# CRC Program in C

CRC (Cyclic Redundancy Check) is an error-detection algorithm that is used to detect any errors that may have occurred during the transmission or storage of data. The basic idea behind CRC is to divide the data to be sent by a predetermined divisor, and then send the remainder along with the data. The receiver then performs the same operation and compares the remainder with the one that was sent. If they match, the data is considered to be error-free.

Here is an example of a simple implementation of the CRC algorithm in C:

## Algorithm

``````unsigned int crc32(unsigned char *message) {
int i, j;
unsigned int byte, crc, mask;

i = 0;
crc = 0xFFFFFFFF;
while (message[i] != 0) {
byte = message[i];            // Get next byte.
crc = crc ^ byte;
for (j = 7; j >= 0; j--) {    // Do eight times.
mask = -(crc& 1);
crc = (crc>> 1) ^ (0xEDB88320 & mask);
}
i = i + 1;
}
return ~crc;
}
``````

Here is a more detailed implementation of the CRC algorithm in C:

Example:

``````#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define POLYNOMIAL 0x04C11DB7  // Standard CRC-32 polynomial

unsigned int crcTable[256];   // Table of precalculated values

// Function to generate the table of precalculated values
void generateCRCTable() {
unsigned int remainder;

// Perform modulo-2 division, a bit at a time
for (int dividend = 0; dividend < 256; dividend++) {
// Start with the dividend followed by zeros
remainder = dividend << 24;

// Perform modulo-2 division, a bit at a time
for (int bit = 0; bit < 8; bit++) {
// Try to divide the current data bit
if (remainder & 0x80000000) {
remainder = (remainder << 1) ^ POLYNOMIAL;
} else {
remainder = (remainder << 1);
}
}

// Store the result in the table
crcTable[dividend] = remainder;
}
}

// Function to calculate the CRC of a given data block
unsigned int calculateCRC(unsigned char *data, int length) {
unsigned int remainder = 0xFFFFFFFF;  // Start with all 1's

// Perform modulo-2 division, a byte at a time
for (int byte = 0; byte < length; byte++) {
// Bring the next byte into the remainder
remainder = (remainder << 8) ^ crcTable[(remainder >> 24) ^ data[byte]];
}

// The final remainder is the CRC result
return remainder;
}

int main(int argc, char *argv[]) {
if (argc != 2) {
printf("Usage: %s <string>\n", argv[0]);
return 1;
}

generateCRCTable();  // Generate the table of precalculated values

// Calculate the CRC of the given string
unsigned int crc = calculateCRC((unsigned char *)argv[1], strlen(argv[1]));

printf("CRC: 0x%08X\n", crc);

return 0;
}
``````

Output:

`Usage: ./a.out<string>`

This implementation uses a table of precalculated values to speed up the calculation of the CRC. The table is generated using the standard CRC-32 polynomial (0x04C11DB7) and the generateCRCTable() function. The calculateCRC() function takes a pointer to the data and its length as arguments and returns the calculated CRC. The main() function takes a string as a command line argument and calculates its CRC.

In this example, the crc is calculated in the little endian format, if you want it in big endian format, you should reverse the byte order of the final remainder before returning it.

## Complexity of CRC Program in C

The time complexity of the implementation of the CRC algorithm in C that is provided earlier is O(n), where n is the length of the data block. This is because the algorithm performs a modulo-2 division, a byte (or a bit) at a time, for the entire data block.

The space complexity of the implementation is O(1) because it only uses a fixed-size table (256 entries) of precalculated values to store the intermediate results, regardless of the size of the data block.

The table generation of precalculated values has a time complexity of O(256*8) = O(2048) which is a constant time and can be done before the actual calculation of the crc.

It is important to note that the time complexity will change if the width of the crc or the polynomial is changed.

One way to implement a CRC using bit manipulation is to use a pre-defined "CRC polynomial" and perform a series of bitwise operations on the data, such as XOR and shift, to calculate a fixed-size "CRC value" that can be appended to the data to form a "CRC code." Here's some example pseudo code for a basic implementation of a CRC using bit manipulation:

``````functioncrc(data, polynomial) {
crc = 0
for each bit in data {
if (crc& 0x8000) {
crc = (crc<< 1) ^ polynomial
} else {
crc = crc<< 1
}
}
returncrc& 0xFFFF
}
``````

In this example, "data" is the raw data that we want to create a CRC code for, and "polynomial" is the pre-defined polynomial that you are using for the calculation. The function first initializes a "crc" variable to 0, then iterates through each bit in the data. For each bit, it checks if the most significant bit of the "crc" variable is set (i.e. if the crc& 0x8000 is true), and if so, it performs an XOR operation with the polynomial and a left shift. If the most significant bit is not set, it simply performs a left shift. Finally, the function returns the "crc" variable, masked with 0xFFFF to return the 16-bit value.