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+// 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
+//
+// http://code.google.com/p/cityhash/
+//
+// This file provides a few functions for hashing strings.  All of them are
+// high-quality functions in the sense that they pass standard tests such
+// as Austin Appleby's SMHasher.  They are also fast.
+//
+// For 64-bit x86 code, on short strings, we don't know of anything faster than
+// CityHash64 that is of comparable quality.  We believe our nearest competitor
+// is Murmur3.  For 64-bit x86 code, CityHash64 is an excellent choice for hash
+// tables and most other hashing (excluding cryptography).
+//
+// For 64-bit x86 code, on long strings, the picture is more complicated.
+// On many recent Intel CPUs, such as Nehalem, Westmere, Sandy Bridge, etc.,
+// CityHashCrc128 appears to be faster than all competitors of comparable
+// quality.  CityHash128 is also good but not quite as fast.  We believe our
+// nearest competitor is Bob Jenkins' Spooky.  We don't have great data for
+// other 64-bit CPUs, but for long strings we know that Spooky is slightly
+// faster than CityHash on some relatively recent AMD x86-64 CPUs, for example.
+// Note that CityHashCrc128 is declared in citycrc.h.
+//
+// For 32-bit x86 code, we don't know of anything faster than CityHash32 that
+// is of comparable quality.  We believe our nearest competitor is Murmur3A.
+// (On 64-bit CPUs, it is typically faster to use the other CityHash variants.)
+//
+// Functions in the CityHash family are not suitable for cryptography.
+//
+// Please see CityHash's README file for more details on our performance
+// measurements and so on.
+//
+// WARNING: This code has been only lightly tested on big-endian platforms!
+// It is known to work well on little-endian platforms that have a small penalty
+// for unaligned reads, such as current Intel and AMD moderate-to-high-end CPUs.
+// It should work on all 32-bit and 64-bit platforms that allow unaligned reads;
+// bug reports are welcome.
+//
+// By the way, for some hash functions, given strings a and b, the hash
+// of a+b is easily derived from the hashes of a and b.  This property
+// doesn't hold for any hash functions in this file.
+
+#pragma once
+
+#include <utility>
+#include <stdint.h>
+#include <stdlib.h> // for size_t.
+
+namespace Common {
+
+typedef std::pair<uint64_t, uint64_t> uint128;
+
+inline uint64_t Uint128Low64(const uint128& x) {
+    return x.first;
+}
+inline uint64_t Uint128High64(const uint128& x) {
+    return x.second;
+}
+
+// Hash function for a byte array.
+uint64_t CityHash64(const char* buf, 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* buf, 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* buf, size_t len, uint64_t seed0, uint64_t seed1);
+
+// Hash function for a byte array.
+uint128 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 CityHash128WithSeed(const char* s, size_t len, uint128 seed);
+
+// Hash 128 input bits down to 64 bits of output.
+// This is intended to be a reasonably good hash function.
+inline uint64_t Hash128to64(const uint128& x) {
+    // Murmur-inspired hashing.
+    const uint64_t kMul = 0x9ddfea08eb382d69ULL;
+    uint64_t a = (Uint128Low64(x) ^ Uint128High64(x)) * kMul;
+    a ^= (a >> 47);
+    uint64_t b = (Uint128High64(x) ^ a) * kMul;
+    b ^= (b >> 47);
+    b *= kMul;
+    return b;
+}
+
+} // namespace Common