<?php

use JetBrains\PhpStorm\Pure;

$input_file = '../inputs/day8.txt';
if (file_exists($input_file)) {
	$input = file_get_contents($input_file);
	if ($input != null) {
		$instructions = explode("\n", $input);
		print 'Part 1 answer: ' . solve_part_one($instructions) . "\n";
		print 'Part 2 answer: ' . solve_part_two($instructions) . "\n";
	}
}

function solve_part_one(array $instructions): int
{
	return get_acc(parse_instructions($instructions));
}

function solve_part_two(array $instructions): int
{
	$original_instructions = parse_instructions($instructions);

	foreach ($original_instructions as $idx => $instruction) {
		$altered_instructions = $original_instructions;
		switch ($instruction[0]) {
			case 'jmp':
				$altered_instructions[$idx][0] = 'nop';
				break;
			case 'nop':
				$altered_instructions[$idx][0] = 'jmp';
				break;
		}

		if (terminates_correctly($altered_instructions))
			return get_acc($altered_instructions);
	}

	return 0;
}

#[Pure] function terminates_correctly(array $instructions): bool
{
	$acc = 0;
	$ip = 0;
	$execution_count = [];

	while ($ip < count($instructions)) {
		if (array_key_exists($ip, $execution_count) && $execution_count[$ip] != 0)
			return false;
		$execution_count[$ip] = 1;
		[$acc, $ip] = run_instruction($instructions[$ip], [$acc, $ip]);
	}

	return true;
}

#[Pure] function get_acc(array $instructions): int
{
	$acc = 0;
	$ip = 0;
	$execution_count = [];

	while ($ip < count($instructions)) {
		if (array_key_exists($ip, $execution_count) && $execution_count[$ip] != 0)
			return $acc;
		$execution_count[$ip] = 1;
		[$acc, $ip] = run_instruction($instructions[$ip], [$acc, $ip]);
	}

	return $acc;
}

function run_instruction(array $instruction, array $regs): array
{
	[$inst, $arg] = $instruction;
	[$acc, $ip] = $regs;
	switch ($inst) {
		case 'acc':
			$acc += $arg;
			$ip += 1;
			break;
		case 'jmp':
			$ip += $arg;
			break;
		case 'nop':
			$ip += 1;
			break;
	}

	// print 'Executed ' . $inst . ', $acc = ' . $acc . ', $ip = ' . $ip . "\n";

	return [$acc, $ip];
}

function parse_instructions(array $instructions): array
{
	$parsed_instructions = [];

	foreach ($instructions as $instruction) {
		if (preg_match('/^(acc|jmp|nop) ([+\-\d]+)$/', trim($instruction), $matches)) {
			[, $inst, $arg] = $matches;
			$parsed_instructions[] = [$inst, $arg];
		}
	}

	return $parsed_instructions;
}