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

#define FRAME_START 0x7E // Start of Frame
#define FRAME_END 0x7E   // End of Frame
#define STUFFING_CHAR 0x7D // Escape character
#define ESCAPED_CHAR 0x20  // Value to append after STUFFING_CHAR

void stuffCharacters(char *input, char *output) {
    int i, j = 0;
    int length = strlen(input);

    // Adding the frame start character
    output[j++] = FRAME_START;

    for (i = 0; i < length; i++) {
        if (input[i] == FRAME_START || input[i] == FRAME_END || input[i] == STUFFING_CHAR) {
            output[j++] = STUFFING_CHAR; // Stuffing character
            output[j++] = input[i] ^ ESCAPED_CHAR; // Escape the character
        } else {
            output[j++] = input[i]; // Normal character
        }
    }

    // Adding the frame end charactera
    output[j++] = FRAME_END;
    output[j] = '\0'; // Null-terminate the output string
}

void unstuffCharacters(char *input, char *output) {
    int i, j = 0;
    int length = strlen(input);

    // Skip frame start
    i = 1; 

    while (i < length - 1) { // Skip frame end
        if (input[i] == STUFFING_CHAR) {
            // Unstuffing character
            output[j++] = input[i + 1] ^ ESCAPED_CHAR;
            i += 2; // Move past the stuffed character
        } else {
            output[j++] = input[i++];
        }
    }

    output[j] = '\0'; // Null-terminate the output string
}

int main() {
    char input[256]; // Buffer for input message
    char stuffed[512]; // Buffer for stuffed message (larger to accommodate potential expansion)
    char unstuffed[256]; // Buffer for unstuffed message

    printf("Enter your message: ");
    fgets(input, sizeof(input), stdin); // Read input from the keyboard

    // Remove newline character if present
    input[strcspn(input, "\n")] = 0;

    printf("Original Message: %s\n", input);

    // Perform character stuffing
    stuffCharacters(input, stuffed);
    printf("Stuffed Message: ");
    for (int i = 0; stuffed[i] != '\0'; i++) {
        printf("%02X ", (unsigned char)stuffed[i]);
    }
    printf("\n");

    // Perform character unstuffing
    unstuffCharacters(stuffed, unstuffed);
    printf("Unstuffed Message: %s\n", unstuffed);

    return 0;
}