Find Pi using Wallis formula

wallis.php

<?php

/**
 * Wallis formula
 *
 *           2     2     4     4     6     6     8     8
 * pi = 2 x --- x --- x --- x --- x --- x --- x --- x --- x ...
 *           1     3     3     5     5     7     7     9
 */

declare(strict_types=1);

namespace App;

include __DIR__ . '/../vendor/autoload.php';

use Generator;
use loophp\collection\Collection;

$precision = 10 ** -7;

$pi = Collection::unfold(static fn (int $n = 0): int => $n + 1)
    ->map(
        static fn (int $i): int => $i * 2,
        static fn (int $i): array => [$i, $i],
        static fn (array $chunk): array => [$chunk[0] / ($chunk[0] - 1), $chunk[1] / ($chunk[1] + 1)]
    )
    ->unwrap()
    ->reduction(
        static fn (float $carry, float $i): float => $carry * $i,
        2
    )
    ->window(1)
    ->drop(1)
    ->until( // Expliquer que x(n+1) - x(n) tend vers 0 ... voirs cours math elem Troestler
        static fn (array $v): bool => abs($v[0] - $v[1]) < ($precision / 2)
    )
    ->unwindow()
    ->normalize()
    ->last();

dump($pi->current()); // 3.1415926035884