2.x branch optimization

[springmeyer]

I noticed that layer_map<CoordinateType> decode_tile(std::string const& buffer) at

vector-tile/include/mapbox/vector_tile.hpp

Lines 62 to 84 in 95c240b

	layer_map<CoordinateType> decode_tile(std::string const& buffer)
	{
	layer_map<CoordinateType> m;
	vtzero::vector_tile tile(buffer);
	while (auto layer = tile.next_layer())
	{
	mapbox::feature::feature_collection<CoordinateType> fc;
	while (auto feature = layer.next_feature())
	{
	auto f = extract_feature<CoordinateType>(feature);
	if (!f.geometry.template is<mapbox::geometry::empty>())
	{
	fc.push_back(f);
	}
	}
	if (!fc.empty())
	{
	m.emplace(std::string(layer.name()), std::move(fc));
	}
	}
	return m;
	}

does not leverage pre-allocation optimizations. We should likely do:

diff --git a/include/mapbox/vector_tile.hpp b/include/mapbox/vector_tile.hpp
index c61640b..0e8c672 100644
--- a/include/mapbox/vector_tile.hpp
+++ b/include/mapbox/vector_tile.hpp
@@ -66,12 +66,13 @@ layer_map<CoordinateType> decode_tile(std::string const& buffer)
     while (auto layer = tile.next_layer())
     {
         mapbox::feature::feature_collection<CoordinateType> fc;
+        fc.reserve(layer.num_features());
         while (auto feature = layer.next_feature())
         {
             auto f = extract_feature<CoordinateType>(feature);
             if (!f.geometry.template is<mapbox::geometry::empty>())
             {
-                fc.push_back(f);
+                fc.push_back(std::move(f));
             }
         }

/cc @flippmoke to review and apply if this looks good.