Tests and documentation for vtzero::key_index/value_index

[springmeyer]

@joto - I noticed that you are working on a more flexible way to control how the tables are built for keys and values during the building of a new vector tile.

The new interface I notice in https://github.com/mapbox/vtzero/blob/master/include/vtzero/index.hpp looks powerful - the idea of allowing the programmer to control whether a std::map or std::unordered_map (or something else like a sso optimized map - https://github.com/greg7mdp/sho) is used it excellent.

I'd like to put a request in to prioritize this code for documentation and tests because I anticipate that @GretaCB and I will want to use it, and benchmark plugging in different containers.

Currently in our code I'm seeing that the default implementation (which appears to iterate the protobuf data each time a key/value is added?) is a bottleneck. So I'm hoping a hashmap will be faster:

refs

vtzero/include/vtzero/builder_impl.hpp

Lines 66 to 79 in 094cd4b

	static index_value find_in_table(const data_view text, const std::string& data) {
	uint32_t index = 0;
	protozero::pbf_message<detail::pbf_layer> pbf_table{data};
	while (pbf_table.next()) {
	const auto v = pbf_table.get_view();
	if (v == text) {
	return index_value{index};
	}
	++index;
	}
	return index_value{};
	}

[springmeyer]

done in d90bb18, thank you @joto