I am trying to create a CRC-15 check in c and the output is never correct for each line of the file. I am trying to output the CRC for each line cumulatively next to each line. I use: #define POLYNOMIAL 0xA053 for the divisor and text for the dividend. I need to represent numbers as 32-bit unsigned integers. I have tried printing out the hex values to keep track and flipping different shifts around. However, I just can't seem to figure it out! I have a feeling it has something to do with the way I am padding things. Is there a flaw to my logic?
The CRC is to be represented in four hexadecimal numbers, that sequence will have four leading 0's. For example, it will look like 0000xxxx where the x's are the hexadecimal digits. The polynomial I use is 0xA053.
I thought about using a temp variable and do 4 16 bit chunks of code per line every XOR, however, I'm not quite sure how I could use shifts to accomplish this so I settled for a checksum of the letters on the line and then XORing that to try to calculate the CRC code.
I am testing my code using the following input and padding with . until the string is of length 504 because that is what the pad character needs to be via the requirements given: "This is the lesson: never give in, never give in, never, never, never, never - in nothing, great or small, large or petty - never give in except to convictions of honor and good sense. Never yield to force; never yield to the apparently overwhelming might of the enemy."
The CRC of the first 64 char line ("This is the lesson: never give in, never give in, never, never,) is supposed to be 000015fa and I am getting bfe6ec00.
My logic:
In CRCCalculation I add each character to a 32-bit unsigned integer and after 64 (the length of one line) I send it into the XOR function.
If it the top bit is not 1, I shift the number to the left one causing 0s to pad the right and loop around again.
If the top bit is 1, I XOR the dividend with the divisor and then shift the dividend to the left one.
After all calculations are done, I return the dividend shifted to the left four ( to add four zeros to the front) to the calculation function
Add result to the running total of the result
Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <ctype.h>
#define POLYNOMIAL 0xA053
void crcCalculation(char *text, int length)
{
int i;
uint32_t dividend = atoi(text);
uint32_t result;
uint32_t sumText = 0;
// Calculate CRC
printf("\nCRC 15 calculation progress:\n");
i = length;
// padding
if(i < 504)
{
for(; i!=504; i++)
{
// printf("i is %d\n", i);
text[i] = '.';
}
}
// Try calculating by first line of crc by summing the values then calcuating, then add in the next line
for (i = 0; i < 504; i++)
{
if(i%64 == 0 && i != 0)
{
result = XOR(POLYNOMIAL, sumText);
printf(" - %x\n",result);
}
sumText +=(uint32_t)text[i];
printf("%c", text[i]);
}
printf("\n\nCRC15 result : %x\n", result);
}
uint32_t XOR(uint32_t divisor, uint32_t dividend)
{
uint32_t divRemainder = dividend;
uint32_t currentBit;
// Note: 4 16 bit chunks
for(currentBit = 32; currentBit > 0; --currentBit)
{
// if topbit is 1
if(divRemainder & 0x80)
{
//divRemainder = (divRemainder << 1) ^ divisor;
divRemainder ^= divisor;
printf("%x %x\n", divRemainder, divisor);
}
// else
// divisor = divisor >> 1;
divRemainder = (divRemainder << 1);
}
//return divRemainder; , have tried shifting to right and left, want to add 4 zeros to front so >>
//return divRemainder >> 4;
return divRemainder >> 4;
}
enumstatement or#definestatements to give those 'magic' numbers meaningful names, then use those meaningful names throughout the code. – user3629249