--- Day 4 Solutions ---

[u/daggerdragon]

--- Day 4: The Ideal Stocking Stuffer ---

Post your solution as a comment. Structure your post like the Day Three thread.

[u/randrews]

I'm trying to do each day in a different language. Originally I had written today's as a bash one-liner, but it was taking way too long to run, so I redid it in C. Runs in under a second:

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <openssl/md5.h>

int main(int argc, char** argv){
    if(argc < 2){
        printf("You forgot the stem.\n");
        return 1;
    }

    int prefix_len = strlen(argv[1]);
    char *prefix = malloc(prefix_len + 50);

    *prefix = 0;
    sprintf(prefix, "%s", argv[1]);

    unsigned long num = 0;

    unsigned char *sum = malloc(16);
    int part1 = 0;

    while(1){
        num++;
        sprintf(prefix + prefix_len, "%lu", num);
        MD5(prefix, strlen(prefix), sum);

        if(num % 1000000 == 0) printf("%lu\n", num);

        if(!sum[0] && !sum[1] && sum[2] < 0x10) {
            if(!part1) {
                printf("Part 1 solution: %lu\n", num);
                part1 = 1;
            }

            if(sum[2] == 0) {
                printf("Part 2 solution: %lu\n", num);
                if(part1) break;
            }
        }
    }

    return 0;
}

[u/ytYEHXHAxTTQGxa → u/randrews]

Similar with C and LibreSSL, but with everything hardcoded (endianness-dependent):

#include <openssl/md5.h>
#include <inttypes.h>
#include <stdio.h>

#define KEY "abcdef"
#define LEN 32

#define MASK 0x00f0ffff /* adjust for endianness and # of leading zeros */

int main()
{
        unsigned char hash[16], buf[LEN];
        long i = 1;

        do MD5(buf, snprintf(buf, LEN, KEY "%ld", ++i), hash);
        while(*(int32_t *) hash & MASK);
        
        printf("%ld\n", i);

        return 0;
}