Create crc.c

2024_Betriebssysteme_und_Rechnernetze/crc.c

@@ -0,0 +1,107 @@
+// ~ $ gcc main.c -o crc       
+// ./crc 011100111110 11011
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+// Function to calculate the degree (highest exponent) of the generator polynomial
+int calculate_degree(const char *bitstream, int len) {
+    for (int i = 0; i < len; i++) {
+        if (bitstream[i] == '1') {
+            return len - i - 1;
+        }
+    }
+    return -1;
+}
+
+// Function to append zeros to the bitstream
+char* appendZerosToBitstream(const char *bitstream, int amount) {
+    int originalLen = strlen(bitstream);
+    char *newBitstream = (char *)malloc(originalLen + amount + 1);
+    if (newBitstream == NULL) {
+        printf("Memory allocation failed!\n");
+        return NULL;
+    }
+    
+    strcpy(newBitstream, bitstream);
+    memset(newBitstream + originalLen, '0', amount);
+    newBitstream[originalLen + amount] = '\0';
+    
+    return newBitstream;
+}
+
+// Function to perform bitwise XOR division
+char* perform_xor_division(const char *bitstream, const char *generator) {
+    int bitstreamLen = strlen(bitstream);
+    int generatorLen = strlen(generator);
+
+    // Copy the bitstream into a mutable buffer
+    char *remainder = (char *)malloc(bitstreamLen + 1);
+    if (remainder == NULL) {
+        printf("Memory allocation failed!\n");
+        return NULL;
+    }
+    strcpy(remainder, bitstream);
+
+    for (int i = 0; i <= bitstreamLen - generatorLen; i++) {
+        if (remainder[i] == '1') {  // Perform XOR only when leading bit is 1
+            for (int j = 0; j < generatorLen; j++) {
+                remainder[i + j] = (remainder[i + j] == generator[j]) ? '0' : '1';
+            }
+        }
+    }
+
+    // Extract the remainder from the last (generatorLen - 1) bits
+    char *crc = (char *)malloc(generatorLen);
+    if (crc == NULL) {
+        printf("Memory allocation failed!\n");
+        free(remainder);
+        return NULL;
+    }
+    strcpy(crc, remainder + bitstreamLen - generatorLen + 1);
+    free(remainder);
+
+    return crc;
+}
+
+int main(int argc, char *argv[]) {
+    if (argc == 3) {
+        char *bitstream = argv[1];
+        printf("Bitstream: %s\n", bitstream);
+        char *generatorpolynom = argv[2];
+
+        // Calculate how many zeros need to be appended
+        int zerosToAppend = calculate_degree(generatorpolynom, strlen(generatorpolynom));
+        if (zerosToAppend < 0) {
+            printf("Invalid generator polynomial.\n");
+            return 1;
+        }
+
+        // Append zeros to the bitstream
+        char *zerodBitstream = appendZerosToBitstream(bitstream, zerosToAppend);
+        if (zerodBitstream == NULL) {
+            return 1;
+        }
+
+        printf("Modified bitstream (with zeros): %s\n", zerodBitstream);
+
+        // Perform bitwise division
+        char *crc = perform_xor_division(zerodBitstream, generatorpolynom);
+        if (crc == NULL) {
+            free(zerodBitstream);
+            return 1;
+        }
+
+        printf("CRC remainder: %s\n", crc);
+        printf("HDLC Header: %s\n",strcat(bitstream,crc));
+
+        free(zerodBitstream);
+        free(crc);
+
+        return 0;
+    } else {
+        printf("Usage: %s <bitstream> <generator polynomial>\n", argv[0]);
+        return 1;
+    }
+}