crc24.c

/*-------------------------------------------------------------------
Function: Decode ADS-B Message
Usage:     crc24  28chars
Example:   crc24 8D40621D58C382D690C8AC2863A7

midaszhou@qq.com
-------------------------------------------------------------------*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h> //-strlen()
#include <stdint.h> //uint32_t
#include "adsb_crc.h"


/*------------------   function  strmid()   ------------------------------
to copy 'n' chars from 'src' to 'dst', starting from 'm'_th char in 'src'
------------------------------------------------------------------------*/
static char* strmid(char *dst,char *src,int n,int m)  
{
 char *p=src;
 char *q=dst;
 unsigned int len=strlen(src);
 if(n>len) n=len-m;
 if(m<0) m=0;
 if(m>len) return NULL;
 p+=m;
 while(n--) *(q++)=*(p++);
 *(q++)='\0';
 return dst;
}




/*=====================================================================
                              MAIN 
=====================================================================*/
void main(int argc, char* argv[])
{

//------------  test code -----------

char str_HEX_CODE[32]=""; //4*8=32 > 28
//strcpy(str_HEX_CODE,"8D40621D58C382D690C8AC2863A7"); //--28 chars for str_HEX code
//strcpy(str_HEX_CODE,"8D40621D58C386435CC412692AD6");
//strcpy(str_HEX_CODE,"8D4840D6202CC371C32CE0576098");

int i,j,k;
char str_Temp[8]; 
//int n_div=4;
uint32_t crc32_GP=0xFFFA0480;//0x04C11DB7; // CRC Generator Polynomial alread inlucdes the highest first bit, while standard CRC usually omitted. 
char str_ICAO24[6+1]=""; //--4*6=24bits
int  int_ICAO24;
uint32_t bin32_code[4]; //store full binary ADS-B CODE in 4 groups of 32bit array
uint32_t bin32_divid[4]={0,0,0,0};//dividend = message(88)+crc(24)=112bits

//--------------- get str_HEX_CODE from input -----------------
 if(argc<1)
  {
    printf("please enter 28bits ADS-B code!\n");
    return;
  }
 if(strlen(argv[1])<28)
 {  
     printf("input data: %s\n",argv[1]);
     printf("ADS-B Code not correct!\n"); 
     return; 
 }
 strcpy(str_HEX_CODE,argv[1]);

//--------------  convert hex code to bin code in string -----------
for(i=0;i<4;i++)
 {
    strmid(str_Temp,str_HEX_CODE,8,i*8);
    bin32_code[i]=strtoul(str_Temp,NULL,16);
    if(i==3)
      bin32_code[i]=(bin32_code[i]<<16);  //--shift the last 16bits to left
    printf("%08x",bin32_code[i]); 
 }
 printf("\n");

//-------------- try to find and fix ibit error in bin32_code -----------
adsb_fixerror_slow(bin32_code);

//--------------call the function to calculate CRC ------------------------
 printf("88bits  CRC24: %06x \n",adsb_crc(bin32_code,88));
 printf("112bits CRC24: %06x \n",adsb_crc(bin32_code,112));

//--------------------- CRC calculation ----------------------
/*
  //------- prepare dividend for CRC calculation 
  bin32_divid[0]=bin32_code[0];
  bin32_divid[1]=bin32_code[1];
  bin32_divid[2]=bin32_code[2]&0xffffff00;
  printf("CRC divid: %08x%08x%08x \n",bin32_divid[0],bin32_divid[1],bin32_divid[2]);

for(j=0;j<88;j++)
 {
    if(bin32_divid[0] & 0x80000000)
         bin32_divid[0]^=crc32_GP; // xor CRC generator polynomial
    bin32_divid[0]=(bin32_divid[0]<<1);
    if(bin32_divid[1] & 0x80000000)  bin32_divid[0]|=1; // shift code[1] to code[0]
    bin32_divid[1]=(bin32_divid[1]<<1);
    if(bin32_divid[2] & 0x80000000)  bin32_divid[1]|=1; // shift code[2] to code[1]
    bin32_divid[2]=(bin32_divid[2]<<1);
 }
//  printf("CRC: %08x \n",bin32_divid[0]); 
  printf("CRC24: %06x \n",bin32_divid[0]>>8);
//  printf("CRC: %08x \n",bin32_divid[0]<<1); //--CRC Width = Generator Golynomial - 1
*/


} //// end of main()