reactos/drivers/lib/ip/network/checksum.c

   1 /*
   2  * COPYRIGHT:   See COPYING in the top level directory
   3  * PROJECT:     ReactOS TCP/IP protocol driver
   4  * FILE:        tcpip/checksum.c
   5  * PURPOSE:     Checksum routines
   6  * NOTES:       The checksum routine is from RFC 1071
   7  * PROGRAMMERS: Casper S. Hornstrup (chorns@users.sourceforge.net)
   8  * REVISIONS:
   9  *   CSH 01/08-2000 Created
  10  */
  11
  12 #include "precomp.h"
  13
  14
  15 ULONG ChecksumFold(
  16   ULONG Sum)
  17 {
  18   /* Fold 32-bit sum to 16 bits */
  19   while (Sum >> 16)
  20     {
  21       Sum = (Sum & 0xFFFF) + (Sum >> 16);
  22     }
  23
  24   return Sum;
  25 }
  26
  27 ULONG ChecksumCompute(
  28   PVOID Data,
  29   UINT Count,
  30   ULONG Seed)
  31 /*
  32  * FUNCTION: Calculate checksum of a buffer
  33  * ARGUMENTS:
  34  *     Data  = Pointer to buffer with data
  35  *     Count = Number of bytes in buffer
  36  *     Seed  = Previously calculated checksum (if any)
  37  * RETURNS:
  38  *     Checksum of buffer
  39  */
  40 {
  41   register ULONG Sum = Seed;
  42
  43   while (Count > 1)
  44     {
  45       Sum += *(PUSHORT)Data;
  46       Count -= 2;
  47       Data = (PVOID)((ULONG_PTR) Data + 2);
  48     }
  49
  50   /* Add left-over byte, if any */
  51   if (Count > 0)
  52     {
  53       Sum += *(PUCHAR)Data;
  54     }
  55
  56   return Sum;
  57 }