codernight.php

<?php

$lines = getLines();
$ones = getOnes($lines);
$groups = buildGroups($ones);

$groupCounts = array_map(function ($group) {
    return count($group);
}, $groups);

sort($groupCounts);

echo end($groupCounts), PHP_EOL;

function getLines()
{
    $lines = [];

    while($f = fgets(STDIN)){
        $lines[] = explode(' ', $f);
    }

    return array_slice($lines, 2);
}

function getOnes($matrix)
{
    $ones = [];

    foreach ($matrix as $rowIndex => $row) {
        foreach ($row as $columnIndex => $column) {
            $cell = (int) trim($column);

            if ($cell === 1) {
                $index = oneKey($rowIndex, $columnIndex);

                $ones[$index] = [$rowIndex, $columnIndex];
            }
        }
    }

    return $ones;
}

function oneKey($row, $column)
{
    return "{$row}-{$column}";
}

function buildGroups($ones)
{
    $groups = [];

    foreach ($ones as $key => $coords) {
        $groups[] = findNeighbors($ones, $coords, [$key => $coords]);
    }

    return $groups;
}

function findNeighbors($ones, $coords, $foundNeighbors)
{
    $neighbors = neighbors($coords);

    foreach ($neighbors as $neighborKey => $neighborArr) {
        if (isset($ones[$neighborKey])) {
            unset($ones[$neighborKey]);

            $foundNeighbors[$neighborKey] = $neighborArr;
            $foundNeighbors = findNeighbors($ones, $neighborArr, $foundNeighbors);
        }

    }

    return $foundNeighbors;
}

function neighbors($position)
{
    list($x, $y) = $position;

    $northY = $y - 1;
    $southY = $y + 1;
    $eastX = $x + 1;
    $westX = $x - 1;

    $northeast = [$eastX, $northY];
    $southeast = [$eastX, $southY];
    $southwest = [$westX, $southY];
    $northwest = [$westX, $northY];
    $north = [$x, $northY];
    $east = [$eastX, $y];
    $south = [$x, $southY];
    $west = [$westX, $y];

    return [
        oneKey(...$northeast) => $northeast,
        oneKey(...$east) => $east,
        oneKey(...$southeast) => $southeast,
        oneKey(...$south) => $south,
        oneKey(...$southwest) => $southwest,
        oneKey(...$west) => $west,
        oneKey(...$northwest) => $northwest,
        oneKey(...$north) => $north,
    ];
}