import BTree from '../b+tree';
/**
 * Efficiently unions two trees, reusing subtrees wherever possible without mutating either input.
 *
 * Complexity is O(N + M) when the trees overlap heavily, and additionally bounded by O(log(N + M) * D)
 * where `D` is the number of disjoint key ranges, because disjoint subtrees are skipped entirely.
 * In practice, that means for keys of random distribution the performance is linear and for keys with significant
 * numbers of non-overlapping key ranges it is much faster.
 * @param treeA First tree to union.
 * @param treeB Second tree to union.
 * @param combineFn Called for keys that appear in both trees. Return the desired value, or
 *        `undefined` to omit the key from the result. Note: symmetric difference can be achieved by always returning `undefined`.
 * @returns A new BTree that contains the unioned key/value pairs.
 * @throws Error if the trees were created with different comparators or max node sizes.
 */
export default function union<TBTree extends BTree<K, V>, K, V>(treeA: TBTree, treeB: TBTree, combineFn: (key: K, leftValue: V, rightValue: V) => V | undefined): TBTree;