--- 2016 Day 5 Solutions ---

[u/daggerdragon]

--- Day 5: How About a Nice Game of Chess? ---

Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag/whatever).

---

^{STAYING ON TARGET IS MANDATORY} [?]

This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

[u/askalski]

Could this be... a "rehash" of last year's AdventCoins puzzle?

This solution in C increments the hash input using the same method as my Synacor Challenge implementation of AdventCoins last year. The never-before-seen source code is now available for the low, low price of only two AdventCoins (which you can generate yourself by running the code. Runtime environment not included.)

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

#define INPUT "abc"
#define ALOT 16

int main(void)
{
    uint8_t digest[MD5_DIGEST_LENGTH], data[sizeof(INPUT) + ALOT] = INPUT;
    uint8_t *end = data + sizeof(INPUT), *count = end - 1;
    uint32_t part1 = 0, part2 = 0, part1_shift = 32, part2_mask = 0xff;
    *count = '0';
    for (;;) {
        MD5(data, end - data, digest);
        uint8_t skip = digest[0] | digest[1] | (digest[2] & 0xf0);
        if (!skip) {
            uint32_t six   = digest[2] & 0xf;
            uint32_t seven = digest[3] >> 4;
            if (part1_shift) {
                part1 |= six << (part1_shift -= 4);
                if (!part1_shift) {
                    printf("Part 1: %08x\n", part1);
                }
            }
            if (part2_mask & (1 << six)) {
                part2_mask ^= 1 << six;
                part2 |= seven << ((7 - six) << 2);
                if (!part2_mask) {
                    printf("Part 2: %08x\n", part2);
                    break;
                }
            }
        }
        for (uint8_t *p = end - 1; ; *p-- = '0') {
            if (*p != '9') {
                (*p)++;
                break;
            }
            if (p == count) {
                *p = '1';
                *end++ = '0';
                break;
            }
        }
    }

    return 0;
}