C# hashcode for array of ints

Question

I have a class that internally is just an array of integers. Once constructed the array never changes. I'd like to pre-compute a good hashcode so that this class can be very efficiently used as a key in a Dictionary. The length of the array is less than about 30 items, and the integers are between -1000 and 1000 in general.

Answer 1

Hi,

I think choosing a good hash-algorithm would have to be based on the distribution (in a probability sense) of the integer values.

Have a look at Wikipedia for a list of algorithms

Answer 2

Not very clever, but sufficient for most practical purposes:

EDIT: changed due to comment of Henk Holterman, thanks for that.

int hc=array.Length;
for(int i=0;i<array.Length;++i)
{
     hc=unchecked(hc*314159 +array[i]);
}
return hc;

If you need something more sophisticated, look here.

Answer 3

Any CRC (or even XOR) should be ok.

Answer 4

For an array of values generally between -1000 and 1000, I would probably use something like this:

static int GetHashCode(int[] values)
{
   int result = 0;
   int shift = 0;
   for (int i = 0; i < values.Length; i++)
   {
      shift = (shift + 11) % 21;
      result ^= (values[i]+1024) << shift;
   }
   return result;
}

Answer 5

You may use CRC32 checksum. Here is the code:

[CLSCompliant(false)]
public class Crc32 {
    uint[] table = new uint[256];
    uint[] Table { get { return table; } }

    public Crc32() {
        MakeCrcTable();
    }
    void MakeCrcTable() {
        for (uint n = 0; n < 256; n++) {
            uint value = n;
            for (int i = 0; i < 8; i++) {
                if ((value & 1) != 0)
                    value = 0xedb88320 ^ (value >> 1);
                else
                    value = value >> 1;
            }
            Table[n] = value;
        }
    }
    public uint UpdateCrc(uint crc, byte[] buffer, int length) {
        uint result = crc;
        for (int n = 0; n < length; n++) {
            result = Table[(result ^ buffer[n]) & 0xff] ^ (result >> 8);
        }
        return result;
    }
    public uint Calculate(Stream stream) {
        long pos = stream.Position;
        const int size = 0x32000;
        byte[] buf = new byte[size];
        int bytes = 0;
        uint result = 0xffffffff;
        do {
            bytes = stream.Read(buf, 0, size);
            result = UpdateCrc(result, buf, bytes);
        }
        while (bytes == size);
        stream.Position = pos;
        return ~result;
    }
}