aureooms/js-integer-big-endian

import assert from 'assert';

/**
 * Adds a big endian array to another.
 * Wraps on overflow. |A| >= |B|.
 *
 * @param {Number} r base (radix)
 * @param {Array} a first operand
 * @param {Number} ai a left
 * @param {Number} aj a right
 * @param {Array} b second operand
 * @param {Number} bi b left
 * @param {Number} bj b right
 */

export default function _iadd(r, a, ai, aj, b, bi, bj) {
    assert(r >= 2);
    assert(ai >= 0 && aj <= a.length);
    assert(bi >= 0 && bj <= b.length);
    assert(aj - ai >= bj - bi);

    let C = 0;

    while (--bj >= bi) {
        const T = a[--aj] + b[bj] + C;
        a[aj] = T % r;
        C = (T >= r) | 0;
    }

    if (C !== 0) {
        while (--aj >= ai && a[aj] === r - 1) a[aj] = 0;
        if (aj >= ai) ++a[aj];
    }
}