/**
 * Copyright 2018 Google Inc. All Rights Reserved.
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *     http://www.apache.org/licenses/LICENSE-2.0
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import { Transform } from "../types.js";

export function distinct<T>(
  f: (a: T, b: T) => boolean = (a, b) => a === b
): Transform<T> {
  let last: T;
  let hasLast = false;
  return new TransformStream<T, T>({
    transform(chunk, controller) {
      if (!hasLast) {
        last = chunk;
        hasLast = true;
        controller.enqueue(chunk);
        return;
      }
      if (!f(chunk, last)) {
        controller.enqueue(chunk);
      }
      last = chunk;
    }
  });
}