1 files changed, 83 insertions, 0 deletions
diff --git a/src/rocksdb/util/hash.cc b/src/rocksdb/util/hash.cc
new file mode 100644
index 000000000..d72be8a45
--- /dev/null
+++ b/src/rocksdb/util/hash.cc
@@ -0,0 +1,83 @@
+//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+//
+// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+
+#include <string.h>
+#include "util/coding.h"
+#include "util/hash.h"
+#include "util/util.h"
+#include "util/xxhash.h"
+
+namespace ROCKSDB_NAMESPACE {
+
+uint32_t Hash(const char* data, size_t n, uint32_t seed) {
+  // MurmurHash1 - fast but mediocre quality
+  // https://github.com/aappleby/smhasher/wiki/MurmurHash1
+  //
+  const uint32_t m = 0xc6a4a793;
+  const uint32_t r = 24;
+  const char* limit = data + n;
+  uint32_t h = static_cast<uint32_t>(seed ^ (n * m));
+
+  // Pick up four bytes at a time
+  while (data + 4 <= limit) {
+    uint32_t w = DecodeFixed32(data);
+    data += 4;
+    h += w;
+    h *= m;
+    h ^= (h >> 16);
+  }
+
+  // Pick up remaining bytes
+  switch (limit - data) {
+    // Note: The original hash implementation used data[i] << shift, which
+    // promotes the char to int and then performs the shift. If the char is
+    // negative, the shift is undefined behavior in C++. The hash algorithm is
+    // part of the format definition, so we cannot change it; to obtain the same
+    // behavior in a legal way we just cast to uint32_t, which will do
+    // sign-extension. To guarantee compatibility with architectures where chars
+    // are unsigned we first cast the char to int8_t.
+    case 3:
+      h += static_cast<uint32_t>(static_cast<int8_t>(data[2])) << 16;
+      FALLTHROUGH_INTENDED;
+    case 2:
+      h += static_cast<uint32_t>(static_cast<int8_t>(data[1])) << 8;
+      FALLTHROUGH_INTENDED;
+    case 1:
+      h += static_cast<uint32_t>(static_cast<int8_t>(data[0]));
+      h *= m;
+      h ^= (h >> r);
+      break;
+  }
+  return h;
+}
+
+// We are standardizing on a preview release of XXH3, because that's
+// the best available at time of standardizing.
+//
+// In testing (mostly Intel Skylake), this hash function is much more
+// thorough than Hash32 and is almost universally faster. Hash() only
+// seems faster when passing runtime-sized keys of the same small size
+// (less than about 24 bytes) thousands of times in a row; this seems
+// to allow the branch predictor to work some magic. XXH3's speed is
+// much less dependent on branch prediction.
+//
+// Hashing with a prefix extractor is potentially a common case of
+// hashing objects of small, predictable size. We could consider
+// bundling hash functions specialized for particular lengths with
+// the prefix extractors.
+uint64_t Hash64(const char* data, size_t n, uint64_t seed) {
+  return XXH3p_64bits_withSeed(data, n, seed);
+}
+
+uint64_t Hash64(const char* data, size_t n) {
+  // Same as seed = 0
+  return XXH3p_64bits(data, n);
+}
+
+}  // namespace ROCKSDB_NAMESPACE