basetrans.js

function basetrans(srcchars, dstchars){
	var srcbase = isUnique(srcchars) && baseNum(srcchars, false),
		dstbase = isUnique(dstchars) && baseNum(dstchars, true),
		srcbase_bitlen = Math.log2(srcbase),
		dstbase_bitlen = Math.log2(dstbase)
	if(!srcbase || !dstbase){
		throw new Error("argument string is not unique chars")
	}
	if(!srcbase_bitlen || !dstbase_bitlen){
		throw new Error("length of argument string is too short")
	}

	function isUnique(str){
		return !str.match(/(.).*\1/)
	}

	function isPowerOfTwo(x) {
		return (x & (x - 1)) == 0;
	}
	
	function zeropad(str, digit){
		return ("0".repeat(digit) + str).substr(-digit)
	}
	
	function baseNum(str, is_floor){
		var l = str.length
		return isPowerOfTwo(l) ? l : Math.pow(2, l.toString(2).length - is_floor)
	}
	
	
	function encode(str){
		return str.replace(/./g, function(e){
			return zeropad(srcchars.indexOf(e).toString(2), srcbase_bitlen)
		}).replace(new RegExp(`(.{1,${dstbase_bitlen}}$)|.{${dstbase_bitlen}}`, "g"), function(m, a){
			if(a){
				return dstchars.charAt(parseInt(m, 2)) + dstchars.charAt(m.length)
			}else{
				return dstchars.charAt(parseInt(m, 2))
			}
		})
	}

	function decode(str){
		return str.replace(/(.)(.)$|./g, function(m, a, b){
			if(a){
				return zeropad(dstchars.indexOf(a).toString(2), dstbase_bitlen).substr(-dstchars.indexOf(b))
			}else{
				return zeropad(dstchars.indexOf(m).toString(2), dstbase_bitlen)
			}
		}).replace(new RegExp(`.{${srcbase_bitlen}}`, "g"), function(e){
			return srcchars.charAt(parseInt(e, 2))
		})
	}

	return {
		encode,
		decode,
		srcbase,
		dstbase,
		srcbase_bitlen,
		dstbase_bitlen
	}
}

usage.js

var barc = basetrans("0123", "0123456789abcdefghijklmnopqrstuvwxyz")
var d = "330120"
var e = barc.encode(d)
var r = barc.decode(e)
console.log(d, e, r)