[core] Evict unused font stacks from GlyphManager

src/mbgl/util/font_stack.cpp

@@ -1,10 +1,14 @@
 #include <mbgl/util/font_stack.hpp>
+#include <mbgl/util/logging.hpp>
+#include <mbgl/style/layers/symbol_layer_impl.hpp>
 
 #include <boost/functional/hash.hpp>
 #include <boost/algorithm/string/join.hpp>
 
 namespace mbgl {
 
+using namespace style;
+
 std::string fontStackToString(const FontStack& fontStack) {
     return boost::algorithm::join(fontStack, ",");
 }
@@ -13,4 +17,40 @@ std::size_t FontStackHash::operator()(const FontStack& fontStack) const {
     return boost::hash_range(fontStack.begin(), fontStack.end());
 }
 
+std::set<FontStack> fontStacks(const std::vector<Immutable<style::Layer::Impl>>& layers) {
+    std::set<FontStack> result;
+
+    for (const auto& layer : layers) {
+        if (layer->type != LayerType::Symbol) {
+            continue;
+        }
+
+        const SymbolLayer::Impl& impl = dynamic_cast<const SymbolLayer::Impl&>(*layer);
+        if (impl.layout.get<TextField>().isUndefined()) {
+            continue;
+        }
+
+        impl.layout.get<TextFont>().match(
+            [&] (Undefined) {
+                result.insert({"Open Sans Regular", "Arial Unicode MS Regular"});
+            },
+            [&] (const FontStack& constant) {
+                result.insert(constant);
+            },
+            [&] (const auto& function) {
+                for (const auto& value : function.possibleOutputs()) {
+                    if (value) {
+                        result.insert(*value);
+                    } else {
+                        Log::Warning(Event::ParseStyle, "Layer '%s' has an invalid value for text-font and will not render text. Output values must be contained as literals within the expression.", impl.id.c_str());
+                        break;
+                    }
+                }
+            }
+        );
+    }
+
+    return result;
+}
+
 } // namespace mbgl