import errorCodes from '../error';

const {
  ERR_INVALID_OPT_VALUE,
} = errorCodes;

interface ChunkLinkListItem {
  chunk: unknown;
  callback?: (err?: unknown) => void;
  next: null | ChunkLinkListItem;
}

const DEFAULT_HWM = 100;

function getHighWaterMark(hwm: number) {
  if (hwm != null) {
    if (!(isFinite(hwm) && Math.floor(hwm) === hwm) || hwm < 0) {
      throw new ERR_INVALID_OPT_VALUE('highWaterMark', hwm);
    }

    return Math.floor(hwm);
  }


  return DEFAULT_HWM;
}

export interface PoolOptions {
  highWaterMark?: number;
  forceDiscard?: boolean;
}

export class Pool {
  // 水位警戒值
  protected readonly highWaterMark: number;
  // 强制丢弃（如果当前挤压的数据 >= highWaterMark，值为true时则强制丢掉新的数据）
  protected readonly forceDiscard: boolean;

  // 采用单向链表缓存数据
  protected chunkLinkListLen = 0;
  protected chunkLinkListStart: ChunkLinkListItem;
  protected chunkLinkListEnd: ChunkLinkListItem;

  constructor({
    highWaterMark,
    forceDiscard,
  }: PoolOptions) {
    this.highWaterMark = getHighWaterMark(highWaterMark);
    this.forceDiscard = !!forceDiscard;
  }

  // 从链头开始获取x个数据
  private getOriginItems(num = 1): Array<ChunkLinkListItem> {
    const arr = [];

    for (let i = 0, start = this.chunkLinkListStart; i < num; i++) {
      if (!start) {
        break;
      }

      arr.push(start);
      start = start.next;
    }

    return arr;
  }

  /**
   * @return boolean 当前待处理的chunk数量是否超过阈值
   * */
  addChunk(chunk: unknown, callback?: ChunkLinkListItem['callback']): boolean {
    // 超过阈值并且设置了"强制丢弃"flag，则丢弃
    if (this.chunkLinkListLen + 1 > this.highWaterMark && this.forceDiscard) {
      return false;
    }

    this.chunkLinkListLen += 1;

    const item: ChunkLinkListItem = {
      chunk,
      callback,
      next: null,
    }
    const latestItem = this.chunkLinkListEnd;

    this.chunkLinkListEnd = item;

    if (latestItem) {
      latestItem.next = item;
    } else {
      this.chunkLinkListStart = this.chunkLinkListEnd;
    }

    return this.chunkLinkListLen < this.highWaterMark;
  }

  delChunks(num: number) {
    const items = this.getOriginItems(num);

    if (items.length > 0) {
      this.chunkLinkListStart = items[items.length - 1].next;
      this.chunkLinkListLen -= items.length;

      items.forEach(item => item.next = null);
    }

    // 链表里没有任何数据了
    if (!this.chunkLinkListStart) {
      this.chunkLinkListEnd = this.chunkLinkListStart;
    }
  }

  getChunks(num: number) {
    const items = this.getOriginItems(num);

    return items.map(item => ({
      ...item,
      next: null,
    }));
  }
}