Largest All One Square in PHP

<?php
declare(strict_types=1);
// Largest All One Square in PHP
//
// PURPOSE
// Each all-one square ending at a cell extends the smallest neighboring square from above,
// left, and upper-left.
//
// METHOD
// Use a single padded row and retain the previous diagonal. A zero cell resets its entry; a
// one cell adds one to the minimum of three neighbors.
//
// COMPLEXITY
// O(rows * cols) time, O(cols) auxiliary space.
//
// CONTRACT
// Flattened 0/1 matrix; rows,cols>=0. Returns SIDE LENGTH, not area.
// Public APIs infer array lengths except in C, which accepts pointer/length pairs. Vertex
// counts and matrix dimensions remain explicit. Arrays must contain signed 32-bit integers.
// Sorted order, correct tree topology, and caller-owned data-structure invariants are
// preconditions; checking them on every query would change query complexity.
// Scalar and shape checks are included. Value-range checks are included where needed to
// prevent invalid indexing or nontermination. Allocation sizes are capped at 4,000,000
// workspace entries. Arithmetic outside Python raises an error on 32-bit overflow; Python
// uses arbitrary precision. Native allocation failures propagate. In-place outputs may be
// partly updated after arithmetic failure. These are single-threaded, in-memory
// implementations.
//
// LANGUAGE DETAILS
// PHP 8.1+ on a 64-bit runtime: packed arrays, strict scalar types, read-only inputs by
// value using copy-on-write, references only for outputs, and checked 32-bit arithmetic.
// Invalid input and overflow throw exceptions. PHP arrays have higher overhead than native
// packed buffers in C/Java.
//
// EXAMPLE
// Expected return: 2.
// Run: php example.php
// Tests for this exact source artifact run before publication. The accompanying source
// archive includes unit cases, runtime results, and source hashes.

function alg_require(bool $valid): void { if (!$valid) throw new InvalidArgumentException("invalid algorithm input"); }
function alg_checked(int|float $v): int { if ($v < -2147483648 || $v > 2147483647 || floor($v) != $v) throw new OverflowException("32-bit result overflow"); return (int)$v; }
function alg_add(int $a,int $b): int { return alg_checked($a+$b); }
function alg_sub(int $a,int $b): int { return alg_checked($a-$b); }
function alg_mul(int $a,int $b): int { return alg_checked($a*$b); }
function alg_size(int $n): int { alg_require($n>=0 && $n<=4000000); return $n; }

function largest_square(array $grid, int $rows, int $cols): int {
    alg_require(array_is_list($grid));
    alg_require((count($grid) == alg_mul($rows, $cols)));
    alg_require((($rows >= 0) && ($cols >= 0) && ($rows <= 1000000) && ($cols <= 1000000)));
    $dp = array_fill(0, alg_size(alg_add($cols, 1)), 0);
    $r = 0;
    $c = 0;
    $diagonal = 0;
    $old = 0;
    $best = 0;
    while (($r < $rows)) {
        $diagonal = 0;
        $c = 1;
        while (($c <= $cols)) {
            $old = $dp[$c];
            if (($grid[alg_sub(alg_add(alg_mul($r, $cols), $c), 1)] == 1)) {
                $dp[$c] = alg_add(1, min($dp[$c], min($dp[alg_sub($c, 1)], $diagonal)));
                $best = max($best, $dp[$c]);
            } else {
                $dp[$c] = 0;
            }
            $diagonal = $old;
            $c = alg_add($c, 1);
        }
        $r = alg_add($r, 1);
    }
    return $best;
}

$grid = [1, 1, 0, 1, 1, 1, 0, 1, 1];
$rows = 3;
$cols = 3;
$result = largest_square($grid, $rows, $cols);
echo $result, PHP_EOL;