DecodePacketNumber(largest_pn, truncated_pn, pn_nbits):
   expected_pn  = largest_pn + 1
   pn_win       = 1 << pn_nbits
   pn_hwin      = pn_win / 2
   pn_mask      = pn_win - 1
   // The incoming packet number should be greater than
   // expected_pn - pn_hwin and less than or equal to
   // expected_pn + pn_hwin
   //
   // This means we cannot just strip the trailing bits from
   // expected_pn and add the truncated_pn because that might
   // yield a value outside the window.
   //
   // The following code calculates a candidate value and
   // makes sure it's within the packet number window.
   // Note the extra checks to prevent overflow and underflow.
   candidate_pn = (expected_pn & ~pn_mask) | truncated_pn
   if candidate_pn <= expected_pn - pn_hwin and
      candidate_pn < (1 << 62) - pn_win:
      return candidate_pn + pn_win
   if candidate_pn > expected_pn + pn_hwin and
      candidate_pn >= pn_win:
      return candidate_pn - pn_win
   return candidate_pn