reactos/drivers/net/tcpip/tcpip/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 #include <tcpip.h>
  12 #include <checksum.h>
  13
  14
  15 ULONG ChecksumCompute(
  16     PVOID Data,
  17     UINT Count,
  18     ULONG Seed)
  19 /*
  20  * FUNCTION: Calculate checksum of a buffer
  21  * ARGUMENTS:
  22  *     Data  = Pointer to buffer with data
  23  *     Count = Number of bytes in buffer
  24  *     Seed  = Previously calculated checksum (if any)
  25  * RETURNS:
  26  *     Checksum of buffer
  27  */
  28 {
  29     /* FIXME: This should be done in assembler */
  30
  31     register ULONG Sum = Seed;
  32
  33     while (Count > 1) {
  34         Sum             += *(PUSHORT)Data;
  35         Count           -= 2;
  36         (ULONG_PTR)Data += 2;
  37     }
  38
  39     /* Add left-over byte, if any */
  40     if (Count > 0)
  41         Sum += *(PUCHAR)Data;
  42
  43     /* Fold 32-bit sum to 16 bits */
  44     while (Sum >> 16)
  45         Sum = (Sum & 0xFFFF) + (Sum >> 16);
  46
  47     return ~Sum;
  48 }
  49
  50 /* EOF */