1 files changed, 85 insertions, 0 deletions
diff --git a/src/mbgl/renderer/frame_history.cpp b/src/mbgl/renderer/frame_history.cpp
new file mode 100644
index 0000000000..8b69162a23
--- /dev/null
+++ b/src/mbgl/renderer/frame_history.cpp
@@ -0,0 +1,85 @@
+#include <mbgl/renderer/frame_history.hpp>
+
+using namespace mbgl;
+
+// Record frame history that will be used to calculate fading params
+void FrameHistory::record(timestamp now, float zoom) {
+    // first frame ever
+    if (!history.size()) {
+        history.emplace_back(FrameSnapshot{0, zoom});
+        history.emplace_back(FrameSnapshot{0, zoom});
+    }
+
+    if (history.size() > 0 || history.back().z != zoom) {
+        history.emplace_back(FrameSnapshot{now, zoom});
+    }
+}
+
+bool FrameHistory::needsAnimation(const timestamp duration) const {
+    if (!history.size()) {
+        return false;
+    }
+
+    // If we have a value that is older than duration and whose z value is the
+    // same as the most current z value, and if all values inbetween have the
+    // same z value, we don't need animation, otherwise we probably do.
+    const FrameSnapshot &pivot = history.back();
+
+    int i = -1;
+    while ((int)history.size() > i + 1 && history[i + 1].t + duration < pivot.t) {
+        i++;
+    }
+
+    if (i < 0) {
+        // There is no frame that is older than the duration time, so we need to
+        // check all frames.
+        i = 0;
+    }
+
+    // Make sure that all subsequent snapshots have the same zoom as the last
+    // pivot element.
+    for (; (int)history.size() > i; i++) {
+        if (history[i].z != pivot.z) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+FadeProperties FrameHistory::getFadeProperties(timestamp duration)
+{
+    const timestamp currentTime = util::now();
+
+    // Remove frames until only one is outside the duration, or until there are only three
+    while (history.size() > 3 && history[1].t + duration < currentTime) {
+        history.pop_front();
+    }
+
+    if (history[1].t + duration < currentTime) {
+        history[0].z = history[1].z;
+    }
+
+    // Find the range of zoom levels we want to fade between
+    float startingZ = history.front().z;
+    const FrameSnapshot lastFrame = history.back();
+    float endingZ = lastFrame.z;
+    float lowZ = std::fmin(startingZ, endingZ);
+    float highZ = std::fmax(startingZ, endingZ);
+
+    // Calculate the speed of zooming, and how far it would zoom in terms of zoom levels in one
+    // duration
+    float zoomDiff = endingZ - history[1].z, timeDiff = lastFrame.t - history[1].t;
+    float fadedist = zoomDiff / (timeDiff / duration);
+
+    // At end of a zoom when the zoom stops changing continue pretending to zoom at that speed
+    // bump is how much farther it would have been if it had continued zooming at the same rate
+    float bump = (currentTime - lastFrame.t) / duration * fadedist;
+
+    return FadeProperties {
+        fadedist,
+        lowZ,
+        highZ,
+        bump
+    };
+}