project-euler-solutions/C/p079.c

/* A common security method used for online banking is to ask the user for three random characters from a passcode.
 * For example, if the passcode was 531278, they may ask for the 2nd, 3rd, and 5th characters; the expected reply would be: 317.
 *
 * The text file, keylog.txt, contains fifty successful login attempts.
 *
 * Given that the three characters are always asked for in order, analyse the file so as to determine the shortest possible
 * secret passcode of unknown length.*/

#define _POSIX_C_SOURCE 199309L

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

int compare(void *a, void *b);
int check_passcode(int **passcode, int len, int **logins, int n);

int main(int argc, char **argv)
{
    int i, j, keylog, len = 4, found = 0, digits[10] = {0}, passcode_digits[10] = {0}, **passcode, **logins;
    char line[5];
    FILE *fp;
    double elapsed;
    struct timespec start, end;

    clock_gettime(CLOCK_MONOTONIC, &start);

    if((fp = fopen("keylog.txt", "r")) == NULL)
    {
        fprintf(stderr, "Error while opening file %s\n", "keylog.txt");
        return 1;
    }

    if((logins = (int **)malloc(50*sizeof(int*))) == NULL)
    {
        fprintf(stderr, "Error while allocating memory\n");
        return 1;
    }

    for(i = 0; i < 50; i++)
    {
        if((logins[i] = (int *)malloc(3*sizeof(int))) == NULL)
        {
            fprintf(stderr, "Error while allocating memory\n");
            return 1;
        }
    }

    i = 0;

    while(fscanf(fp, "%s", line) != EOF)
    {
        j = 2;
        keylog = atoi(line);

        while(keylog > 0)
        {
            logins[i][j--] = keylog % 10;
            /* Check which digits are present in the login attempts.*/
            digits[keylog%10]++;
            keylog /= 10;
        }

        i++;
    }

    fclose(fp);

    j = 0;
    for(i = 0; i < 10; i++)
    {
        /* To generate the passcode, only use the digits present in the
         * login attempts.*/
        if(digits[i] > 0)
        {
            passcode_digits[j++] = i;
        }
    }

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

        for(i = 0; i < len; i++)
        {
            if((passcode[i] = (int *)malloc(sizeof(int))) == NULL)
            {
                fprintf(stderr, "Error while allocating memory\n");
                return 1;
            }
            /* For the current length, generate the first passcode with the
             * digits in order.*/
            *passcode[i] = passcode_digits[i];
        }

        /* Check if the passcode is compatible with the login attempts.*/
        if(check_passcode(passcode, len, logins, 50))
        {
            found = 1;
            break;
        }

        /* For the given length, check every permutation until the correct
         * passcode has been found, or all the permutations have been tried.*/
        while(next_permutation((void **)passcode, len, compare) != -1)
        {
            if(check_passcode(passcode, len, logins, 50))
            {
                printf("Project Euler, Problem 79\n");
                printf("Answer: ");
                for(i = 0; i < len; i++)
                    printf("%d", *passcode[i]);
                printf("\n");

                found = 1;
                break;
            }
        }

        for(i = 0; i < len; i++)
        {
            free(passcode[i]);
        }
        free(passcode);

        /* If the passcode has not yet been found, try a longer passcode.*/
        len++;
    }

    for(i = 0; i < 50; i++)
    {
        free(logins[i]);
    }

    free(logins);

    clock_gettime(CLOCK_MONOTONIC, &end);

    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;
}

int check_passcode(int **passcode, int len, int **logins, int n)
{
    int i, j, k;

    /* For every login attempt, check if all the digits appear in the
     * passcode in the correct order. Return 0 if a login attempt
     * incompatible with the current passcode is found.*/
    for(i = 0; i < n; i++)
    {
        k = 0;
        for(j = 0; j < len; j++)
        {
            if(*passcode[j] == logins[i][k])
            {
                k++;

                if(k == 3)
                {
                    break;
                }
            }
        }

        if(k < 3)
        {
            return 0;
        }
    }

    return 1;
}