Today we needed to hash one unsigned integer into another to allow anonymising of user data. I found http://www.concentric.net/~Ttwang/tech/inthash.htm which has a list of hashing algorithms which can be easily used in c#. It’s important to test these hashing algorithms to see if duplicate hashes are created, as they were by the first 2 algorithms tried. The algorithm used in the end was tested for an input integer of 1 to 1,000,000 without any duplicates.

        static void Main(string[] args)
        {
            List<Int32> results = new List<int>();

            for (int c = 1; c < 1000000; c++)
                results.Add(hash32shiftmult(c));

            Console.WriteLine("Added hashes to results table, looking for matches.");

            for (int i = 0; i < results.Count; i++)
            {
                if(i%10000 == 0)
                    Console.Write(i + " ");

                for (int j = 0; j < results.Count; j++)
                {
                    if (i != j)
                        if (results[i].Equals(results[j]))
                            Console.WriteLine("rnfound at " + i + " (" + results[i] + ") and " + j + " (" + results[j] + ")");
                }
            }

            Console.ReadLine();
        }

        static int mix(int a, int b, int c)
        {
          a=a-b;  a=a-c;  a=a^(c >> 13);
          b=b-c;  b=b-a;  b=b^(a << 8);
          c=c-a;  c=c-b;  c=c^(b >> 13);
          a=a-b;  a=a-c;  a=a^(c >> 12);
          b=b-c;  b=b-a;  b=b^(a << 16);
          c=c-a;  c=c-b;  c=c^(b >> 5);
          a=a-b;  a=a-c;  a=a^(c >> 3);
          b=b-c;  b=b-a;  b=b^(a << 10);
          c=c-a;  c=c-b;  c=c^(b >> 15);
          return c;
        }

        static int hash(int a)
        {
           a = (a+0x7ed55d6) + (a<<12);
           a = (a^0xc761c2c) ^ (a>>19);
           a = (a+0x165667b1) + (a<<5);
           a = (a+0xd3a264c) ^ (a<<9);
           a = (a+0xfd70465) + (a<<3);
           a = (a^0xb55a4f9) ^ (a>>16);
           return a;
        }

        static int hash32shiftmult(int key)
        {
          int c2=0x27d4eb2d; // a prime or an odd constant
          key = (key ^ 61) ^ (key >> 16);
          key = key + (key << 3);
          key = key ^ (key >> 4);
          key = key * c2;
          key = key ^ (key >> 15);
          return key;
        }