Print out the Bukkit/Spigot “lastKnownName” tag if it exists.

nbtologist.c

/* "Where my people at?"
 *
 * NBTologist: search through an un-gzipped player.dat NBT file for the player's
 * X/Y/Z coordinates.
 */

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BUF_SIZE 128

/*
 * Read the given `source` to EOF and place its contents into `dest`.
 *
 * Arguments:   source, the source file
 *              dest, the destination block of memory; expected to be null
 * Returns:     the number of bytes read (i.e., the useful size of dest)
 */
size_t consume_file(FILE *source, char **dest)
{
    size_t allocated = 0;
    size_t size = 0;

    int c;
    while ((c = fgetc(source)) != EOF)
    {
        if (size == allocated)
        {
            if (allocated == 0) allocated = BUF_SIZE;
            else allocated *= 2;
            *dest = realloc(*dest, allocated);
        }

        (*dest)[size++] = (char) c;
    }

    return size;
}

/*
 * Find the first occurrence of a binary string in another binary string.
 *
 * Arguments:   haystack, the string within which to search
 *              haystack_size, the size of the haystack
 *              needle, the search string
 *              needle_size, the size of the search string
 * Returns:     the first index of `needle` within `haystack` or -1 if not found
 */
int memfind(const char *haystack, size_t haystack_size, const char *needle, size_t needle_size)
{
    for (int i = 0; i < haystack_size - needle_size; i++)
        if (memcmp(haystack + i, needle, needle_size) == 0)
            return i;

    return -1;
}

/*
 * Retrieve the big endian short integer at `pos` in `ptr`.
 */
uint16_t short_at(char *ptr, size_t pos)
{
    return __bswap_16(*(uint16_t *) &ptr[pos]);
}

/*
 * Retrieve the big endian double at `pos` in `ptr`.
 */
double double_at(char *ptr, size_t pos)
{
    double val;
    *((uint64_t *) &val) = __bswap_64(*(uint64_t *) &ptr[pos]);
    return val;
}

int main(int argc, char *argv[])
{
    /* Read the (uncompressed) NBT data from stdin. */
    char *player = NULL;
    size_t size = consume_file(stdin, &player);

    /* Figure out where the "Pos" list is. */
    int pos = memfind(player, size, "Pos\x06\x00\x00\x00\x03", 8);
    if (pos < 0)
        return EXIT_FAILURE;

    /* Print out the values. */
    printf("%f %f %f",
        double_at(player, pos + 8),
        double_at(player, pos + 8 + sizeof(uint64_t)),
        double_at(player, pos + 8 + sizeof(uint64_t) + sizeof(uint64_t)));

    /* See if the Bukkit "lastKnownName" tag exists. */
    int name_pos = memfind(player, size, "lastKnownName", 13);
    if (name_pos >= 0)
    {
        int name_length = short_at(player, name_pos + 13);
        printf(" %.*s", name_length, player + name_pos + 15);
    }

    putchar('\n');

    return EXIT_SUCCESS;
}