The algorithm of Arcaea's lowest.world/connect

lowest-world-connect.c

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

int main() {
    time_t time_val = time(NULL);
    struct tm *t = gmtime(&time_val);

    size_t year = t->tm_year + 1900;
    size_t month = t->tm_mon;
    size_t day = t->tm_mday;

    char s[100];
    size_t l = sprintf(s, "%zuori%zuwol%zuoihs%zuotas", year, month, day, day);

    printf("s = %s\n", s);

    char digest[16];
    MD5(s, l, digest);

    char digest_string[32 + 1];
    for (size_t i = 0; i < 16; i++) {
        sprintf(&digest_string[i * 2], "%2.2hhx", digest[i]);
    }
    digest_string[32] = 0;

    printf("md5 = %s\n", digest_string);

    const char *table = "qwertyuiopasdfghjklzxcvbnm1234567890";
    const size_t tl = strlen(table);

    char transformed[32 + 1];
    for (size_t i = 0; i < 32; i++)
        transformed[i] = table[digest_string[i] % tl];
    transformed[32] = 0;
    printf("transformed = %s\n", transformed);

    const size_t pos[] = { 1, 20, 4, 30, 2, 11, 23 };
    const size_t pos_count = sizeof(pos) / sizeof(pos[0]);

    char code[10], *p = code;
    for (size_t i = 0; i < pos_count; i++) {
        size_t j = pos[i];
        *p++ = transformed[j];
    }
    *p = 0;

    printf("code = %s\n", code);
}