Advent of code 2021/24

Ajax Direct

/**
 * The program consists of 14 repeats of the following :
 *    $x = ($z % 26) + $b;
 *    $z = (int) ($z / $c);
 *    if ($w != $x) $z = ($z * 26) + ($w + $a);
 *    return $z;
 * With $w as the nth number and $a, $b, $c as numbers from the input commands.
 *
 * We have two cases :
 *    1. $b is positive and $c is 1 => $z will always increase
 *    2. $b is negative and $c is 26 => $z should always decrease, $a is not used
 * There are 7 of each, which should balance eachother to reach 0 at the end.
 */

$cmd = $input->split("inp")->filter()->map(function ($block) {
    $n = $block->numbers();
    $d = $n[2] == 26;
    return [$d ? null : $n[9], $d ? $n[3] : null];
})->values();

function solve($cmd, $p2 = false) {
    $num = [];
    $stack = [];
    $digit = $p2 ? 1 : 9;
    $calc = $p2 ? "max" : "min";

    foreach ($cmd as $i=>[$a, $b]) {
        if ($a) {
            $stack[] = [$i, $a];
        } else {
            [$ia, $a] = array_pop($stack);
            $diff = $a + $b;
            $num[$ia] = $calc($digit, $digit - $diff);
            $num[$i] = $calc($digit, $digit + $diff);
        }
    }

    return set($num)->sortKeys()->join("");
}

// ==================================================
// > SOLUTION
// ==================================================
$solution_1 = solve($cmd);
$solution_2 = solve($cmd, true);