src/merge.js

import prepend from './prepend.js';

/**
 * Merge: compare the two root elements, the smaller remains the root of the
 * result, the larger element and its subtree is appended as a child of this
 * root.
 *
 * /!\ Precondition:
 *   1. A != null and A.next = A.prev = null
 *   2. B != null and B.next = B.prev = null
 *
 * @param {Function} compare Comparison functions for keys.
 * @param {Node} A The first input node.
 * @param {Node} B The second input node.
 * @returns {Node} The input node with smallest key with .next = .prev = null.
 */
export default function merge(compare, A, B) {
	if (compare(A.value, B.value) < 0) return prepend(A, B);

	return prepend(B, A);
}