aureooms/js-prime

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

export function atos(i, n) {
    const s = Math.sqrt(i) | 0;
    const l = s;
    const r = s + 1;
    const L = l * l;
    const R = r * r;
    let x = ((n - 1) / 2) | 0;
    let y = (n / 2) | 0;

    if (s % 2 === 1) {
        if ((i < (L + R) / 2) | 0) {
            x += ((s / 2) | 0) + 1;
            y += ((l / 2) | 0) + L - i;
        } else {
            x -= ((r / 2) | 0) - R + i;
            y -= ((s / 2) | 0) + 1;
        }
    } else if ((i < (L + R) / 2) | 0) {
        x -= (s / 2) | 0;
        y -= ((l / 2) | 0) + L - i;
    } else {
        x += ((r / 2) | 0) + 1 - R + i;
        y += (s / 2) | 0;
    }

    return n * y + x;
}