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

#define MAX_LEN 28

char t[MAX_LEN], cs[MAX_LEN], g[MAX_LEN];
int a, e, c, b;

void xor() {
    for (c = 1; c < strlen(g); c++)
        cs[c] = ((cs[c] == g[c]) ? '0' : '1');
}

void crc() {
    for (e = 0; e < strlen(g); e++)
        cs[e] = t[e];
    
    do {
        if (cs[0] == '1') {
            xor();
        }
        
        for (c = 0; c < strlen(g) - 1; c++)
            cs[c] = cs[c + 1];
        
        cs[c] = t[e++];
    } while (e <= a + strlen(g) - 1);
}

int main() {
    int flag = 0;
    do {
        printf("\n1. CRC-12\n2. CRC-16\n3. CRC-CCITT\n4. Exit\n\nEnter your option: ");
        scanf("%d", &b);

        switch (b) {
            case 1: 
                strcpy(g, "1100000001111");
                break;
            case 2: 
                strcpy(g, "11000000000000101");
                break;
            case 3: 
                strcpy(g, "10001000000100001");
                break;
            case 4: 
                return 0;
            default:
                printf("Invalid option. Please try again.\n");
                continue;
        }

        printf("\nEnter data: ");
        scanf("%s", t);
        printf("\n-----------------------\n");
        printf("Generating polynomial: %s\n", g);
        a = strlen(t);

        // Append zeros to the data
        for (e = a; e < a + strlen(g) - 1; e++)
            t[e] = '0';
        t[e] = '\0';  // Null terminate the string

        printf("--------------------------\n");
        printf("Modified data is: %s\n", t);
        printf("-----------------------\n");
        crc();
        printf("Checksum is: %s\n", cs);
        
        // Prepare the final codeword
        for (e = a; e < a + strlen(g) - 1; e++)
            t[e] = cs[e - a];
        printf("-----------------------\n");
        printf("Final codeword is: %s\n", t);
        printf("------------------------\n");

        // Error detection option
        printf("Test error detection (0: yes, 1: no)?: ");
        scanf("%d", &e);
        if (e == 0) {
            do {
                printf("\n\tEnter the position where error is to be inserted: ");
                scanf("%d", &e);
            } while (e <= 0 || e > a + strlen(g) - 1);
            
            t[e - 1] = (t[e - 1] == '0') ? '1' : '0'; // Toggle the bit
            printf("-----------------------\n");
            printf("\nErroneous data: %s\n", t);
        }

        crc();
        for (e = 0; (e < strlen(g) - 1) && (cs[e] != '1'); e++);
        if (e < strlen(g) - 1)
            printf("Error detected\n\n");
        else
            printf("No error detected\n\n");
        
        printf("-----------------------\n");
    } while (flag != 1);

    return 0;
}