/* An irrational decimal fraction is created by concatenating the positive integers:
 *
 * 0.123456789101112131415161718192021...
 *
 * It can be seen that the 12th digit of the fractional part is 1.
 *
 * If d_n represents the nth digit of the fractional part, find the value of the following expression.
 *
 * d_1 × d_10 × d_100 × d_1000 × d_10000 × d_100000 × d_1000000*/

#define _POSIX_C_SOURCE 199309L

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

int main(int argc, char **argv)
{
    int i, value, n;
    int digits[1000005];
    double elapsed;
    struct timespec start, end;

    clock_gettime(CLOCK_MONOTONIC, &start);

    i = 1;
    value = 1;

    /* Loop on all numbers and put the digits in the right place
     * in an array. Use modulo and division to get the digits
     * for numbers with more than one digit.*/
    while(i <= 1000000)
    {
        if(value < 10)
        {
            digits[i-1] = value;
            i++;
        }
        else if(value < 100)
        {
            digits[i-1] = value / 10;
            digits[i] = value % 10;
            i += 2;
        }
        else if(value < 1000)
        {
            digits[i-1] = value / 100;
            digits[i] = (value / 10) % 10;
            digits[i+1] = value % 10;
            i += 3;
        }
        else if(value < 10000)
        {
            digits[i-1] = value / 1000;
            digits[i] = (value / 100) % 10;
            digits[i+1] = (value / 10) % 10;
            digits[i+2] = value % 10;
            i += 4;
        }
        else if(value < 100000)
        {
            digits[i-1] = value / 10000;
            digits[i] = (value / 1000) % 10;
            digits[i+1] = (value / 100) % 10;
            digits[i+2] = (value / 10) % 10;
            digits[i+3] = value % 10;
            i += 5;
        }
        else if(value < 1000000)
        {
            digits[i-1] = value / 100000;
            digits[i] = (value / 10000) % 10;
            digits[i+1] = (value / 1000) % 10;
            digits[i+2] = (value / 100) % 10;
            digits[i+3] = (value / 10) % 10;
            digits[i+4] = value % 10;
            i += 6;
        }
        value++;
    }

    /* Calculate the product.*/
    n = digits[0] * digits[9] * digits[99] * digits[999] * digits[9999] * digits[99999] * digits[999999];

    clock_gettime(CLOCK_MONOTONIC, &end);

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

    printf("Project Euler, Problem 40\n");
    printf("Answer: %d\n", n);

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

    return 0;
}