/// <summary>
/// Computes the 128-bit city hash and are tuned for strings of at least a few hundred bytes using
/// the specified <paramref name="seed"/> starting at a <paramref name="offset"/> position.
/// </summary>
/// <param name="value">The encoded string.</param>
/// <param name="seed">The seed used by the hash alrorithm.</param>
/// <param name="offset">The offset position in the byte array.</param>
/// <returns>The 128-bit city hash.</returns>
/// <remarks>
/// The city hash is designed to compute hash for STRINGs only!
/// The city hash "works" with other types of data, but keep in mind it was not built for it.
/// </remarks>
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)
});
}
/// <summary>
/// Computes the 128-bit city hash and are tuned for strings of at least a few hundred bytes using
/// the specified <paramref name="seed"/>.
/// </summary>
/// <param name="value">The string value.</param>
/// <param name="seed">The seed used by the city hash algorithm.</param>
/// <returns>The 128-bit city hash.</returns>
/// <remarks>This function encodes the string using the unicode block (ISO/IEC 8859-1).</remarks>
public static UInt128 CityHash128(string value, UInt128 seed)
{
return(CityHash128(Encoding.GetEncoding("ISO-8859-1").GetBytes(value), seed, 0));
}
