aureooms/js-integer-big-endian

import assert from 'assert';

/**
 * Subtracts two big endian arrays, |C| >= |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
 * @param {array} c result, must be 0 initialized
 * @param {Number} ci c left
 * @param {Number} cj c right
 */

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

    let T;
    let C = 0;

    while (--bj >= bi) {
        --aj;
        --cj;
        T = C;
        C = (a[aj] < b[bj] + T) | 0;
        c[cj] = a[aj] - b[bj] + (C * r - T);
    }

    while (--aj >= ai) {
        --cj;
        T = C;
        C = (a[aj] < T) | 0;
        c[cj] = a[aj] + (C * r - T);
    }

    if (C) {
        while (--cj >= ci) {
            c[cj] = r - 1;
        }
    }
}