docs: private static int[] trickyGetPairsFromWindow(final byte[] window, final int k, final int numPairsToGet,
        final int emptySpace) {
      final int outputLength = emptySpace + numPairsToGet;
      final int[] pairs = new int[outputLength];
      int rowIndex = 0;
      int pairIndex = emptySpace;
      for (rowIndex = 0; rowIndex < k; rowIndex++) {
        int wByte = window[rowIndex] & 0XFF;
        while (wByte != 0) {
          final int colIndex = Integer.numberOfTrailingZeros(wByte);
          //      assert (colIndex < 8);
          wByte ^= (1 << colIndex); // erase the 1
          pairs[pairIndex++] = (rowIndex << 6) | colIndex;
        }
      }
      assert (pairIndex == outputLength);
      return (pairs);
    }