<?php
$input_file = '../inputs/day13.txt';
if (file_exists($input_file)) {
	$input = file_get_contents($input_file);
	if ($input != null) {
		$in = array_map('trim', explode("\n", $input));
		print 'Part 1 answer: ' . solve_part_one($in) . "\n";
		print 'Part 2 answer: ' . solve_part_two($in) . "\n";
	}
}

function solve_part_one(array $in): int
{
	[$arrival_time, $lines] = $in;
	$trimmed_bus_lines = array_map('trim', explode(',', $lines));
	$active_lines = array_filter($trimmed_bus_lines, fn ($line) => $line != 'x');
	$bus_arrival_times = array_map(get_closest_arrival_time_to($arrival_time), $active_lines);
	[$min_key] = array_keys($bus_arrival_times, min($bus_arrival_times));
	return $active_lines[$min_key] * ($bus_arrival_times[$min_key] - $arrival_time);
}

function solve_part_two(array $in): int
{
	[, $lines] = $in;
	$trimmed_bus_lines = array_map('trim', explode(',', $lines));
	$active_lines = array_filter($trimmed_bus_lines, fn ($line) => $line != 'x');
	return get_first_consecutive_time($active_lines)[0];
}

function get_closest_arrival_time_to(int $target): callable
{
	return function (int $x) use ($target): int {
		$i = 1;
		while ($i * $x < $target) {
			$i += 1;
		}
		return $i * $x;
	};
}

function get_first_consecutive_time(array $lines): array
{
	if (count($lines) == 1)
		return [0, $lines[0]];
	else {
		$keys = array_keys($lines);
		$last_key = $keys[count($keys) - 1];
		$last_elem = array_pop($lines);
		[$previous_time, $previous_lcm] = get_first_consecutive_time($lines);
		$timestamp = $previous_time;
		while (true) {
			if (($timestamp + $last_key) % $last_elem === 0)
				break;
			else
				$timestamp += $previous_lcm;
		}
		return [$timestamp, $previous_lcm * $last_elem];
	}
}