retention

retention.php

<?php

//$retentionByMonth = [100, 20, 25, 20]; // 3
$retentionByMonth = [100, 80, 75, 72, 70]; // 4
//$usersByMonth = array_fill(0, 12 * 10, 4000);
$usersByMonth = array_fill(0, 12 * 10, 1000);

$fill = $retentionByMonth[count($retentionByMonth) - 1];
for ($i = 0, $c = count($usersByMonth) - count($retentionByMonth); $i < $c; $i++) {
    $retentionByMonth[] = $fill;
}

$matrix = [];
for ($row = 0, $c = count($retentionByMonth); $row < $c; $row++) {
    for ($column = 0; $column < $c; $column++) {
        if ($column === $row) {
            $matrix[$column][$row] = $usersByMonth[$row];
        } else {
            $matrix[$column][$row] = null;
        }
    }
}


for ($row = 0, $ic = count($matrix); $row < $ic; $row++) {
    for ($column = 0, $jc = count($matrix[$column]); $column < $jc; $column++) {
        if ($column === $row) {
            break;
        }
        foreach ($matrix[$column] as $key => $value) {
            if ($value !== null) {
                break;
            }
        }
        $matrix[$column][$row] = $matrix[$column][$key] / 100 * $retentionByMonth[$row - $key];
    }
}

$sum = [];
for ($row = 0, $ic = count($matrix); $row < $ic; $row++) {
    $sum[$row] = array_sum(array_column($matrix, $row));
}


echo implode(", ", $sum);