<?php
$input_file = '../inputs/day5.txt';
if (file_exists($input_file)) {
	$input = file_get_contents($input_file);
	if ($input != null) {
		$passes = explode("\n", $input);
		print 'Part 1 answer: ' . solve_part_one($passes) . "\n";
		print 'Part 2 answer: ' . solve_part_two($passes) . "\n";
	}
}

function solve_part_one(array $passes): int
{
	$valid_passes = parse_passes($passes);
	$max_pass = 0;

	foreach ($valid_passes as $pass)
		if ($pass[2] > $max_pass)
			$max_pass = $pass[2];

	return $max_pass;
}

function solve_part_two(array $passes): int
{
	$valid_passes = parse_passes($passes);
	$sorted_ids = array_map(fn ($pass) => $pass[2], $valid_passes);
	sort($sorted_ids);

	for ($i = 0; $i < count($sorted_ids); $i++)
		if ($sorted_ids[$i] - $sorted_ids[0] != $i)
			return $sorted_ids[$i] - 1;

	return 0;
}

function parse_passes(array $passes): array
{
	$parsed_passes = [];

	foreach ($passes as $pass) {
		if (preg_match('/^([FB]{7})([LR]{3})$/', trim($pass), $matches) > 0) {
			$min_row = 0;
			$max_row = 127;
			$min_col = 0;
			$max_col = 7;

			for ($i = 0; $i < strlen($matches[1]); $i++) {
				$tot_rows = $max_row + 1 - $min_row;
				$new_tot_rows = $tot_rows / 2;
				if ($matches[1][$i] === 'F') {
					$max_row -= $new_tot_rows;
				} else {
					$min_row += $new_tot_rows;
				}
			}

			for ($i = 0; $i < strlen($matches[2]); $i++) {
				$tot_cols = $max_col + 1 - $min_col;
				$new_tot_cols = $tot_cols / 2;
				if ($matches[2][$i] === 'L') {
					$max_col -= $new_tot_cols;
				} else {
					$min_col += $new_tot_cols;
				}
			}

			$parsed_passes[] = [$min_row, $min_col, $min_row * 8 + $min_col];
		}
	}

	return $parsed_passes;
}