1 files changed, 138 insertions, 0 deletions
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
+