Checksum Validation

Code

#include <stdio.h>
#include <stdint.h>

int validate_checksum(int *mem, int n) 
{
    if ((n < 2) || (mem == NULL))
    {
        return 0;
    }

    int result = 0;
    int check_sum = mem[0];
    uint8_t index = 1;
    
    for ( ; index < (n - 1); index++)
    {
        check_sum ^= mem[index];
    }

    if (check_sum == mem[n - 1])
    {
        result = 1;
    }
    else
    {
        result = 0;
    }

    return result;
}

int main() 
{
    int n, arr[100];
    scanf("%d", &n);
    
    for (int i = 0; i < n; i++) 
    {
        scanf("%d", &arr[i]);
    }

    int result = validate_checksum(arr, n);
    printf("%d", result);

    return 0;
}

Solving Approach

This C program is designed to verify the integrity of a data set using a Checksum logic. Specifically, it uses the XOR (Exclusive OR) bitwise operation to determine if the data has been corrupted or if it matches the provided "check value."

1. High-Level Logic

The program assumes that the input array (the "packet") is structured as follows:

• The Data: All elements from index 0 up to index n-2.

• The Checksum: The very last element in the array (mem[n-1]).

The function calculates its own version of the checksum by XORing all data elements together. If the calculated value matches the stored last element, the data is considered valid.

2. Key Components Explained

The validate_checksum Function

This is the core logic of the program. Here is how it breaks down:

• Safety Checks: ```c

  if ((n < 2) || (mem == NULL)) return 0;

  It first ensures there are at least two elements (one piece of data and one checksum) and that the memory pointer isn't empty.
  

• The Accumulator: It starts with the first element (mem[0]) and then loops through the rest of the data.

• The XOR Operation (^=):

  XOR is ideal for checksums because it is reversible and fast. If you XOR a sequence of numbers, any single bit change in the data will result in a different final check_sum.

• The Comparison:

  Finally, it compares the calculated check_sum against the value stored at mem[n-1]. It returns 1 for a match (Success) and 0 for a mismatch (Failure).

The main Function

This acts as the driver:

1. It asks for the number of elements (n).

2. It populates an array (arr) with user input.

3. It calls the validation function and prints the result.

3. A Practical Example

Imagine you input n = 4 with the values: 10, 20, 30, 0.

4. Potential Improvements

While the code is functional, here are two things to keep in mind:

• Variable Type: The index is declared as uint8_t. This limits the loop to a maximum of 255 iterations. If the user inputs n = 300, the loop will wrap around (integer overflow) and cause an infinite loop or a crash. Using int or size_t for the index is safer.

• Scalability: The arr[100] in main is fixed. If the user enters an n greater than 100, the program will experience a buffer overflow.