/* A permutation is an ordered arrangement of objects. For example, 3124 is one possible permutation of the digits 1, 2, 3 and 4.
 * If all of the permutations are listed numerically or alphabetically, we call it lexicographic order. The lexicographic permutations of 0, 1 and 2 are:
 *
 * 012   021   102   120   201   210
 *
 * What is the millionth lexicographic permutation of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9?*/

#define _POSIX_C_SOURCE 199309L

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

int compare(void *a, void *b);

int main(int argc, char **argv)
{
    int i, res[10];
    int **perm;
    double elapsed;
    struct timespec start, end;

    clock_gettime(CLOCK_MONOTONIC, &start);

    if((perm = (int **)malloc(10*sizeof(int *))) == NULL)
    {
        fprintf(stderr, "Error while allocating memory\n");
        return 1;
    }

    for(i = 0; i < 10; i++)
    {
        if((perm[i] = (int *)malloc(sizeof(int))) == NULL)
        {
            fprintf(stderr, "Error while allocating memory\n");
            return 1;
        }
        *perm[i] = i;
    }

    for(i = 0; i < 999999; i++)
    {
        /* Function that generates permutations in lexicographic order.
         * Finish when the 1000000th is found.*/
        next_permutation((void **)perm, 10, compare);
    }

    for(i = 0; i < 10; i++)
    {
        res[i] = *perm[i];
        free(perm[i]);
    }

    free(perm);

    clock_gettime(CLOCK_MONOTONIC, &end);

    printf("Project Euler, Problem 24\n");
    printf("Answer: ");

    for(i = 0; i < 10; i++)
    {
        printf("%d", res[i]);
    }

    printf("\n");

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

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

    return 0;
}

int compare(void *a, void *b)
{
    int *n1, *n2;

    n1 = (int *)a;
    n2 = (int *)b;

    return *n1 - *n2;
}