data-structure-algebra/doubly-linked-list

import assert from 'assert';

import Node from './Node.js';
import _rotate_to from './_rotate_to.js';

/**
 * Rotate list to the left k steps. The parameter k must be positive and
 * smaller than the length of the list.
 *
 * @param {Node} x The current first node.
 * @param {Node} z The current last node.
 * @param {number} k MUST be positive and smaller than the length of the list.
 * @return {[Node, Node]} The new first and last nodes.
 */
export default function _rotate_left(x, z, k) {
    assert(Number.isInteger(k));
    assert(k > 0);
    assert(x instanceof Node);
    assert(x.prev === null);
    assert(z instanceof Node);
    assert(z.next === null);
    assert(x !== z);
    let y = z;
    while (--k) {
        y = y.prev;
        assert(y instanceof Node);
    }

    const w = y.prev;
    _rotate_to(x, y, z);
    return [y, w];
}