1 files changed, 54 insertions, 0 deletions
diff --git a/include/mbgl/style/function/exponential_stops.hpp b/include/mbgl/style/function/exponential_stops.hpp
new file mode 100644
index 0000000000..7bd8783614
--- /dev/null
+++ b/include/mbgl/style/function/exponential_stops.hpp
@@ -0,0 +1,54 @@
+#pragma once
+
+#include <mbgl/util/feature.hpp>
+#include <mbgl/util/interpolate.hpp>
+
+#include <map>
+
+namespace mbgl {
+namespace style {
+
+template <class T>
+class ExponentialStops {
+public:
+    using Stops = std::map<float, T>;
+
+    Stops stops;
+    float base = 1.0f;
+
+    ExponentialStops() = default;
+    ExponentialStops(Stops stops_, float base_ = 1.0f)
+        : stops(std::move(stops_)),
+          base(base_) {
+    }
+
+    T evaluate(const Value& value) const {
+        if (stops.empty()) {
+            assert(false);
+            return T();
+        }
+
+        optional<float> z = numericValue<float>(value);
+        if (!z) {
+            return T();
+        }
+
+        auto it = stops.upper_bound(*z);
+        if (it == stops.end()) {
+            return stops.rbegin()->second;
+        } else if (it == stops.begin()) {
+            return stops.begin()->second;
+        } else {
+            return util::interpolate(std::prev(it)->second, it->second,
+                util::interpolationFactor(base, { std::prev(it)->first, it->first }, *z));
+        }
+    }
+
+    friend bool operator==(const ExponentialStops& lhs,
+                           const ExponentialStops& rhs) {
+        return lhs.stops == rhs.stops && lhs.base == rhs.base;
+    }
+};
+
+} // namespace style
+} // namespace mbgl