108 lines
3.1 KiB
C
108 lines
3.1 KiB
C
// ~ $ gcc main.c -o crc
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// ./crc 011100111110 11011
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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// Function to calculate the degree (highest exponent) of the generator polynomial
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int calculate_degree(const char *bitstream, int len) {
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for (int i = 0; i < len; i++) {
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if (bitstream[i] == '1') {
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return len - i - 1;
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}
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}
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return -1;
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}
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// Function to append zeros to the bitstream
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char* appendZerosToBitstream(const char *bitstream, int amount) {
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int originalLen = strlen(bitstream);
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char *newBitstream = (char *)malloc(originalLen + amount + 1);
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if (newBitstream == NULL) {
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printf("Memory allocation failed!\n");
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return NULL;
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}
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strcpy(newBitstream, bitstream);
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memset(newBitstream + originalLen, '0', amount);
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newBitstream[originalLen + amount] = '\0';
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return newBitstream;
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}
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// Function to perform bitwise XOR division
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char* perform_xor_division(const char *bitstream, const char *generator) {
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int bitstreamLen = strlen(bitstream);
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int generatorLen = strlen(generator);
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// Copy the bitstream into a mutable buffer
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char *remainder = (char *)malloc(bitstreamLen + 1);
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if (remainder == NULL) {
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printf("Memory allocation failed!\n");
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return NULL;
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}
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strcpy(remainder, bitstream);
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for (int i = 0; i <= bitstreamLen - generatorLen; i++) {
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if (remainder[i] == '1') { // Perform XOR only when leading bit is 1
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for (int j = 0; j < generatorLen; j++) {
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remainder[i + j] = (remainder[i + j] == generator[j]) ? '0' : '1';
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}
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}
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}
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// Extract the remainder from the last (generatorLen - 1) bits
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char *crc = (char *)malloc(generatorLen);
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if (crc == NULL) {
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printf("Memory allocation failed!\n");
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free(remainder);
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return NULL;
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}
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strcpy(crc, remainder + bitstreamLen - generatorLen + 1);
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free(remainder);
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return crc;
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}
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int main(int argc, char *argv[]) {
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if (argc == 3) {
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char *bitstream = argv[1];
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printf("Bitstream: %s\n", bitstream);
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char *generatorpolynom = argv[2];
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// Calculate how many zeros need to be appended
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int zerosToAppend = calculate_degree(generatorpolynom, strlen(generatorpolynom));
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if (zerosToAppend < 0) {
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printf("Invalid generator polynomial.\n");
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return 1;
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}
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// Append zeros to the bitstream
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char *zerodBitstream = appendZerosToBitstream(bitstream, zerosToAppend);
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if (zerodBitstream == NULL) {
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return 1;
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}
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printf("Modified bitstream (with zeros): %s\n", zerodBitstream);
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// Perform bitwise division
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char *crc = perform_xor_division(zerodBitstream, generatorpolynom);
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if (crc == NULL) {
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free(zerodBitstream);
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return 1;
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}
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printf("CRC remainder: %s\n", crc);
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printf("HDLC Header: %s\n",strcat(bitstream,crc));
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free(zerodBitstream);
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free(crc);
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return 0;
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} else {
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printf("Usage: %s <bitstream> <generator polynomial>\n", argv[0]);
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return 1;
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}
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}
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