#!/usr/local/bin/php
<?php

class JoltageAdapters {
    public $adapters = [];
    public int $phone;
    private $cache = [];

    public function __construct() {
        $this->adapters = file(__DIR__.'/input.txt');
        $this->adapters = array_map('intval', $this->adapters);
        $this->phone = max($this->adapters) + 3;
        array_push($this->adapters, $this->phone);
        sort($this->adapters);
    }

    public function part1() : int
    {
        $jolts = 0;
        $three_gaps = 0;
        $one_gaps = 0;
        foreach ($this->adapters as $adapter) {
            if ($adapter - $jolts < 1 || $adapter - $jolts > 3) {
                throw new Exception("no chain possible between $adapter and $jolts", 1);
            }
            if ($adapter - $jolts == 1) {
                $one_gaps += 1;
            } elseif ($adapter - $jolts == 3) {
                $three_gaps += 1;
            }
            $jolts = $adapter;
        }
        return $three_gaps * $one_gaps;
    }

    public function part2() : int {
        return $this->count_combos(0, $this->phone);
    }

    private function count_combos($start, $end) : int {
        if ($start + 3 == $end) {
            return 1;
        } elseif ($start + 3 > $end) {
            throw new Exception("should be unreachable", 1);
        }

        $combos = 0;
        foreach ([1, 2, 3] as $offset) {
            if (in_array($start + $offset, $this->adapters)) {
                if (!isset($this->cache[$start + $offset])) {
                    $this->cache[$start + $offset] = $this->count_combos($start + $offset, $end);
                }
                $combos += $this->cache[$start + $offset];
            }
        }
        return $combos;
    }
}

$ja = new JoltageAdapters();
$part1 = $ja->part1();
echo("Part 1 solution: $part1\n");

$part2 = $ja->part2($ja->adapters);
echo("Part 2 solution: $part2\n");