Find maximum possible time HH:MM by permuting four given digits

Question

I recently took a coding test for a promotion at work. This was one of the tasks I really struggled with and was wondering what is the best way to do this. I used a load of

Answer 1

It's not elegant or pretty, but it seems to do the trick!

const NOT_POSSIBLE = 'NOT POSSIBLE';

function generate(A, B, C, D) {
	var args = [A, B, C, D];
	var idx = -1;
	var out = NOT_POSSIBLE;
	var firstN, secondN;

	MAIN: {
		args.sort(NUMERIC_ASCENDING);
		// number has to start with 0, 1 or 2
		if (args[0] > 2) break MAIN;

		while (args[++idx] < 3) {}

		// take the higest 2, 1, or 0
		firstN = args[--idx];
		args = pop(args, idx);

		if (firstN === 2) {
			// make sure that the first number doesn't exceed 23 and
			// the second number 59
			if (args[0] > 3 || args[0] > 1 && args[1] > 5)
				break MAIN;
			// advance to the first number < 3 or the length
			idx = 0;
			while (args[++idx] < 3){}
		} else {
			// much simpler if we have a 0 or 1, take the biggest n remaining
			idx = args.length;
		}

		secondN = args[--idx];
		args = pop(args, idx);
		// if minutes number is too large, swap
		if (args[0] > 5) {
			out = '' + secondN + args[1] + ':' + firstN + args[0];
		} else {
			// if bottom number is low enough, swap for more minutes
			out = '' + firstN + secondN + (args[1] < 6 ? ':' + args[1] + args[0] : ':' + args[0] + args[1]);
		}
	}
	return out;
}

// numeric comparator for sort
function NUMERIC_ASCENDING(x, y) {
	return x > y ? 1 : y > x ? -1 : 0;
}

// specialized "array pop" I wrote out longhand that's very optimized; might be cheating =D
function pop(arr, target) {
	switch (arr.length) {
	case 3:
		switch (target) {
		case 0: return [arr[1], arr[2]];
		case 1: return [arr[0], arr[2]];
		default: return [arr[0], arr[1]];
		}
	case 4:
		switch (target) {
		case 0: return [arr[1], arr[2], arr[3]];
		case 1: return [arr[0], arr[2], arr[3]];
		case 2: return [arr[0], arr[1], arr[3]];
		default: return [arr[0], arr[1], arr[2]];
		}
	}
}

/* --------------- Start Speed Test --------------------- */
let startTime = Math.floor(Date.now());
let times = 10000;
let timesHolder = times;

while (times--) {
  let A = randNum();
  let B = randNum();
  let C = randNum();
  let D = randNum();
  generate(A, B, C, D);
  if (times == 0) {
    let totalTime = Math.floor(Date.now()) - startTime;
    let msg = timesHolder + ' Call Finished Within -> ' + totalTime + ' ms <-';
    console.log(msg);
  }
}
function randNum() {
  return Math.floor(Math.random() * (9 - 0 + 1)) + 0;
}
/* --------------- END Speed Test --------------------- */