Don't copy VT keys / values into each feature

Instead, maintain a key index ⇢ value index map and look up the
value in getValue().

src/mbgl/map/vector_tile.cpp

@@ -29,7 +29,8 @@ Value parseValue(pbf data) {
     return false;
 }
 
-VectorTileFeature::VectorTileFeature(pbf feature_pbf, const VectorTileLayer& layer) {
+VectorTileFeature::VectorTileFeature(pbf feature_pbf, const VectorTileLayer& layer_)
+    : layer(layer_) {
     while (feature_pbf.next()) {
         if (feature_pbf.tag == 1) { // id
             id = feature_pbf.varint<uint64_t>();
@@ -43,16 +44,16 @@ VectorTileFeature::VectorTileFeature(pbf feature_pbf, const VectorTileLayer& lay
                     throw std::runtime_error("feature referenced out of range key");
                 }
 
-                if (tags) {
-                    uint32_t tag_val = tags.varint();
-                    if (layer.values.size() <= tag_val) {
-                        throw std::runtime_error("feature referenced out of range value");
-                    }
-
-                    properties.emplace(layer.keys[tag_key], layer.values[tag_val]);
-                } else {
+                if (!tags) {
                     throw std::runtime_error("uneven number of feature tag ids");
                 }
+
+                uint32_t tag_val = tags.varint();
+                if (layer.values.size() <= tag_val) {
+                    throw std::runtime_error("feature referenced out of range value");
+                }
+
+                properties.emplace(tag_key, tag_val);
             }
         } else if (feature_pbf.tag == 3) { // type
             type = (FeatureType)feature_pbf.varint();
@@ -65,11 +66,15 @@ VectorTileFeature::VectorTileFeature(pbf feature_pbf, const VectorTileLayer& lay
 }
 
 mapbox::util::optional<Value> VectorTileFeature::getValue(const std::string& key) const {
-    auto it = properties.find(key);
-    if (it != properties.end()) {
-        return it->second;
+    auto keyIter = layer.keys.find(key);
+    if (keyIter == layer.keys.end()) {
+        return mapbox::util::optional<Value>();
     }
-    return mapbox::util::optional<Value>();
+    auto propIter = properties.find(keyIter->second);
+    if (propIter == properties.end()) {
+        return mapbox::util::optional<Value>();
+    }
+    return layer.values[propIter->second];
 }
 
 GeometryCollection VectorTileFeature::getGeometries() const {
@@ -143,7 +148,7 @@ VectorTileLayer::VectorTileLayer(pbf layer_pbf) {
         } else if (layer_pbf.tag == 2) { // feature
             features.push_back(layer_pbf.message());
         } else if (layer_pbf.tag == 3) { // keys
-            keys.emplace_back(layer_pbf.string());
+            keys.emplace(layer_pbf.string(), keys.size());
         } else if (layer_pbf.tag == 4) { // values
             values.emplace_back(std::move(parseValue(layer_pbf.message())));
         } else if (layer_pbf.tag == 5) { // extent

src/mbgl/map/vector_tile.hpp

@@ -4,7 +4,7 @@
 #include <mbgl/map/geometry_tile.hpp>
 #include <mbgl/util/pbf.hpp>
 
-#include <map>
+#include <unordered_map>
 
 namespace mbgl {
 
@@ -19,9 +19,10 @@ public:
     GeometryCollection getGeometries() const override;
 
 private:
+    const VectorTileLayer& layer;
     uint64_t id = 0;
     FeatureType type = FeatureType::Unknown;
-    std::map<std::string, Value> properties;
+    std::unordered_map<uint32_t, uint32_t> properties;
     pbf geometry_pbf;
 };
 
@@ -38,7 +39,7 @@ private:
 
     std::string name;
     uint32_t extent = 4096;
-    std::vector<std::string> keys;
+    std::unordered_map<std::string, uint32_t> keys;
     std::vector<Value> values;
     std::vector<pbf> features;
 };
@@ -50,7 +51,7 @@ public:
     util::ptr<const GeometryTileLayer> getLayer(const std::string&) const override;
 
 private:
-    std::map<std::string, util::ptr<const GeometryTileLayer>> layers;
+    std::unordered_map<std::string, util::ptr<const GeometryTileLayer>> layers;
 };
 
 }