1 files changed, 68 insertions, 0 deletions
diff --git a/src/mbgl/style/function_properties.cpp b/src/mbgl/style/function_properties.cpp
new file mode 100644
index 0000000000..69466c1f64
--- /dev/null
+++ b/src/mbgl/style/function_properties.cpp
@@ -0,0 +1,68 @@
+#include <mbgl/style/function_properties.hpp>
+#include <mbgl/style/types.hpp>
+#include <mbgl/util/interpolate.hpp>
+
+#include <cmath>
+
+namespace mbgl {
+
+template <typename T>
+inline T defaultStopsValue();
+
+template <> inline bool defaultStopsValue() { return true; }
+template <> inline float defaultStopsValue() { return 1.0f; }
+template <> inline Color defaultStopsValue() { return {{ 0, 0, 0, 1 }}; }
+
+
+template <typename T>
+T StopsFunction<T>::evaluate(float z) const {
+    bool smaller = false;
+    float smaller_z = 0.0f;
+    T smaller_val = T();
+    bool larger = false;
+    float larger_z = 0.0f;
+    T larger_val = T();
+
+    for (uint32_t i = 0; i < values.size(); i++) {
+        float stop_z = values[i].first;
+        T stop_val = values[i].second;
+        if (stop_z <= z && (!smaller || smaller_z < stop_z)) {
+            smaller = true;
+            smaller_z = stop_z;
+            smaller_val = stop_val;
+        }
+        if (stop_z >= z && (!larger || larger_z > stop_z)) {
+            larger = true;
+            larger_z = stop_z;
+            larger_val = stop_val;
+        }
+    }
+
+    if (smaller && larger) {
+        if (larger_z == smaller_z || larger_val == smaller_val) {
+            return smaller_val;
+        }
+        const float zoomDiff = larger_z - smaller_z;
+        const float zoomProgress = z - smaller_z;
+        if (base == 1.0f) {
+            const float t = zoomProgress / zoomDiff;
+            return util::interpolate(smaller_val, larger_val, t);
+        } else {
+            const float t = (std::pow(base, zoomProgress) - 1) / (std::pow(base, zoomDiff) - 1);
+            return util::interpolate(smaller_val, larger_val, t);
+        }
+    } else if (larger) {
+        return larger_val;
+    } else if (smaller) {
+        return smaller_val;
+    } else {
+        // No stop defined.
+        return defaultStopsValue<T>();
+    }
+}
+
+template bool StopsFunction<bool>::evaluate(float z) const;
+template float StopsFunction<float>::evaluate(float z) const;
+template Color StopsFunction<Color>::evaluate(float z) const;
+
+}