XDecimal

sample.js

var xd = new XDecimal("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ");

console.log(xd.toString(10));
console.log(xd.parse("a"));
console.log(xd.toString(1445598572.04283));
console.log(xd.parse("1zPAe0.175"));

XDecimal.js

function XDecimal(seed){
	var SEED = seed.split("");
	var SEED_LENGTH = SEED.length;

	function toString(value) {
		if(value % 1 != 0){
			var p = value.toString().split(".");
			return toString(p[0]) + "." + toString(p[1]);
		}
		var v = value;
		var c = [];
		while (0 < v) {
			var m = v % SEED_LENGTH;
			c.push(SEED[m]);
			v = (v - m) / SEED_LENGTH;
		}
		return c.reverse().join("");
	}

	function parse(value) {
		if(0 < value.indexOf(".")){
			var p = value.split(".");
			return parseFloat(parse(p[0]) + "." + parse(p[1]));
		}
		var v = 0;
		var c = value.split("");
		for(var i = 0; i < c.length; i++) {
			var m = SEED.indexOf(c[i]);
			if(m < 0){
				throw new Error();
			}
			v *= SEED_LENGTH;
			v += m;
		}
		return v;
	}

	this.toString = toString;
	this.parse = parse;

}