[core] Move expression.cpp to expression directory

src/mbgl/style/expression/expression.cpp

@@ -0,0 +1,38 @@
+#include <mbgl/style/expression/expression.hpp>
+#include <mbgl/style/expression/compound_expression.hpp>
+#include <mbgl/tile/geometry_tile_data.hpp>
+
+namespace mbgl {
+namespace style {
+namespace expression {
+
+class GeoJSONFeature : public GeometryTileFeature {
+public:
+    const Feature& feature;
+
+    GeoJSONFeature(const Feature& feature_) : feature(feature_) {}
+
+    FeatureType getType() const override  {
+        return apply_visitor(ToFeatureType(), feature.geometry);
+    }
+    PropertyMap getProperties() const override { return feature.properties; }
+    optional<FeatureIdentifier> getID() const override { return feature.id; }
+    GeometryCollection getGeometries() const override { return {}; }
+    optional<mbgl::Value> getValue(const std::string& key) const override {
+        auto it = feature.properties.find(key);
+        if (it != feature.properties.end()) {
+            return optional<mbgl::Value>(it->second);
+        }
+        return optional<mbgl::Value>();
+    }
+};
+
+
+EvaluationResult Expression::evaluate(optional<float> zoom, const Feature& feature, optional<double> heatmapDensity) const {
+    GeoJSONFeature f(feature);
+    return this->evaluate(EvaluationContext(zoom, &f, heatmapDensity));
+}
+
+} // namespace expression
+} // namespace style
+} // namespace mbgl