checksum.h

00001 #ifdef __cplusplus
00002 extern "C" {
00003 #endif
00004 
00005 #ifndef _CHECKSUM_H_
00006 #define _CHECKSUM_H_
00007 
00008 #include "inttypes.h"
00009 
00010 
00011 /**
00012  *
00013  *  CALCULATE THE CHECKSUM
00014  *
00015  */
00016 
00017 #define X25_INIT_CRC 0xffff
00018 #define X25_VALIDATE_CRC 0xf0b8
00019 
00020 /**
00021  * @brief Accumulate the X.25 CRC by adding one char at a time.
00022  *
00023  * The checksum function adds the hash of one char at a time to the
00024  * 16 bit checksum (uint16_t).
00025  *
00026  * @param data new char to hash
00027  * @param crcAccum the already accumulated checksum
00028  **/
00029 static inline void crc_accumulate(uint8_t data, uint16_t *crcAccum)
00030 {
00031         /*Accumulate one byte of data into the CRC*/
00032         uint8_t tmp;
00033 
00034         tmp=data ^ (uint8_t)(*crcAccum &0xff);
00035         tmp^= (tmp<<4);
00036         *crcAccum = (*crcAccum>>8) ^ (tmp<<8) ^ (tmp <<3) ^ (tmp>>4);
00037 }
00038 
00039 /**
00040  * @brief Initiliaze the buffer for the X.25 CRC
00041  *
00042  * @param crcAccum the 16 bit X.25 CRC
00043  */
00044 static inline void crc_init(uint16_t* crcAccum)
00045 {
00046         *crcAccum = X25_INIT_CRC;
00047 }
00048 
00049 
00050 /**
00051  * @brief Calculates the X.25 checksum on a byte buffer
00052  *
00053  * @param  pBuffer buffer containing the byte array to hash
00054  * @param  length  length of the byte array
00055  * @return the checksum over the buffer bytes
00056  **/
00057 static inline uint16_t crc_calculate(uint8_t* pBuffer, int length)
00058 {
00059 
00060         // For a "message" of length bytes contained in the unsigned char array
00061         // pointed to by pBuffer, calculate the CRC
00062         // crcCalculate(unsigned char* pBuffer, int length, unsigned short* checkConst) < not needed
00063 
00064         uint16_t crcTmp;
00065         //uint16_t tmp;
00066         uint8_t* pTmp;
00067         int i;
00068 
00069         pTmp=pBuffer;
00070         
00071 
00072         /* init crcTmp */
00073         crc_init(&crcTmp);
00074 
00075         for (i = 0; i < length; i++){
00076                 crc_accumulate(*pTmp++, &crcTmp);
00077         }
00078 
00079         /* This is currently not needed, as only the checksum over payload should be computed
00080         tmp = crcTmp;
00081         crcAccumulate((unsigned char)(~crcTmp & 0xff),&tmp);
00082         crcAccumulate((unsigned char)((~crcTmp>>8)&0xff),&tmp);
00083         *checkConst = tmp;
00084         */
00085         return(crcTmp);
00086 }
00087 
00088 
00089 
00090 
00091 #endif /* _CHECKSUM_H_ */
00092 
00093 #ifdef __cplusplus
00094 }
00095 #endif