next big number

next_big_number.js

var num = "9123";

var digits = num.split("");

function nextBig(position, digits) {
	var currentNumber = digits[position];
	for (var i = position-1; i >= 0; i--) {
		if (currentNumber > digits[i]) {
			currentNumber = digits[i];
			return i;
		}
	}

	return -1;
}

function nextFinder(j, digits) {
	if (j < 1 || digits.length <= 1) {
		return digits.join("");
	}
	for (var i = j; i >= 0; i--) {
		var pos = nextBig(i, digits);
		if (pos == -1) {
			return digits.join("");
		}
		if (pos !== i) {
			//swap
			var nextBigNum = digits[i];
			digits[i] = digits[pos];
			digits[pos] = nextBigNum;
			return digits.join("");
		}
	}

	return nextFinder(j--, digits);
}

console.log(nextFinder(digits.length - 1, digits));