advent_of_code_2020/day_11/part_2.php

<?php

$seats = explode("\n", file_get_contents('input.txt'));

array_walk($seats, static function(&$seat) {
    $seat = str_split($seat);
});

function checkAdjacentSeats($seats, $row, $col)
{
    $adjacent_seats = [
        'L' => 0,
        '.' => 0,
        '#' => 0,
    ];
    // count left
    $i = $col - 1;
    while ($i >= 0) {
        $new_seat = $seats[$row][$i];
        $adjacent_seats[$new_seat]++;
        if ('.' !== $new_seat) {
            break;
        }
        $i--;
    }
    // count up
    $i = $row - 1;
    while ($i >= 0) {
        $new_seat = $seats[$i][$col];
        $adjacent_seats[$new_seat]++;
        if ('.' !== $new_seat) {
            break;
        }
        $i--;
    }
    // count right
    $i = $col + 1;
    while ($i < count($seats[$row])) {
        $new_seat = $seats[$row][$i];
        $adjacent_seats[$new_seat]++;
        if ('.' !== $new_seat) {
            break;
        }
        $i++;
    }
    // count bottom
    $i = $row + 1;
    while ($i < count($seats)) {
        $new_seat = $seats[$i][$col];
        $adjacent_seats[$new_seat]++;
        if ('.' !== $new_seat) {
            break;
        }
        $i++;
    }
    // diag up left
    $i = $row - 1;
    $j = $col - 1;
    while (array_key_exists($i, $seats) && array_key_exists($j, $seats[$i])) {
        $new_seat = $seats[$i][$j];
        $adjacent_seats[$new_seat]++;
        if ('.' !== $new_seat) {
            break;
        }
        $i--;
        $j--;
    }
    // diag up right
    $i = $row - 1;
    $j = $col + 1;
    while (array_key_exists($i, $seats) && array_key_exists($j, $seats[$i])) {
        $new_seat = $seats[$i][$j];
        $adjacent_seats[$new_seat]++;
        if ('.' !== $new_seat) {
            break;
        }
        $i--;
        $j++;
    }
    // diag bottom right
    $i = $row + 1;
    $j = $col + 1;
    while (array_key_exists($i, $seats) && array_key_exists($j, $seats[$i])) {
        $new_seat = $seats[$i][$j];
        $adjacent_seats[$new_seat]++;
        if ('.' !== $new_seat) {
            break;
        }
        $i++;
        $j++;
    }
    // diag bottom left
    $i = $row + 1;
    $j = $col - 1;
    while (array_key_exists($i, $seats) && array_key_exists($j, $seats[$i])) {
        $new_seat = $seats[$i][$j];
        $adjacent_seats[$new_seat]++;
        if ('.' !== $new_seat) {
            break;
        }
        $i++;
        $j--;
    }

    return $adjacent_seats;
}

function makeARound($seats)
{
    $new_seats = $seats;
    for ($row = 0, $row_count = count($seats); $row < $row_count; $row++) {
        for ($col = 0, $col_count = count($seats[0]); $col < $col_count; $col++) {
            $current = $seats[$row][$col];
            if ('.' === $current) {
                continue;
            }

            $adjacent_seats = checkAdjacentSeats($seats, $row, $col);
            if ('L' === $current) {
                // check adjacent seat, if all empty => it becomes # else nothing
                if ($adjacent_seats['#'] === 0) {
                    $new_seats[$row][$col] = '#';
                }
            }

            if ('#' === $current) {
                // check adjacent seat, if count # >= 4 => it becomes L else nothing
                if ($adjacent_seats['#'] >= 5) {
                    $new_seats[$row][$col] = 'L';
                }
            }
        }
    }
    return $new_seats;
}

function countOccupiedSeats($seats)
{
    $occupied_seats_count = 0;
    foreach ($seats as $row) {
        $row_values = array_count_values($row);
        $occupied_seats_count += array_key_exists('#', $row_values) ? $row_values['#'] : 0;
    }
    return $occupied_seats_count;
}

function seatsAreIdentical($seats, $new_seats)
{
    foreach ($seats as $row_i => $row) {
        foreach ($row as $cell_i => $cell) {
            if ($new_seats[$row_i][$cell_i] !== $cell) {
                return false;
            }
        }
    }

    return true;
}

function displaySeats($seats)
{
    foreach ($seats as $row) {
        echo implode('', $row). "\n";
    }
    echo str_repeat('=', count($seats[0]))."\n";
}

$new_seats = [];
$count_loop = 0;
while (true) {
    $new_seats = makeARound($seats);
    if (seatsAreIdentical($seats, $new_seats)) {
        echo sprintf('%u occupied seats found in %u round', countOccupiedSeats($new_seats), $count_loop);
        exit;
    }
    $seats = $new_seats;
    $count_loop++;
}