This December I participated in Advent of Code.

One of those days, where I probably grumbled a lot. Part 1 was easy. I chose an algorithm, where I went through a list of all the expressions, checked whether the gate’s output could be calculated (and if yes, deleted the expression from the list), looped back and did it again, until the list was empty. Part 2… I tried several different approaches. Reading hints from others, I ultimately used a free online diagram tool, Mermaid, that accepts text input. Perfect. I wrote a small program to turn the input into something Mermaid could understand (every value, value, gate, value, turned into 2 x value, arrow, value). Then I tried to analyze this diagram. Printed a lot of full adders. And began copying the diagram on to these. Every time the diagram wouldn’t fit, I’d found a swap. Then it took some thinking to figure out how to swap back. But I got there!

Part 1:

<?php

///////////////////////////////////////

$input = file_get_contents('./d24input2.txt', true);

foreach(preg_split("/((\r?\n)|(\r\n?))/", $input) as $line) {
  if(strlen($line)>2) {
    // x00: 1
    if(preg_match("/^(\w+): (\d)$/", $line, $m)) {
      $values[$m[1]] = $m[2];
    }
    // ntg XOR fgs -> mjb
    if(preg_match("/->/", $line, $m)) {
      $m = explode(" ", $line);
      $expr[] = $m;
    }
  }
}

while(sizeof($expr) > 0) {
  foreach($expr as $key => $exp) {
    $val1 = $exp[0];
    $val2 = $exp[2];
    if(isset($values[$val1]) && isset($values[$val2])) {
      $gate = $exp[1];
      $val3 = $exp[4];
      switch($gate) {
        case "AND":
          $values[$val3] =
            $values[$val1] * $values[$val2];
          break;
        case "OR":
          $values[$val3] =
            $values[$val1] + $values[$val2] 
            - $values[$val1] * $values[$val2];
          break;
        case "XOR":
          $values[$val3] =
            ($values[$val1] + $values[$val2]) % 2;
          break;
      }
      unset($expr[$key]);
    }
  }
}

$allzs = $values;
foreach($allzs as $key => $posz) {
  if(!preg_match("/z/", $key, $m)) {
    unset($allzs[$key]);
  }
}

print_r($allzs);

$sum = 0;
foreach($allzs as $key => $z) {
  $pos = (int) str_replace("z", "", $key);
  $sum += $z * pow(2, $pos);
}

print("$sum\n");
?>

Part 2:

Big diagram. Small diagrams.