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C# hashcode for array of ints

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.

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freddy smith Avatar asked Aug 04 '10 10:08

freddy smith


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2 Answers

Not very clever, but sufficient for most practical purposes:

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

  int hc = array.Length;
  foreach (int val in array)
  {
      hc = unchecked(hc * 314159 + val);
  }

If you need something more sophisticated, look here.

like image 133
Doc Brown Avatar answered Oct 01 '22 06:10

Doc Brown


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;
}
like image 34
BlueMonkMN Avatar answered Oct 01 '22 08:10

BlueMonkMN