+#include "asl/testing/testing.hpp"
+#include "asl/hashing/hash.hpp"
+#include "asl/strings/string_view.hpp"
+#include "asl/strings/string.hpp"
+#include "asl/containers/buffer.hpp"
+#include "asl/types/box.hpp"
+#include "asl/types/option.hpp"
+#include "asl/types/status.hpp"
+#include "asl/types/status_or.hpp"
+
+static_assert(!asl::hashable<int*>);
+static_assert(!asl::hashable<int[]>);
+static_assert(!asl::hashable<int[9]>);
+
+static_assert(asl::hashable<uint8_t>);
+static_assert(asl::hashable<uint16_t>);
+static_assert(asl::hashable<uint32_t>);
+static_assert(asl::hashable<uint64_t>);
+static_assert(asl::hashable<uint128_t>);
+
+static_assert(asl::hashable<int8_t>);
+static_assert(asl::hashable<int16_t>);
+static_assert(asl::hashable<int32_t>);
+static_assert(asl::hashable<int64_t>);
+
+ASL_TEST(integers)
+{
+    uint64_t a = asl::hash_value<uint16_t>(45);
+    uint64_t b = asl::hash_value<uint16_t>(45);
+    uint64_t c = asl::hash_value<uint16_t>(46);
+    uint64_t d = asl::hash_value<uint32_t>(45);
+
+    ASL_TEST_EXPECT(a == b);
+    ASL_TEST_EXPECT(a != c);
+    ASL_TEST_EXPECT(a != d);
+}
+
+static_assert(asl::hashable<bool>);
+
+ASL_TEST(bool)
+{
+    ASL_TEST_EXPECT(asl::hash_value(true) == asl::hash_value(true));
+    ASL_TEST_EXPECT(asl::hash_value(false) == asl::hash_value(false));
+    ASL_TEST_EXPECT(asl::hash_value(true) != asl::hash_value(false));
+}
+
+static_assert(asl::hashable<asl::string_view>);
+static_assert(asl::hashable<asl::string<>>);
+
+ASL_TEST(strings)
+{
+    ASL_TEST_EXPECT(asl::hash_value("hello"_sv) == asl::hash_value("hello"_sv));
+    ASL_TEST_EXPECT(asl::hash_value("hello"_sv) != asl::hash_value("hello "_sv));
+    ASL_TEST_EXPECT(asl::hash_value("hello"_sv) != asl::hash_value("HELLO"_sv));
+
+    ASL_TEST_EXPECT(asl::hash_value(asl::string("hello"_sv)) == asl::hash_value(asl::string("hello"_sv)));
+    ASL_TEST_EXPECT(asl::hash_value(asl::string("hello"_sv)) != asl::hash_value(asl::string("hello "_sv)));
+    ASL_TEST_EXPECT(asl::hash_value(asl::string("hello"_sv)) != asl::hash_value(asl::string("HELLO"_sv)));
+
+    ASL_TEST_EXPECT(asl::hash_value("hello"_sv) == asl::hash_value(asl::string("hello"_sv)));
+}
+
+static_assert(asl::hashable<asl::span<const int>>);
+static_assert(!asl::hashable<asl::span<const int*>>);
+static_assert(asl::hashable<asl::span<asl::string_view>>);
+
+ASL_TEST(span)
+{
+    int ints1[] = {1, 2, 3};
+    int ints2[] = {1, 2, 3};
+    int ints3[] = {1, 2};
+    int ints4[] = {3, 2, 1};
+
+    ASL_TEST_EXPECT(asl::hash_value(asl::span<int>(ints1)) == asl::hash_value(asl::span<int>(ints2)));
+    ASL_TEST_EXPECT(asl::hash_value(asl::span<int>(ints1)) != asl::hash_value(asl::span<int>(ints3)));
+    ASL_TEST_EXPECT(asl::hash_value(asl::span<int>(ints1)) != asl::hash_value(asl::span<int>(ints4)));
+
+    asl::string_view strs1[] = {"a", "abc", "hello"};
+    asl::string_view strs2[] = {"a", "abc", "hello"};
+    asl::string_view strs3[] = {"a", "abc"};
+    asl::string_view strs4[] = {"a", "abc", "hello", "what"};
+
+    ASL_TEST_EXPECT(asl::hash_value(asl::span<asl::string_view>(strs1)) == asl::hash_value(asl::span<asl::string_view>(strs2)));
+    ASL_TEST_EXPECT(asl::hash_value(asl::span<asl::string_view>(strs1)) != asl::hash_value(asl::span<asl::string_view>(strs3)));
+    ASL_TEST_EXPECT(asl::hash_value(asl::span<asl::string_view>(strs1)) != asl::hash_value(asl::span<asl::string_view>(strs4)));
+}
+
+static_assert(asl::hashable<asl::buffer<int>>);
+static_assert(!asl::hashable<asl::buffer<int*>>);
+
+ASL_TEST(buffer)
+{
+    asl::buffer<int> ints1;
+    ints1.push(1);
+    ints1.push(2);
+    ints1.push(3);
+
+    asl::buffer<int> ints2;
+    ints2.push(1);
+    ints2.push(2);
+    ints2.push(3);
+
+    asl::buffer<int> ints3;
+    ints3.push(1);
+    ints3.push(2);
+
+    asl::buffer<int> ints4;
+    ints4.push(1);
+    ints4.push(2);
+    ints4.push(4);
+
+    ASL_TEST_EXPECT(asl::hash_value(ints1) == asl::hash_value(ints2));
+    ASL_TEST_EXPECT(asl::hash_value(ints1) != asl::hash_value(ints3));
+    ASL_TEST_EXPECT(asl::hash_value(ints1) != asl::hash_value(ints4));
+    ASL_TEST_EXPECT(asl::hash_value(ints1) == asl::hash_value(ints1.as_span()));
+
+    asl::buffer<asl::string_view> strs1;
+    strs1.push("Hello");
+    strs1.push("World");
+
+    asl::buffer<asl::string_view> strs2;
+    strs2.push("Hello");
+    strs2.push("World");
+
+    asl::buffer<asl::string_view> strs3;
+    strs3.push("Hello");
+    strs3.push("world");
+
+    asl::buffer<asl::string_view> strs4;
+    strs4.push("Hello");
+    strs4.push("World");
+    strs4.push("World");
+
+    ASL_TEST_EXPECT(asl::hash_value(strs1) == asl::hash_value(strs2));
+    ASL_TEST_EXPECT(asl::hash_value(strs1) != asl::hash_value(strs3));
+    ASL_TEST_EXPECT(asl::hash_value(strs1) != asl::hash_value(strs4));
+    ASL_TEST_EXPECT(asl::hash_value(strs1) == asl::hash_value(strs1.as_span()));
+}
+
+enum Enum1 {};
+enum class Enum2 {};
+
+static_assert(asl::hashable<Enum1>);
+static_assert(asl::hashable<Enum2>);
+
+static_assert(!asl::hashable<asl::box<int*>>);
+static_assert(asl::hashable<asl::box<asl::string_view>>);
+
+ASL_TEST(box)
+{
+    auto b1 = asl::make_box<asl::string_view>("Hello, world!");
+    auto b2 = asl::make_box<asl::string_view>("Hello, world!");
+    auto b3 = asl::make_box<asl::string_view>("Hello, world! 2");
+
+    ASL_TEST_EXPECT(asl::hash_value(b1) == asl::hash_value(b2));
+    ASL_TEST_EXPECT(asl::hash_value(b1) != asl::hash_value(b3));
+    ASL_TEST_EXPECT(asl::hash_value(b1) == asl::hash_value("Hello, world!"_sv));
+}
+
+struct NonZero
+{
+    int value;
+
+    constexpr explicit NonZero(int x) : value(x)
+    {
+        ASL_ASSERT(x != 0);
+    }
+
+    constexpr explicit NonZero(asl::niche_t) : value(0) {}
+
+    constexpr bool operator==(asl::niche_t) const { return value == 0; }
+};
+
+namespace asl { template<> struct is_uniquely_represented<NonZero> : true_type {}; }
+static_assert(asl::has_niche<NonZero>);
+static_assert(asl::uniquely_represented<NonZero>);
+
+static_assert(asl::hashable<asl::option<int>>);
+static_assert(!asl::hashable<asl::option<int*>>);
+static_assert(asl::hashable<asl::option<asl::string_view>>);
+static_assert(asl::hashable<asl::option<NonZero>>);
+static_assert(asl::uniquely_represented<asl::option<NonZero>>);
+
+ASL_TEST(option)
+{
+    asl::option<int> int1 = 0;
+    asl::option<int> int2 = 0;
+    asl::option<int> int3 = 1;
+    asl::option<int> int4 = asl::nullopt;
+
+    ASL_TEST_EXPECT(asl::hash_value(int1) == asl::hash_value(int2));
+    ASL_TEST_EXPECT(asl::hash_value(int1) != asl::hash_value(int3));
+    ASL_TEST_EXPECT(asl::hash_value(int1) != asl::hash_value(int4));
+
+    asl::option<NonZero> noz1{8};
+    asl::option<NonZero> noz2{8};
+    asl::option<NonZero> noz3{9};
+    asl::option<NonZero> noz4 = asl::nullopt;
+
+    ASL_TEST_EXPECT(asl::hash_value(noz1) == asl::hash_value(noz2));
+    ASL_TEST_EXPECT(asl::hash_value(noz1) != asl::hash_value(noz3));
+    ASL_TEST_EXPECT(asl::hash_value(noz1) != asl::hash_value(noz4));
+}
+
+static_assert(asl::hashable<asl::status>);
+
+ASL_TEST(status)
+{
+    asl::status s1 = asl::ok();
+    asl::status s2 = asl::ok();
+    asl::status s3 = asl::internal_error();
+    asl::status s4 = asl::internal_error();
+    asl::status s5 = asl::runtime_error();
+    asl::status s6 = asl::internal_error("Oh, no!");
+    asl::status s7 = asl::internal_error("Oh, no!");
+    asl::status s8 = asl::internal_error("Oh, no");
+    asl::status s9 = asl::runtime_error("Oh, no!");
+
+    ASL_TEST_EXPECT(asl::hash_value(s1) == asl::hash_value(s2));
+    ASL_TEST_EXPECT(asl::hash_value(s3) == asl::hash_value(s4));
+    ASL_TEST_EXPECT(asl::hash_value(s6) == asl::hash_value(s7));
+
+    ASL_TEST_EXPECT(asl::hash_value(s1) != asl::hash_value(s3));
+    ASL_TEST_EXPECT(asl::hash_value(s1) != asl::hash_value(s5));
+    ASL_TEST_EXPECT(asl::hash_value(s1) != asl::hash_value(s6));
+    ASL_TEST_EXPECT(asl::hash_value(s1) != asl::hash_value(s9));
+
+    ASL_TEST_EXPECT(asl::hash_value(s3) != asl::hash_value(s5));
+    ASL_TEST_EXPECT(asl::hash_value(s3) != asl::hash_value(s6));
+    ASL_TEST_EXPECT(asl::hash_value(s3) != asl::hash_value(s8));
+    ASL_TEST_EXPECT(asl::hash_value(s3) != asl::hash_value(s9));
+
+    ASL_TEST_EXPECT(asl::hash_value(s6) != asl::hash_value(s8));
+    ASL_TEST_EXPECT(asl::hash_value(s6) != asl::hash_value(s9));
+}
+
+static_assert(asl::hashable<asl::status_or<int>>);
+static_assert(asl::hashable<asl::status_or<asl::string_view>>);
+static_assert(!asl::hashable<asl::status_or<int*>>);
+
+ASL_TEST(status_or)
+{
+    asl::status_or<int> s1 = 42;
+    asl::status_or<int> s2 = 42;
+    asl::status_or<int> s3 = 43;
+    asl::status_or<int> s4 = asl::runtime_error();
+    asl::status_or<int> s5 = asl::runtime_error();
+    asl::status_or<int> s6 = asl::runtime_error("Hello");
+    asl::status_or<int> s7 = asl::runtime_error("Hello");
+
+    ASL_TEST_EXPECT(asl::hash_value(s1) == asl::hash_value(s2));
+    ASL_TEST_EXPECT(asl::hash_value(s4) == asl::hash_value(s5));
+    ASL_TEST_EXPECT(asl::hash_value(s6) == asl::hash_value(s7));
+
+    ASL_TEST_EXPECT(asl::hash_value(s1) != asl::hash_value(s3));
+    ASL_TEST_EXPECT(asl::hash_value(s1) != asl::hash_value(s4));
+    ASL_TEST_EXPECT(asl::hash_value(s1) != asl::hash_value(s6));
+
+    ASL_TEST_EXPECT(asl::hash_value(s4) != asl::hash_value(s6));
+}