arithmetic-operations-for/naturals-big-endian

import assert from 'assert';

/**
 * Subtracts B from A, |A| >= |B|.
 * Wraps.
 *
 * @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 _isub(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) {
        --aj;
        const T = C;
        C = (a[aj] < b[bj] + T) | 0;
        a[aj] = a[aj] - b[bj] + (C * r - T);
    }

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