Hashing a string between two integers with a good distribution (Uniform Hash)

Question

I'm trying to hash some strings between 0 and a very low n in order to give one color per user.

Here is my (working) code:

 function nameToColor(name) {
            var colors = ['red', 'blue', 'green', 'purple', 'orange', 'darkred', 'darkblue', 'darkgreen', 'cadetblue', 'darkpurple'];
            var hash = hashStr(name);
            var index = hash % colors.length;
            return colors[index];
        }

        //djb2 hash
        function hashStr(str) {
            var hash = 5381;
            for (var i = 0; i < str.length; i++) {
                var charCode = str.charCodeAt(i);
                hash = ((hash << 5) + hash) + charCode; /* hash * 33 + c */
            }
            return hash;    
        }

Unfortunately the low numbers are massively over-represented.

Question:

How can I write a deterministic javascript function that takes any string as argument and returns with a good (as uniform as possible) distribution a number between 0 and n?

Answer 1

Hogan gave in comment a link to several hash implementation in javascript. It turns out that the most simple is the most appropriate:

function nameToColor(name) {
                var colors = ['red', 'blue', 'green', 'purple', 'orange', 'darkred', 'darkblue', 'darkgreen', 'cadetblue', 'darkpurple'];
                var hash = hashStr(name);
                var index = hash % colors.length;
                return colors[index];
        }

        //very simple hash
        function hashStr(str) {
            var hash = 0;
            for (var i = 0; i < str.length; i++) {
                var charCode = str.charCodeAt(i);
                hash += charCode;
            }
            return hash;
        }

I think it works well because it only uses the addition (no shift or multiplications) which leave the modulo unchanged, so the initial quality of distribution is conserved.

I also found this on wikipedia, but did not have to use it:

In many applications, the range of hash values may be different for each run of the program, or may change along the same run (for instance, when a hash table needs to be expanded). In those situations, one needs a hash function which takes two parameters—the input data z, and the number n of allowed hash values.

A common solution is to compute a fixed hash function with a very large range (say, 0 to 232 − 1), divide the result by n, and use the division's remainder. If n is itself a power of 2, this can be done by bit masking and bit shifting. When this approach is used, the hash function must be chosen so that the result has fairly uniform distribution between 0 and n − 1, for any value of n that may occur in the application. Depending on the function, the remainder may be uniform only for certain values of n, e.g. odd or prime numbers.

We can allow the table size n to not be a power of 2 and still not have to perform any remainder or division operation, as these computations are sometimes costly. For example, let n be significantly less than 2b. Consider a pseudo random number generator (PRNG) function P(key) that is uniform on the interval [0, 2b − 1]. A hash function uniform on the interval [0, n-1] is n P(key)/2b. We can replace the division by a (possibly faster) right bit shift: nP(key)>> b.