GetDistance.cs

public static int GetDistance(ArraySegment<byte> bs1, ArraySegment<byte> bs2)
{
    if (bs1.Count != bs2.Count) throw new ArgumentException("bs1 と b2 の長さが違います。");

    var count = bs1.Count;
    var distance = 0;
    var i = 0;

    if (Vector.IsHardwareAccelerated)
    {
        var vecSize = Vector<byte>.Count;

        for (; i + vecSize <= count; i += vecSize)
        {
            var vec1 = Vector.AsVectorUInt64(new Vector<byte>(bs1.Array, bs1.Offset + i));
            var vec2 = Vector.AsVectorUInt64(new Vector<byte>(bs2.Array, bs2.Offset + i));
            var weight = vec1 ^ vec2;

            // Vector にビットシフトが入ったら全部 Vector でやりたい
            for (var j = 0; j < Vector<ulong>.Count; j++)
                distance += popcount64c(weight[j]);
        }
    }

    while (count - i is var left && left > 0)
    {
        ulong v;
        if (left >= 8)
        {
            v = Unsafe.ReadUnaligned<ulong>(ref bs1.Array[bs1.Offset + i])
                ^ Unsafe.ReadUnaligned<ulong>(ref bs2.Array[bs2.Offset + i]);
            i += 8;
        }
        else if (left >= 4)
        {
            v = Unsafe.ReadUnaligned<uint>(ref bs1.Array[bs1.Offset + i])
                ^ Unsafe.ReadUnaligned<uint>(ref bs2.Array[bs2.Offset + i]);
            i += 4;
        }
        else
        {
            v = (uint)bs1.Array[bs1.Offset + i] ^ bs2.Array[bs2.Offset + i];
            i++;
        }

        distance += popcount64c(v);
    }

    return distance;

    // https://en.wikipedia.org/wiki/Hamming_weight
    int popcount64c(ulong x)
    {
        x -= (x >> 1) & 0x5555555555555555;
        x = (x & 0x3333333333333333) + ((x >> 2) & 0x3333333333333333);
        x = (x + (x >> 4)) & 0x0f0f0f0f0f0f0f0f;
        return (int)((x * 0x0101010101010101) >> 56);
    }
}