day_three_part_one.php

<?php

$number = 361527;

$bottomRoot = floor(sqrt($number));

// Skip even numbers because they aren't corners.
if ($bottomRoot % 2 == 0) {
    $bottomRoot = $bottomRoot-1;
}

$bottomSquare = $bottomRoot * $bottomRoot;
$topSquare = ($bottomRoot + 2) * ($bottomRoot + 2);
$topRoot = sqrt($topSquare);

$verticalOffset = (($bottomRoot - 1) / 2) + 1;

$pointer = ($number - $bottomSquare);

$height = $topRoot - 1;

$middle = $height / 2;

$horizontalOffset = ($pointer % $height) - $middle;

return $horizontalOffset + $verticalOffset;