1 files changed, 59 insertions, 0 deletions
diff --git a/include/mbgl/style/function/source_function.hpp b/include/mbgl/style/function/source_function.hpp
new file mode 100644
index 0000000000..29b1067a19
--- /dev/null
+++ b/include/mbgl/style/function/source_function.hpp
@@ -0,0 +1,59 @@
+#pragma once
+
+#include <mbgl/style/function/exponential_stops.hpp>
+#include <mbgl/style/function/interval_stops.hpp>
+#include <mbgl/style/function/categorical_stops.hpp>
+#include <mbgl/style/function/identity_stops.hpp>
+#include <mbgl/tile/geometry_tile_data.hpp>
+#include <mbgl/util/interpolate.hpp>
+#include <mbgl/util/variant.hpp>
+
+#include <string>
+
+namespace mbgl {
+namespace style {
+
+template <class T>
+class SourceFunction {
+public:
+    using Stops = std::conditional_t<
+        util::Interpolatable<T>,
+        variant<
+            ExponentialStops<T>,
+            IntervalStops<T>,
+            CategoricalStops<T>,
+            IdentityStops<T>>,
+        variant<
+            IntervalStops<T>,
+            CategoricalStops<T>,
+            IdentityStops<T>>>;
+
+    SourceFunction(std::string property_, Stops stops_, optional<T> defaultValue_ = {})
+        : property(std::move(property_)),
+          stops(std::move(stops_)),
+          defaultValue(std::move(defaultValue_)) {
+    }
+
+    T evaluate(const GeometryTileFeature& feature, T finalDefaultValue) const {
+        optional<Value> v = feature.getValue(property);
+        if (!v) {
+            return defaultValue.value_or(finalDefaultValue);
+        }
+        return stops.match([&] (const auto& s) -> T {
+            return s.evaluate(*v).value_or(defaultValue.value_or(finalDefaultValue));
+        });
+    }
+
+    friend bool operator==(const SourceFunction& lhs,
+                           const SourceFunction& rhs) {
+        return std::tie(lhs.property, lhs.stops, lhs.defaultValue)
+            == std::tie(rhs.property, rhs.stops, rhs.defaultValue);
+    }
+
+    std::string property;
+    Stops stops;
+    optional<T> defaultValue;
+};
+
+} // namespace style
+} // namespace mbgl