/// <summary>
/// Computes the 128-bit city hash using a specific <paramref name="seed" />.
/// This algorithm is tuned for strings of at least a few hundred bytes.
/// </summary>
/// <param name="value">The string value.</param>
/// <param name="seed">Specifies the seed for the CityHash algorithm.</param>
/// <returns>The 128-bit city hash.</returns>
/// <exception cref="ArgumentNullException">value</exception>
/// <remarks>This function encodes the string using the unicode block (ISO/IEC 8859-1).</remarks>
public static uint128 GetCityHash128(this string value, uint128 seed)
{
if (value == null)
{
throw new ArgumentNullException("value");
}
return(CityHash.CityHash128(value, seed));
}
private static ulong Hash128to64(uint128 x)
{
const ulong kMul = 0x9ddfea08eb382d69UL;
var a = (x.Low ^ x.High) * kMul;
a ^= (a >> 47);
var b = (x.High ^ a) * kMul;
b ^= (b >> 47);
b *= kMul;
return(b);
}
protected static uint128 CityMurmur(byte[] value, uint128 seed, int offset)
{
var a = seed.Low;
var b = seed.High;
ulong c;
ulong d;
var len = value.Length - offset;
var l = len - 16;
if (l <= 0) // len <= 16
{
a = ShiftMix(a * k1) * k1;
c = b * k1 + HashLen0to16(value, offset);
d = ShiftMix(a + (len >= 8 ? Fetch64(value, offset) : c));
}
else // len > 16
{
c = HashLen16(Fetch64(value, offset + len - 8) + k1, a);
d = HashLen16(b + (ulong)len, c + Fetch64(value, offset + len - 16));
a += d;
var p = offset;
do
{
a ^= ShiftMix(Fetch64(value, p) * k1) * k1;
a *= k1;
b ^= a;
c ^= ShiftMix(Fetch64(value, p + 8) * k1) * k1;
c *= k1;
d ^= c;
p += 16;
l -= 16;
} while (l > 0);
}
a = HashLen16(a, c);
b = HashLen16(d, b);
return(new uint128(a ^ b, HashLen16(b, a)));
}
protected static uint128 CityHash128(byte[] value, uint128 seed, int offset)
{
if (value.Length - offset < 128)
{
return(CityMurmur(value, seed, offset));
}
// We expect len >= 128 to be the common case. Keep 56 bytes of state:
// v, w, x, y, and z.
var len = value.Length - offset;
var x = seed.Low;
var y = seed.High;
var z = (ulong)len * k1;
var v = new uint128
{
Low = Rotate(seed.High ^ k1, 49) * k1 + Fetch64(value, offset)
};
v.High = Rotate(v.Low, 42) * k1 + Fetch64(value, offset + 8);
var w = new uint128
{
Low = Rotate(y + z, 35) * k1 + x,
High = Rotate(seed.Low + Fetch64(value, offset + 88), 53) * k1
};
// This is the same inner loop as CityHash64(), manually unrolled.
var s = offset;
do
{
x = Rotate(x + y + v.Low + Fetch64(value, s + 8), 37) * k1;
y = Rotate(y + v.High + Fetch64(value, s + 48), 42) * k1;
x ^= w.High;
y += v.Low + Fetch64(value, s + 40);
z = Rotate(z + w.Low, 33) * k1;
v = WeakHashLen32WithSeeds(value, s, v.High * k1, x + w.Low);
w = WeakHashLen32WithSeeds(value, s + 32, z + w.High, y + Fetch64(value, s + 16));
Swap(ref z, ref x);
s += 64;
x = Rotate(x + y + v.Low + Fetch64(value, s + 8), 37) * k1;
y = Rotate(y + v.High + Fetch64(value, s + 48), 42) * k1;
x ^= w.High;
y += v.Low + Fetch64(value, s + 40);
z = Rotate(z + w.Low, 33) * k1;
v = WeakHashLen32WithSeeds(value, s, v.High * k1, x + w.Low);
w = WeakHashLen32WithSeeds(value, s + 32, z + w.High, y + Fetch64(value, s + 16));
Swap(ref z, ref x);
s += 64;
len -= 128;
} while (len >= 128);
x += Rotate(v.Low + z, 49) * k0;
y = y * k0 + Rotate(w.High, 37);
z = z * k0 + Rotate(w.Low, 27);
w.Low *= 9;
v.Low *= k0;
// If 0 < len < 128, hash up to 4 chunks of 32 bytes each from the end of s.
for (var tail = 0; tail < len;)
{
tail += 32;
y = Rotate(x + y, 42) * k0 + v.High;
w.Low += Fetch64(value, s + len - tail + 16);
x = x * k0 + w.Low;
z += w.High + Fetch64(value, s + len - tail);
w.High += v.Low;
v = WeakHashLen32WithSeeds(value, s + len - tail, v.Low + z, v.High);
v.Low *= 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.Low);
y = HashLen16(y + z, w.Low);
return(new uint128
{
Low = HashLen16(x + v.High, w.High) + y,
High = HashLen16(x + w.High, y + v.High)
});
}
public static uint128 CityHash128(string value, uint128 seed)
{
return(CityHash128(Encoding.GetEncoding("ISO-8859-1").GetBytes(value), seed, 0));
}
