12th day both parts

12/part2.php

@@ -0,0 +1,31 @@
+<?php
+$input = file('input', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
+$input = array_map(fn($line) => explode('-', $line), $input);
+
+$adjacencyList = [];
+foreach ($input as $key => $item) {
+    if ($item[1] != 'start') $adjacencyList[$item[0]][] = $item[1];
+    if ($item[0] != 'start') $adjacencyList[$item[1]][] = $item[0];
+}
+$history = [];
+echo track($adjacencyList, "start", $history, 0);
+
+function track($adjacencyList, $cave, $history, $doubled)
+{
+    $count = 0;
+    $history[] = $cave;
+    foreach ($adjacencyList[$cave] as $path) {
+        //small cave, not already visited
+        if ($path == "start") {
+        } elseif (strtolower($path) === $path && in_array($path, $history)) {
+            if ($doubled == 0) {
+                $count += track($adjacencyList, $path, $history, 1);
+            }
+        } elseif ($path == "end") {
+            $count += 1;
+        } else {
+            $count += track($adjacencyList, $path, $history, $doubled);
+        }
+    }
+    return $count;
+}