This December I participated in Advent of Code.

This is a case of, part 1 can be brute forced, part 2 can’t. Part 1 is straightforward. Simply keep a list of the numbers from each step. In part 2 it matters, that the order doesn’t matter. If I may say so. Instead of keeping a list of the numbers, I keep a count. How many 0s, how many 1s etc. So e.g. in a step I say, I previously had x 0s, now I have x 1s. I previously had y 1s, now I have y 2024s. I previously had z 2024s, now I have z 20s and z 24s. Etc. Adding everything up, because there might be more than one source for the numbers in the next step.

I liked this one!

Part 1:

<?php

$input = file_get_contents('./d11input2.txt', true);

foreach(preg_split("/((\r?\n)|(\r\n?))/", $input) as $line) {
	if(strlen($line)>2) {
		$num1 = explode(" ", $line);
	}
}

/*

    If the stone is engraved with the number 0, it is replaced by a stone engraved with the number 1.
    If the stone is engraved with a number that has an even number of digits, it is replaced by two stones. The left half of the digits are engraved on the new left stone, and the right half of the digits are engraved on the new right stone. (The new numbers don't keep extra leading zeroes: 1000 would become stones 10 and 0.)
    If none of the other rules apply, the stone is replaced by a new stone; the old stone's number multiplied by 2024 is engraved on the new stone.

*/

for($i=0;$i<25;$i++) { 
	for($j=0;$j<sizeof($num1);$j++) { 
		$num = $num1[$j];
		if($num == 0) {
			$num2[] = 1;
			continue;
		}
		$numl = strlen((string) $num);
		if($numl % 2 == 0) {
			$num2[] = (int) substr((string) $num, 0, $numl/2);
			$num2[] = (int) substr((string) $num, $numl/2, $numl/2);
			continue;
		}
		$num2[] = $num * 2024;
	} 
	$num1 = $num2;
	$num2 = array();
}

print(sizeof($num1)."\n");

?>

Part 2:

<?php

$input = file_get_contents('./d11input2.txt', true);

foreach(preg_split("/((\r?\n)|(\r\n?))/", $input) as $line) {
	if(strlen($line)>2) {
		$num1 = explode(" ", $line);
	}
}

for($j=0;$j<sizeof($num1);$j++) {
	$num = $num1[$j];
	if(isset($num2[$num])) {
		$num2[$num]++;
	} else {
		$num2[$num] = 1;
	}
}
$num1 = $num2;
$num2 = array();

/*

    If the stone is engraved with the number 0, it is replaced by a stone engraved with the number 1.
    If the stone is engraved with a number that has an even number of digits, it is replaced by two stones. The left half of the digits are engraved on the new left stone, and the right half of the digits are engraved on the new right stone. (The new numbers don't keep extra leading zeroes: 1000 would become stones 10 and 0.)
    If none of the other rules apply, the stone is replaced by a new stone; the old stone's number multiplied by 2024 is engraved on the new stone.

*/

for($i=0;$i<75;$i++) { 
	foreach($num1 as $num => $nom) { 
		if($num == 0) {
			if(isset($num2[1])) {
				$num2[1] += $nom;
			} else {
				$num2[1] = $nom;
			}
			continue;
		}
		$numl = strlen((string) $num);
		if($numl % 2 == 0) {
			$numa = (int) substr((string) $num, 0, $numl/2);
			if(isset($num2[$numa])) {
				$num2[$numa] += $nom;
			} else {
				$num2[$numa] = $nom;
			}
			$numb = (int) substr((string) $num, $numl/2, $numl/2);
			if(isset($num2[$numb])) {
				$num2[$numb] += $nom;
			} else {
				$num2[$numb] = $nom;
			}
			continue;
		}
		$numc = $num * 2024;
		if(isset($num2[$numc])) {
			$num2[$numc] += $nom;
		} else {
			$num2[$numc] = $nom;
		}
	} 
	$num1 = $num2;
	$num2 = array();
}

print(array_sum($num1)."\n");

?>