binary-search-tree/red-black-tree

import assert from 'assert';

import BLACK from '../color/BLACK.js';
import RED from '../color/RED.js';
import grandparent from '../family/grandparent.js';
import rotate_left from '../rotate/rotate_left.js';
import rotate_right from '../rotate/rotate_right.js';
import Node from '../types/Node.js';

/**
 * Preconditions:
 *   - n is red.
 *   - n's children are BLACK
 *   - n is not the root of the tree.
 *   - n's parent is red.
 *   - n's uncle is black.
 *   - the path from n to its grandparent makes a left-left or right-right.
 *
 * @param {Node} n - The input node.
 * @return {Node} The root of the modified subtree.
 */
const insert_case4 = (n) => {
    assert(n instanceof Node);
    assert(n._color === RED);
    assert(n.left === null || n.left._color === BLACK);
    assert(n.right === null || n.right._color === BLACK);
    assert(n.parent !== null);
    assert(n.parent._color === RED);
    const g = grandparent(n);

    // Repaint n's parent black, n's grandparent red
    n.parent._color = BLACK;
    g._color = RED;

    if (n === n.parent.left) {
        /**
         * If the path from g to n makes a left-left, {@link rotate_right} at g.
         * We are done.
         *
         *             R                     B
         *           /   \                 /   \
         *         B       B            >R       R
         *        / \     / \   -->     / \     / \
         *      >R   =   -   -         =   =   =   B
         *      / \                               / \
         *     =   =                             -   -
         */
        assert(g.left instanceof Node);
        assert(n === g.left.left);
        assert(g.right === null || g.right._color === BLACK);
        rotate_right(g);
    } else {
        /**
         * If the path from g to n makes a right-right, {@link rotate_left} at g.
         * We are done.
         *
         *             R                     B
         *           /   \                 /   \
         *         B       B             R      >R
         *        / \     / \   -->     / \     / \
         *       -   -   =  >R         B   =   =   =
         *                  / \       / \
         *                 =   =     -   -
         */
        assert(g.right instanceof Node);
        assert(n === g.right.right);
        assert(g.left === null || g.left._color === BLACK);
        rotate_left(g);
    }

    return n.parent;
};

export default insert_case4;