[core] Add a method for statically evaluating font stacks used by a style

src/mbgl/style/function.hpp

@@ -27,6 +27,9 @@ public:
 
     T evaluate(const StyleCalculationParameters&) const;
 
+    float getBase() const { return base; }
+    const std::vector<std::pair<float, T>>& getStops() const { return stops; }
+
 private:
     float base = 1;
     std::vector<std::pair<float, T>> stops;

src/mbgl/style/style_parser.cpp

@@ -18,6 +18,7 @@
 
 #include <algorithm>
 #include <sstream>
+#include <set>
 
 namespace mbgl {
 
@@ -484,4 +485,23 @@ void StyleParser::parseVisibility(StyleLayer& layer, const JSValue& value) {
     layer.visibility = VisibilityTypeClass({ value["visibility"].GetString(), value["visibility"].GetStringLength() });
 }
 
+std::vector<std::string> StyleParser::fontStacks() const {
+    std::set<std::string> result;
+
+    for (const auto& layer : layers) {
+        if (layer->is<SymbolLayer>()) {
+            LayoutProperty<std::string> property = layer->as<SymbolLayer>()->layout.text.font;
+            if (property.parsedValue) {
+                for (const auto& stop : property.parsedValue->getStops()) {
+                    result.insert(stop.second);
+                }
+            } else {
+                result.insert(property.value);
+            }
+        }
+    }
+
+    return std::vector<std::string>(result.begin(), result.end());
+}
+
 } // namespace mbgl

src/mbgl/style/style_parser.hpp

@@ -28,6 +28,9 @@ public:
     std::vector<std::unique_ptr<Source>> sources;
     std::vector<std::unique_ptr<StyleLayer>> layers;
 
+    // Statically evaluate layer properties to determine what font stacks are used.
+    std::vector<std::string> fontStacks() const;
+
     static std::unique_ptr<SourceInfo> parseTileJSON(const std::string& json, const std::string& sourceURL, SourceType);
     static std::unique_ptr<SourceInfo> parseTileJSON(const JSValue&);