/*
 * If not stated otherwise in this file or this component's LICENSE file the
 * following copyright and licenses apply:
 *
 * Copyright 2023 Comcast Cable Communications Management, LLC.
 *
 * 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 type { Texture } from '../textures/Texture.js';

export interface TextureUploadQueue {
  readonly size: number;
  has(texture: Texture): boolean;
  enqueue(texture: Texture): void;
  dequeue(): Texture | undefined;
  remove(texture: Texture): void;
  clear(): void;
}

/**
 * Creates a FIFO queue with O(1) membership check and O(1) cancellation.
 *
 * @remarks
 * Backed by an array (for ordering) and a Set (for fast lookup/cancel).
 * Cancelled entries are tombstoned (set to null) and skipped on dequeue.
 * The array is compacted when the head pointer exceeds half the array length
 * and is at least 64 entries deep, amortising the compaction cost.
 */
export function createTextureUploadQueue(): TextureUploadQueue {
  let list: (Texture | null)[] = [];
  const set: Set<Texture> = new Set();
  let head = 0;

  function compact(): void {
    if (head >= 64 && head >= list.length >> 1) {
      list = list.slice(head);
      head = 0;
    }
  }

  return {
    get size(): number {
      return set.size;
    },

    has(texture: Texture): boolean {
      return set.has(texture);
    },

    enqueue(texture: Texture): void {
      if (set.has(texture) === true) return;
      set.add(texture);
      list.push(texture);
    },

    dequeue(): Texture | undefined {
      while (head < list.length) {
        const t = list[head]!;
        head++;
        if (t !== null && set.has(t) === true) {
          set.delete(t);
          compact();
          return t;
        }
      }
      compact();
      return undefined;
    },

    remove(texture: Texture): void {
      if (set.delete(texture) === false) return;
      const idx = list.indexOf(texture);
      if (idx !== -1) {
        list[idx] = null;
      }
    },

    clear(): void {
      list.length = 0;
      set.clear();
      head = 0;
    },
  };
}