lucene 分配數組大小 (newSize + 1) & 0x7ffffffe

  public static int oversize(int minTargetSize, int bytesPerElement) {

    if (minTargetSize < 0) {
      // catch usage that accidentally overflows int
      throw new IllegalArgumentException("invalid array size " + minTargetSize);
    }

    if (minTargetSize == 0) {
      // wait until at least one element is requested
      return 0;
    }

    if (minTargetSize > MAX_ARRAY_LENGTH) {
      throw new IllegalArgumentException("requested array size " + minTargetSize + " exceeds maximum array in java (" + MAX_ARRAY_LENGTH + ")");
    }

    // asymptotic exponential growth by 1/8th, favors
    // spending a bit more CPU to not tie up too much wasted
    // RAM:
    int extra = minTargetSize >> 3;

    if (extra < 3) {
      // for very small arrays, where constant overhead of
      // realloc is presumably relatively high, we grow
      // faster
      extra = 3;
    }

    int newSize = minTargetSize + extra;

    // add 7 to allow for worst case byte alignment addition below:
    if (newSize+7 < 0 || newSize+7 > MAX_ARRAY_LENGTH) {
      // int overflowed, or we exceeded the maximum array length
      return MAX_ARRAY_LENGTH;
    }

    if (Constants.JRE_IS_64BIT) {
      // round up to 8 byte alignment in 64bit env
      switch(bytesPerElement) {
      case 4:
        // round up to multiple of 2  取整數爲2的倍數
        return (newSize + 1) & 0x7ffffffe;
      case 2:
        // round up to multiple of 4 取整數爲4的倍數
        return (newSize + 3) & 0x7ffffffc;
      case 1:
        // round up to multiple of 8 取整數爲8的倍數
        return (newSize + 7) & 0x7ffffff8;
      case 8:
        // no rounding
      default:
        // odd (invalid?) size
        return newSize;
      }
    } else {
      // round up to 4 byte alignment in 64bit env
      switch(bytesPerElement) {
      case 2:
        // round up to multiple of 2
        return (newSize + 1) & 0x7ffffffe;
      case 1:
        // round up to multiple of 4
        return (newSize + 3) & 0x7ffffffc;
      case 4:
      case 8:
        // no rounding
      default:
        // odd (invalid?) size
        return newSize;
      }
    }
  }