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/**
 * Constructs a ConsArray object. It is used mainly for tree traversal.
 * In this use case we have lots of arrays that we need to iterate
 * sequentally. The internal Array implementation is horribly slow
 * when concatenating on large (10K items) arrays due to memory copying.
 * That's why we avoid copying memory and insead build a linked list
 * of arrays to iterate through.
 *
 * @constructor
 */
export class ConsArray {
  constructor() {
    this.tail_ = new ConsArrayCell(null, null);
    this.currCell_ = this.tail_;
    this.currCellPos_ = 0;
  }
  /**
   * Concatenates another array for iterating. Empty arrays are ignored.
   * This operation can be safely performed during ongoing ConsArray
   * iteration.
   *
   * @param {Array} arr Array to concatenate.
   */
  concat(arr) {
    if (arr.length > 0) {
      this.tail_.data = arr;
      this.tail_ = this.tail_.next = new ConsArrayCell(null, null);
    }
  }

  /**
   * Whether the end of iteration is reached.
   */
  atEnd() {
    return this.currCell_ === null ||
        this.currCell_.data === null ||
        this.currCellPos_ >= this.currCell_.data.length;
  }

  /**
   * Returns the current item, moves to the next one.
   */
  next() {
    const result = this.currCell_.data[this.currCellPos_++];
    if (this.currCellPos_ >= this.currCell_.data.length) {
      this.currCell_ = this.currCell_.next;
      this.currCellPos_ = 0;
    }
    return result;
  }
}


/**
 * A cell object used for constructing a list in ConsArray.
 *
 * @constructor
 */
class ConsArrayCell {
  constructor(data, next) {
    this.data = data;
    this.next = next;
  }
}
