/* The sum of the squares of the first ten natural numbers is,
 *
 * 1^2 + 2^2 + ... + 10^2 = 385
 *
 * The square of the sum of the first ten natural numbers is,
 *
 * (1 + 2 + ... + 10)^2 = 55^2 = 3025
 *
 * Hence the difference between the sum of the squares of the first ten natural numbers and the square of the sum is 3025 − 385 = 2640.
 *
 * Find the difference between the sum of the squares of the first one hundred natural numbers and the square of the sum.*/

#define _POSIX_C_SOURCE 199309L

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(int argc, char **argv)
{
    int i, sum_squares = 0, square_sum = 0;
    double elapsed;
    struct timespec start, end;

    clock_gettime(CLOCK_MONOTONIC, &start);

    /* Straightforward brute-force approach.*/
    for(i = 1; i <= 100; i++)
    {
        sum_squares += i*i;
        square_sum += i;
    }

    square_sum *= square_sum;

    clock_gettime(CLOCK_MONOTONIC, &end);

    elapsed = (end.tv_sec - start.tv_sec) + (double)(end.tv_nsec - start.tv_nsec) / 1000000000;

    printf("Project Euler, Problem 6\n");
    printf("Answer: %d\n", square_sum-sum_squares);

    printf("Elapsed time: %.9lf seconds\n", elapsed);

    return 0;
}