[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&);
 

test/fixtures/style_parser/font_stacks.json

@@ -0,0 +1,38 @@
+{
+  "version": 8,
+  "sources": {
+    "source": {
+      "type": "vector",
+      "tiles": []
+    }
+  },
+  "layers": [{
+    "id": "a",
+    "type": "symbol",
+    "source": "source",
+    "source-layer": "source-layer",
+    "layout": {
+      "text-font": ["a"]
+    }
+  }, {
+    "id": "a,b",
+    "type": "symbol",
+    "source": "source",
+    "source-layer": "source-layer",
+    "layout": {
+      "text-font": {
+        "stops": [[0, ["a", "b"]]]
+      }
+    }
+  }, {
+    "id": "a,b;a,b,c",
+    "type": "symbol",
+    "source": "source",
+    "source-layer": "source-layer",
+    "layout": {
+      "text-font": {
+        "stops": [[0, ["a", "b"]], [1, ["a", "b", "c"]]]
+      }
+    }
+  }]
+}

test/style/style_parser.cpp

@@ -113,3 +113,13 @@ TEST(StyleParser, ParseTileJSONVector) {
     EXPECT_EQ("attribution", result->attribution);
     EXPECT_EQ("http://a.tiles.mapbox.com/mapbox.streets/{z}-{x}-{y}.vector.pbf?access_token=key", result->tiles[0]);
 }
+
+TEST(StyleParser, FontStacks) {
+    StyleParser parser;
+    parser.parse(util::read_file("test/fixtures/style_parser/font_stacks.json"));
+    auto result = parser.fontStacks();
+    ASSERT_EQ(3, result.size());
+    ASSERT_EQ("a", result[0]);
+    ASSERT_EQ("a,b", result[1]);
+    ASSERT_EQ("a,b,c", result[2]);
+}