Rework hashing, use a more extensible framework similar to n3876 proposal

std::hash specialization is a pain to work with, stop using that, and
just specialize a 'size_t hash_value(const T&)' free function.

1 files changed, 68 insertions, 0 deletions

diff --git a/src/hash.cc b/src/hash.cc
new file mode 100644
index 00000000..2ef9262d
--- /dev/null
+++ b/src/hash.cc
@@ -0,0 +1,68 @@
+#include "hash.hh"
+
+#include <cstdint>
+
+namespace Kakoune
+{
+
+[[gnu::always_inline]]
+static inline uint32_t rotl(uint32_t x, int8_t r)
+{
+    return (x << r) | (x >> (32 - r));
+}
+
+[[gnu::always_inline]]
+static inline uint32_t fmix(uint32_t h)
+{
+    h ^= h >> 16;
+    h *= 0x85ebca6b;
+    h ^= h >> 13;
+    h *= 0xc2b2ae35;
+    h ^= h >> 16;
+
+    return h;
+}
+
+// murmur3 hash, based on https://github.com/PeterScott/murmur3
+size_t hash_data(const char* input, size_t len)
+{
+    const uint8_t* data = reinterpret_cast<const uint8_t*>(input);
+    uint32_t hash = 0x1235678;
+    constexpr uint32_t c1 = 0xcc9e2d51;
+    constexpr uint32_t c2 = 0x1b873593;
+
+    const int nblocks = len / 4;
+    const uint32_t* blocks = reinterpret_cast<const uint32_t*>(data + nblocks*4);
+
+    for (int i = -nblocks; i; ++i)
+    {
+        uint32_t key = blocks[i];
+        key *= c1;
+        key = rotl(key, 15);
+        key *= c2;
+
+        hash ^= key;
+        hash = rotl(hash, 13);
+        hash = hash * 5 + 0xe6546b64;
+    }
+
+    const uint8_t* tail = data + nblocks * 4;
+    uint32_t key = 0;
+    switch (len & 3)
+    {
+        case 3: key ^= tail[2] << 16;
+        case 2: key ^= tail[1] << 8;
+        case 1: key ^= tail[0];
+                key *= c1;
+                key = rotl(key,15);
+                key *= c2;
+                hash ^= key;
+    }
+
+    hash ^= len;
+    hash = fmix(hash);
+
+    return hash;
+}
+
+}