number-theoretic/primes

/**
 * Converts an array index to a square Ulam spiral index.
 *
 */

export function stoa(i, n) {
    let line = (i / n) | 0;
    let col = i % n;

    if (n % 2 === 0) {
        line = n - line - 1;
        col = n - col - 1;
    }

    let side_col = 1;
    let side_line = 1;

    let ring_index;
    if (col < n - line) {
        ring_index = Math.min(line, col);
    } else if (col > line) {
        ring_index = n - col - 1;
        side_line = -1;
    } else {
        ring_index = n - line - 1;
        side_col = -1;
    }

    const sub_size = n - ring_index * 2;

    return (
        sub_size * sub_size -
        (sub_size - 1) * 2 +
        side_line * (line - ring_index) -
        side_col * (col - ring_index) -
        1
    );
}