✨ Finit le 15e jour

day_15/part_1.php

@@ -0,0 +1,33 @@
+<?php
+
+$starting_numbers = explode(',', file_get_contents('input.txt'));
+
+$numbers = array_map('intval', $starting_numbers);
+$number = array_pop($numbers);
+for ($i = count($starting_numbers); $i <= 2020; $i++) {
+    $number = newNumber($number);
+}
+
+echo $number;
+
+function newNumber(int $number = 0)
+{
+    global $numbers;
+    if (!in_array($number, $numbers, true)) {
+        $numbers[] = $number;
+
+        return $number;
+    }
+
+    $indexes = array_keys($numbers, $number);
+
+    if (count($indexes) === 1) {
+        $numbers[] = 0;
+        return 0;
+    }
+
+    $new_number = $indexes[count($indexes) - 1] - $indexes[count($indexes) - 2];
+    $numbers[] = $new_number;
+
+    return $new_number;
+}