<?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;
}