/* seek to the n-th entry */
    struct fat16_read_callback_arg arg;
    memset(&arg, 0, sizeof(arg));
    arg.entry_num = entry_num;
    if(!device_read_interval(header->root_dir_offset,
                             buffer,
                             sizeof(buffer),
                             header->cluster_zero_offset - header->root_dir_offset,
                             fat16_dir_entry_seek_callback,
                             &arg) ||
       arg.entry_offset == 0
      )
        return 0;

    /* read entry */
    memset(dir_entry, 0, sizeof(*dir_entry));
    if(!device_read_interval(arg.entry_offset,
                             buffer,
                             sizeof(buffer),
                             arg.byte_count,
                             fat16_dir_entry_read_callback,
                             dir_entry))
        return 0;

    return dir_entry->long_name[0] != '\0' ? 1 : 0;
}

/**
 * \ingroup fat16_fs
 * Reads a directory entry of a given parent directory.
 *
 * \param[in] fs Descriptor of file system to use.
 * \param[in] entry_num The index of the directory entry to read.
 * \param[in] parent Directory entry descriptor in which to read directory entry.
 * \param[out] dir_entry Directory entry descriptor which will get filled.
 * \returns 0 on failure, 1 on success
 * \see fat16_read_root_dir_entry, fat16_read_dir_entry_by_path
 */
u8 fat16_read_sub_dir_entry(const struct fat16_fs_struct* fs, u16 entry_num, const struct fat16_dir_entry_struct* parent, struct fat16_dir_entry_struct* dir_entry)
{
    if(!fs || !parent || !dir_entry)
        return 0;

    /* we are in a parent directory and want to search within its directory entry table */
    if(!(parent->attributes & FAT16_ATTRIB_DIR))
        return 0;
	
    /* loop through all clusters of the directory */
    u8 buffer[32];
    u32 cluster_offset;
    u16 cluster_size = fs->header.cluster_size;
    u16 cluster_num = parent->cluster;
    struct fat16_read_callback_arg arg;

    while(1)
    { 
        /* calculate new cluster offset */
        cluster_offset = fs->header.cluster_zero_offset + (u32) (cluster_num - 2) * cluster_size;

        /* seek to the n-th entry */
        memset(&arg, 0, sizeof(arg));
        arg.entry_num = entry_num;
        if(!fs->partition->device_read_interval(cluster_offset,
                                                buffer,
                                                sizeof(buffer),
                                                cluster_size,
                                                fat16_dir_entry_seek_callback,
                                                &arg)
          )
        return 0;
		
        /* check if we found the entry */
        if(arg.entry_offset)
		    break;
		
        /* get number of next cluster */
        if(!(cluster_num = fat16_get_next_cluster(fs, cluster_num)))
            return 0; /* directory entry not found */
    }

    memset(dir_entry, 0, sizeof(*dir_entry));

    /* read entry */
    if(!fs->partition->device_read_interval(arg.entry_offset,
                                            buffer,
                                            sizeof(buffer),
                                            arg.byte_count,
                                            fat16_dir_entry_read_callback,
                                            dir_entry))
        return 0;

    return dir_entry->long_name[0] != '\0' ? 1 : 0;
}

/**
 * \ingroup fat16_fs
 * Callback function for seeking through subdirectory entries.
 */
u8 fat16_dir_entry_seek_callback(u8* buffer, u32 offset, void* p)
{
    struct fat16_read_callback_arg* arg = p;

    /* skip deleted or empty entries */
    if(buffer[0] == FAT16_DIRENTRY_DELETED || !buffer[0])
        return 1;

    if(arg->entry_cur == arg->entry_num)
    {
        arg->entry_offset = offset;
        arg->byte_count = buffer[11] == 0x0f ?
                          ((buffer[0] & FAT16_DIRENTRY_LFNSEQMASK) + 1) * 32 :
                          32;
        return 0;
    }

    /* if we read a 8.3 entry, we reached a new directory entry */
    if(buffer[11] != 0x0f)
        ++arg->entry_cur;

    return 1;
}

/**
 * \ingroup fat16_fs
 * Callback function for reading a directory entry.
 */
u8 fat16_dir_entry_read_callback(u8* buffer, u32 offset, void* p)
{
    struct fat16_dir_entry_struct* dir_entry = p;

    /* there should not be any deleted or empty entries */
    if(buffer[0] == FAT16_DIRENTRY_DELETED || !buffer[0])
        return 0;

    if(!dir_entry->entry_offset)
        dir_entry->entry_offset = offset;
    
    switch(fat16_interpret_dir_entry(dir_entry, buffer))
    {
        case 0: /* failure */
            return 0;
        case 1: /* buffer successfully parsed, continue */
            return 1;
        case 2: /* directory entry complete, finish */
            return 0;
    }

    return 0;
}

/**
 * \ingroup fat16_fs
 * Interprets a raw directory entry and puts the contained
 * information into the directory entry.
 * 
 * For a single file there may exist multiple directory
 * entries. All except the last one are lfn entries, which
 * contain parts of the long filename. The last directory
 * entry is a traditional 8.3 style one. It contains all
 * other information like size, cluster, date and time.
 * 
 * \param[in,out] dir_entry The directory entry to fill.
 * \param[in] raw_entry A pointer to 32 bytes of raw data.
 * \returns 0 on failure, 1 on success and 2 if the
 *          directory entry is complete.
 */
u8 fat16_interpret_dir_entry(struct fat16_dir_entry_struct* dir_entry, const u8* raw_entry)
{
    if(!dir_entry || !raw_entry || !raw_entry[0])
        return 0;

    char* long_name = dir_entry->long_name;
    if(raw_entry[11] == 0x0f)
    {
        u16 char_offset = ((raw_entry[0] & 0x3f) - 1) * 13;

        if(char_offset + 12 < sizeof(dir_entry->long_name))
        {
            /* Lfn supports unicode, but we do not, for now.
             * So we assume pure ascii and read only every
             * second byte.
             */
            long_name[char_offset + 0] = raw_entry[1];
            long_name[char_offset + 1] = raw_entry[3];
            long_name[char_offset + 2] = raw_entry[5];
            long_name[char_offset + 3] = raw_entry[7];
            long_name[char_offset + 4] = raw_entry[9];
            long_name[char_offset + 5] = raw_entry[14];
            long_name[char_offset + 6] = raw_entry[16];
            long_name[char_offset + 7] = raw_entry[18];
            long_name[char_offset + 8] = raw_entry[20];
            long_name[char_offset + 9] = raw_entry[22];
            long_name[char_offset + 10] = raw_entry[24];
            long_name[char_offset + 11] = raw_entry[28];
            long_name[char_offset + 12] = raw_entry[30];
        }

        return 1;
    }
    else
    {
        /* if we do not have a long name, take the short one */
        if(long_name[0] == '\0')
        {
            u8 i;
            for(i = 0; i < 8; ++i)
            {
                if(raw_entry[i] == ' ')
                    break;
                long_name[i] = raw_entry[i];
            }
            if(long_name[0] == 0x05)
                long_name[0] = (char) FAT16_DIRENTRY_DELETED;

            if(raw_entry[8] != ' ')
            {
                long_name[i++] = '.';

                u8 j = 8;
                for(; j < 11; ++j)
                {
                    if(raw_entry[j] != ' ')
                    {
                        long_name[i++] = raw_entry[j];
                    }
                    else
                    {
                        break;
                    }
                }
            } 

            long_name[i] = '\0';
        }
        
        /* extract properties of file and store them within the structure */
        dir_entry->attributes = raw_entry[11];
        dir_entry->cluster = ((u16) raw_entry[26]) |
                             ((u16) raw_entry[27] << 8);
        dir_entry->file_size = ((u32) raw_entry[28]) |
                               ((u32) raw_entry[29] << 8) |
                               ((u32) raw_entry[30] << 16) |
                               ((u32) raw_entry[31] << 24);

#if FAT16_DATETIME_SUPPORT
        dir_entry->modification_time = ((u16) raw_entry[22]) |
                                       ((u16) raw_entry[23] << 8);
        dir_entry->modification_date = ((u16) raw_entry[24]) |
                                       ((u16) raw_entry[25] << 8);
#endif

        return 2;
    }
}

/**
 * \ingroup fat16_file
 * Retrieves the directory entry of a path.
 *
 * The given path may both describe a file or a directory.
 *
 * \param[in] fs The FAT16 filesystem on which to search.
 * \param[in] path The path of which to read the directory entry.
 * \param[out] dir_entry The directory entry to fill.
 * \returns 0 on failure, 1 on success.
 * \see fat16_read_dir
 */
u8 fat16_get_dir_entry_of_path(struct fat16_fs_struct* fs, const char* path, struct fat16_dir_entry_struct* dir_entry)
{
    if(!fs || !path || path[0] == '\0' || !dir_entry)
        return 0;

    if(path[0] == '/')
        ++path;

    /* begin with the root directory */
    memset(dir_entry, 0, sizeof(*dir_entry));
    dir_entry->attributes = FAT16_ATTRIB_DIR;

    if(path[0] == '\0')
        return 1;

    while(1)
    {
        struct fat16_dir_struct* dd = fat16_open_dir(fs, dir_entry);
        if(!dd)
            break;

        /* extract the next hierarchy we will search for */
        const char* sep_pos = strchr(path, '/');
        if(!sep_pos)
            sep_pos = path + strlen(path);
        u8 length_to_sep = sep_pos - path;
        
        /* read directory entries */
        while(fat16_read_dir(dd, dir_entry))
        {
            /* check if we have found the next hierarchy */
            if((strlen(dir_entry->long_name) != length_to_sep ||
                strncmp(path, dir_entry->long_name, length_to_sep) != 0))
                continue;

            fat16_close_dir(dd);
            dd = 0;

            if(path[length_to_sep] == '\0')
                /* we iterated through the whole path and have found the file */
                return 1;

            if(dir_entry->attributes & FAT16_ATTRIB_DIR)
            {
                /* we found a parent directory of the file we are searching for */
                path = sep_pos + 1;
                break;
            }

            /* a parent of the file exists, but not the file itself */
            return 0;
        }

        fat16_close_dir(dd);
    }
    
    return 0;
}

/**
 * \ingroup fat16_fs
 * Retrieves the next following cluster of a given cluster.
 *
 * Using the filesystem file allocation table, this function returns
 * the number of the cluster containing the data directly following
 * the data within the cluster with the given number.
 *
 * \param[in] fs The filesystem for which to determine the next cluster.
 * \param[in] cluster_num The number of the cluster for which to determine its successor.
 * \returns The wanted cluster number, or 0 on error.
 */
u16 fat16_get_next_cluster(const struct fat16_fs_struct* fs, u16 cluster_num)
{
    if(!fs || cluster_num < 2)
        return 0;

    /* read appropriate fat entry */
    u8 fat_entry[2];
    if(!fs->partition->device_read(fs->header.fat_offset + 2 * cluster_num, fat_entry, 2))
        return 0;

    /* determine next cluster from fat */
    cluster_num = ((u16) fat_entry[0]) |
                  ((u16) fat_entry[1] << 8);
    
    if(cluster_num == FAT16_CLUSTER_FREE ||
       cluster_num == FAT16_CLUSTER_BAD ||
       (cluster_num >= FAT16_CLUSTER_RESERVED_MIN && cluster_num <= FAT16_CLUSTER_RESERVED_MAX) ||
       (cluster_num >= FAT16_CLUSTER_LAST_MIN && cluster_num <= FAT16_CLUSTER_LAST_MAX))
        return 0;
    
    return cluster_num;
}

/**
 * \ingroup fat16_fs
 * Appends a new cluster chain to an existing one.
 *
 * Set cluster_num to zero to create a completely new one.
 *
 * \param[in] fs The file system on which to operate.
 * \param[in] cluster_num The cluster to which to append the new chain.
 * \param[in] count The number of clusters to allocate.
 * \returns 0 on failure, the number of the first new cluster on success.
 */
u16 fat16_append_clusters(const struct fat16_fs_struct* fs, u16 cluster_num, u16 count)
{
#if FAT16_WRITE_SUPPORT
    if(!fs)
        return 0;

    device_read_t device_read = fs->partition->device_read;
    device_write_t device_write = fs->partition->device_write;
    u32 fat_offset = fs->header.fat_offset;
    u16 cluster_max = fs->header.fat_size / 2;
    u16 cluster_next = 0;
    u16 count_left = count;
    u8 buffer[2];

    for(u16 cluster_new = 0; cluster_new < cluster_max; ++cluster_new)
    {
        if(!device_read(fat_offset + 2 * cluster_new, buffer, sizeof(buffer)))
            return 0;

        /* check if this is a free cluster */
        if(buffer[0] == (FAT16_CLUSTER_FREE & 0xff) &&
           buffer[1] == ((FAT16_CLUSTER_FREE >> 8) & 0xff))
        {
            /* allocate cluster */
            if(count_left == count)
            {
                buffer[0] = FAT16_CLUSTER_LAST_MAX & 0xff;
                buffer[1] = (FAT16_CLUSTER_LAST_MAX >> 8) & 0xff;
            }
            else
            {
                buffer[0] = cluster_next & 0xff;
                buffer[1] = (cluster_next >> 8) & 0xff;
            }

            if(!device_write(fat_offset + 2 * cluster_new, buffer, sizeof(buffer)))
                break;

            cluster_next = cluster_new;
            if(--count_left == 0)
                break;
        }
    }

    do
    {
        if(count_left > 0)
            break;