Iterating over map or array returned with MMDB_get_value()

[gonzus]

I am working with the example data from the library's documentation:

{
    "names": {
        "en": "Germany",
        "de": "Deutschland"
    },
    "cities": [ "Berlin", "Frankfurt" ]
}

Say I call

MMDB_get_value(&result.entry, &entry_data, "names", NULL);

This will produce entry_data.type == 7, which is a MAP, as expected. How can I then retrieve the values for specific keys in this map? Because entry_data is a MMDB_entry_data_s and NOT a MMDB_entry_s, I cannot pass it into MMDB_get_value.

Same question would apply for an array:

MMDB_get_value(&result.entry, &entry_data, "cities", NULL);

This will produce with entry_data.type == 11, which is an ARRAY, as expected. How can I then retrieve the values for specific positions in this array?

[oschwald]

To iterate over collection, you will need to use the offset_to_next field.

See this example for maps:

libmaxminddb/src/maxminddb.c

Lines 1255 to 1283 in ed7a425

	while (size-- > 0) {
	MMDB_entry_data_s key, value;
	CHECKED_DECODE_ONE_FOLLOW(mmdb, offset, &key);
	uint32_t offset_to_value = key.offset_to_next;
	if (MMDB_DATA_TYPE_UTF8_STRING != key.type) {
	return MMDB_INVALID_DATA_ERROR;
	}
	if (key.data_size == path_elem_len &&
	!memcmp(path_elem, key.utf8_string, path_elem_len)) {
	DEBUG_MSG("found key matching path elem");
	CHECKED_DECODE_ONE_FOLLOW(mmdb, offset_to_value, &value);
	memcpy(entry_data, &value, sizeof(MMDB_entry_data_s));
	return MMDB_SUCCESS;
	} else {
	/* We don't want to follow a pointer here. If the next element is
	* a pointer we simply skip it and keep going */
	CHECKED_DECODE_ONE(mmdb, offset_to_value, &value);
	int status = skip_map_or_array(mmdb, &value);
	if (MMDB_SUCCESS != status) {
	return status;
	}
	offset = value.offset_to_next;
	}
	}

And for arrays:

libmaxminddb/src/maxminddb.c

Lines 1231 to 1239 in ed7a425

	for (int i = 0; i < array_index; i++) {
	/* We don't want to follow a pointer here. If the next element is a
	* pointer we simply skip it and keep going */
	CHECKED_DECODE_ONE(mmdb, entry_data->offset_to_next, entry_data);
	int status = skip_map_or_array(mmdb, entry_data);
	if (MMDB_SUCCESS != status) {
	return status;
	}
	}

[gonzus]

Thanks for the quick reply @oschwald. Is there a way to directly retrieve, without iteration, the value for a given key in a map, or the value for a given position in an array?

Also, there seems to be no public API to do any of this; the code you pointed to depends on a couple of macros and static functions, which I would have to replicate in my code. Is this correct? If so, are there any plans to expose such APIs as part of the stable public interface of the library? I would love to have the equivalent of at least map_retrieve(key), map_get_keys(), array_get_size(), array_fetch(position).

[oschwald]

You can look up particular map values or array index values directly with MMDB_get_values(), e.g., from your example above:

MMDB_get_value(&result.entry, &entry_data, "names", "en", NULL);

This would look up the value for the en key of the names map.

Similarly, the following would look up the first city:

MMDB_get_value(&result.entry, &entry_data, "cities", "0", NULL);

[gonzus]

Yes, this is how I am doing it today. But if I know before hand I will access several keys inside a map, I think it would be more efficient to first get the map entry, and then get the value for each key, instead of repeatedly searching for "names" / "en", "names" / "es", "names" / "de", etc.

[oschwald]

I see. I don't think we expose any functions to do this directly, although you likely could just create a new MMDB_entry_s with the offset to the map and then use MMDB_get_value on that.

[gonzus]

Ok, thanks for all the hints. I still think it would be nice to have an official API for this kind of access, but I understand if this is out of the scope for the library -- feel free to close the issue if this is not in the plans.

[oschwald]

We improved the docs around this.