advent-of-code/2024/day1/index.php

<?php declare(strict_types=1);

function readLines (): array
{
	$fp = fopen("./input.txt", 'r');
	$lines = [];
	if ($fp)
	{
		while (($buffer = fgets($fp)))
		{
			$lines[] = $buffer;
		}
		
		fclose($fp);
	}
	
	return $lines;
}

function parseLists (): array
{
	$arr1 = [];
	$arr2 = [];
	$lines = readLines();
	
	echo "Number of input lines: " . count($lines) . "\n";
	
	foreach ($lines as $line)
	{
		[$entry1, $entry2] = explode('   ', $line);
		$arr1[] = (int)$entry1;
		$arr2[] = (int)$entry2;
	}
	
	return [$arr1, $arr2];
}

function diffSum (array $list1, array $list2): int
{
	sort($list1, SORT_NUMERIC);
	sort($list2, SORT_NUMERIC);
	
	$total = count($list1);
	$sum = 0;
	
	for ($i = 0; $i < $total; $i++)
	{
		$sum += abs($list1[$i] - $list2[$i]);
	}
	
	return $sum;
}

function similarityScore (array $list1, array $list2): int
{
	$sum = 0;
	foreach ($list1 as $search)
	{
		$rightList = array_filter($list2, fn (int $item) => $item === $search);
		$sum += ($search * count($rightList));
	}
	
	return $sum;
}

[$list1, $list2] = parseLists();

$sumOfDifference = diffSum($list1, $list2);
$similarityScore = similarityScore($list1, $list2);

echo "Part 1: Total distance: $sumOfDifference\n";
echo "Part 2: Similarity score: $similarityScore\n";