Consolidate duplicate interpolation definitions

While here, do not interpolate style properties that
are not transitionable. The previous behavior was to
switch from old to new value halfway through the transition;
now the new value is used immediately, as in JS.

1 files changed, 25 insertions, 0 deletions

diff --git a/include/mbgl/util/interpolate.hpp b/include/mbgl/util/interpolate.hpp
new file mode 100644
index 0000000000..e8c3389350
--- /dev/null
+++ b/include/mbgl/util/interpolate.hpp
@@ -0,0 +1,25 @@
+#ifndef MBGL_UTIL_INTERPOLATE
+#define MBGL_UTIL_INTERPOLATE
+
+namespace mbgl {
+namespace util {
+
+template <typename T>
+T interpolate(const T a, const T b, const double t) {
+    return a * (1.0 - t) + b * t;
+}
+
+template <typename T>
+inline std::array<T, 4> interpolate(const std::array<T, 4>& a, const std::array<T, 4>& b, const double t) {
+    return {{
+        interpolate(a[0], b[0], t),
+        interpolate(a[1], b[1], t),
+        interpolate(a[2], b[2], t),
+        interpolate(a[3], b[3], t)
+    }};
+}
+
+}
+}
+
+#endif