test_hash_function.js

var Hash = {
    /** 
     * Constant: URL_HASH_FACTOR
     * {Float} Used to hash URL param strings for multi-WMS server selection.
     *         Set to the Golden Ratio per Knuth's recommendation.
     */
	URL_HASH_FACTOR: (Math.sqrt(5) - 1) / 2,
    /**
     * Method: selectUrl
     * selectUrl() implements the standard floating-point multiplicative
     *     hash function described by Knuth, and hashes the contents of the 
     *     given param string into a float between 0 and 1. This float is then
     *     scaled to the size of the provided urls array, and used to select
     *     a URL.
     *
     * Parameters:
     * paramString - {String}
     * urls - {Array(String)}
     * 
     * Returns:
     * {String} An entry from the urls array, deterministically selected based
     *          on the paramString.
     */
	selectUrl: function(paramString, urls) {
        var product = 1;
        for (var i=0, len=paramString.length; i<len; i++) { 
            product *= paramString.charCodeAt(i) * this.URL_HASH_FACTOR; 
            product -= Math.floor(product); 
        }
        return urls[Math.floor(product * urls.length)];
    }
};

var urls = [
	"http://tile1.bbc.co.uk/tilechef/{recipeKey}/uk/unknown/composite/201010011500/{sourceKey}.png", 
	"http://tile2.bbc.co.uk/tilechef/{recipeKey}/uk/unknown/composite/201010011500/{sourceKey}.png", 
	"http://tile3.bbc.co.uk/tilechef/{recipeKey}/uk/unknown/composite/201010011500/{sourceKey}.png", 
	"http://tile4.bbc.co.uk/tilechef/{recipeKey}/uk/unknown/composite/201010011500/{sourceKey}.png"
];

var quadKeys   = [ 031311121, 031311120, 031311122, 031311123, 031311132, 031311130, 031311112, 031311103, 031311102 ];

var recipeKeys = [ 21, 20, 22, 23, 32, 30, 12, 03, 02 ];
var sourceKeys = [ 0313111, 0313111, 0313111, 0313111, 0313111, 0313111, 0313111, 0313111, 0313111 ];

quadKeys.forEach( function ( item, index ) {
	console.log( item, Hash.selectUrl( item + '', urls ) );
});