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diff --git a/src/common/cityhash.cpp b/src/common/cityhash.cpp
new file mode 100644
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+++ b/src/common/cityhash.cpp
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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
+//
+// 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 <algorithm>
+#include <string.h> // for memcpy and memset
+#include "cityhash.h"
+#include "common/swap.h"
+
+// #include "config.h"
+#ifdef __GNUC__
+#define HAVE_BUILTIN_EXPECT 1
+#endif
+#ifdef COMMON_BIG_ENDIAN
+#define WORDS_BIGENDIAN 1
+#endif
+
+using namespace std;
+
+typedef uint8_t uint8;
+typedef uint32_t uint32;
+typedef uint64_t uint64;
+
+namespace Common {
+
+static uint64 UNALIGNED_LOAD64(const char* p) {
+    uint64 result;
+    memcpy(&result, p, sizeof(result));
+    return result;
+}
+
+static uint32 UNALIGNED_LOAD32(const char* p) {
+    uint32 result;
+    memcpy(&result, p, sizeof(result));
+    return result;
+}
+
+#ifdef WORDS_BIGENDIAN
+#define uint32_in_expected_order(x) (swap32(x))
+#define uint64_in_expected_order(x) (swap64(x))
+#else
+#define uint32_in_expected_order(x) (x)
+#define uint64_in_expected_order(x) (x)
+#endif
+
+#if !defined(LIKELY)
+#if HAVE_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;
+
+// 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 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 = s[0];
+        uint8 b = s[len >> 1];
+        uint8 c = 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 pair<uint64, uint64> 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 make_pair(a + z, b + c);
+}
+
+// Return a 16-byte hash for s[0] ... s[31], a, and b.  Quick and dirty.
+static pair<uint64, uint64> 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 = swap64((u + v) * mul) + h;
+    uint64 x = Rotate(e + f, 42) + c;
+    uint64 y = (swap64((v + w) * mul) + g) * mul;
+    uint64 z = e + f + c;
+    a = swap64((x + z) * mul + y) + b;
+    b = ShiftMix((z + a) * mul + d + h) * mul;
+    return b + x;
+}
+
+uint64 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));
+    pair<uint64, uint64> v = WeakHashLen32WithSeeds(s + len - 64, len, z);
+    pair<uint64, uint64> 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.first + Fetch64(s + 8), 37) * k1;
+        y = Rotate(y + v.second + Fetch64(s + 48), 42) * k1;
+        x ^= w.second;
+        y += v.first + Fetch64(s + 40);
+        z = Rotate(z + w.first, 33) * k1;
+        v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);
+        w = WeakHashLen32WithSeeds(s + 32, z + w.second, y + Fetch64(s + 16));
+        std::swap(z, x);
+        s += 64;
+        len -= 64;
+    } while (len != 0);
+    return HashLen16(HashLen16(v.first, w.first) + ShiftMix(y) * k1 + z,
+                     HashLen16(v.second, w.second) + x);
+}
+
+uint64 CityHash64WithSeed(const char* s, size_t len, uint64 seed) {
+    return CityHash64WithSeeds(s, len, k2, seed);
+}
+
+uint64 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 = Uint128Low64(seed);
+    uint64 b = Uint128High64(seed);
+    uint64 c = 0;
+    uint64 d = 0;
+    signed long l = static_cast<long>(len) - 16;
+    if (l <= 0) { // len <= 16
+        a = ShiftMix(a * k1) * k1;
+        c = b * k1 + HashLen0to16(s, len);
+        d = ShiftMix(a + (len >= 8 ? Fetch64(s) : c));
+    } else { // len > 16
+        c = HashLen16(Fetch64(s + len - 8) + k1, a);
+        d = HashLen16(b + len, c + Fetch64(s + len - 16));
+        a += d;
+        do {
+            a ^= ShiftMix(Fetch64(s) * k1) * k1;
+            a *= k1;
+            b ^= a;
+            c ^= ShiftMix(Fetch64(s + 8) * k1) * k1;
+            c *= k1;
+            d ^= c;
+            s += 16;
+            l -= 16;
+        } while (l > 0);
+    }
+    a = HashLen16(a, c);
+    b = HashLen16(d, b);
+    return uint128(a ^ b, HashLen16(b, a));
+}
+
+uint128 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.
+    pair<uint64, uint64> v, w;
+    uint64 x = Uint128Low64(seed);
+    uint64 y = Uint128High64(seed);
+    uint64 z = len * k1;
+    v.first = Rotate(y ^ k1, 49) * k1 + Fetch64(s);
+    v.second = Rotate(v.first, 42) * k1 + Fetch64(s + 8);
+    w.first = Rotate(y + z, 35) * k1 + x;
+    w.second = Rotate(x + Fetch64(s + 88), 53) * k1;
+
+    // This is the same inner loop as CityHash64(), manually unrolled.
+    do {
+        x = Rotate(x + y + v.first + Fetch64(s + 8), 37) * k1;
+        y = Rotate(y + v.second + Fetch64(s + 48), 42) * k1;
+        x ^= w.second;
+        y += v.first + Fetch64(s + 40);
+        z = Rotate(z + w.first, 33) * k1;
+        v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);
+        w = WeakHashLen32WithSeeds(s + 32, z + w.second, y + Fetch64(s + 16));
+        std::swap(z, x);
+        s += 64;
+        x = Rotate(x + y + v.first + Fetch64(s + 8), 37) * k1;
+        y = Rotate(y + v.second + Fetch64(s + 48), 42) * k1;
+        x ^= w.second;
+        y += v.first + Fetch64(s + 40);
+        z = Rotate(z + w.first, 33) * k1;
+        v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);
+        w = WeakHashLen32WithSeeds(s + 32, z + w.second, y + Fetch64(s + 16));
+        std::swap(z, x);
+        s += 64;
+        len -= 128;
+    } while (LIKELY(len >= 128));
+    x += Rotate(v.first + z, 49) * k0;
+    y = y * k0 + Rotate(w.second, 37);
+    z = z * k0 + Rotate(w.first, 27);
+    w.first *= 9;
+    v.first *= 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.second;
+        w.first += Fetch64(s + len - tail_done + 16);
+        x = x * k0 + w.first;
+        z += w.second + Fetch64(s + len - tail_done);
+        w.second += v.first;
+        v = WeakHashLen32WithSeeds(s + len - tail_done, v.first + z, v.second);
+        v.first *= 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.first);
+    y = HashLen16(y + z, w.first);
+    return uint128(HashLen16(x + v.second, w.second) + y, HashLen16(x + w.second, y + v.second));
+}
+
+uint128 CityHash128(const char* s, size_t len) {
+    return len >= 16
+               ? CityHash128WithSeed(s + 16, len - 16, uint128(Fetch64(s), Fetch64(s + 8) + k0))
+               : CityHash128WithSeed(s, len, uint128(k0, k1));
+}
+
+} // namespace Common