[core] Add support for data-driven styling

1 files changed, 56 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..e998be082a
--- /dev/null
+++ b/include/mbgl/style/function/source_function.hpp
@@ -0,0 +1,56 @@
+#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_)
+        : property(std::move(property_)),
+          stops(std::move(stops_)) {
+    }
+
+    T evaluate(const GeometryTileFeature& feature) const {
+        optional<Value> v = feature.getValue(property);
+        if (!v) {
+            return T();
+        }
+        return stops.match([&] (const auto& s) {
+            return s.evaluate(*v);
+        });
+    }
+
+    friend bool operator==(const SourceFunction& lhs,
+                           const SourceFunction& rhs) {
+        return lhs.property == rhs.property && lhs.stops == rhs.stops;
+    }
+
+    std::string property;
+    Stops stops;
+};
+
+} // namespace style
+} // namespace mbgl