borrwoed a lot code from https://github.com/foriequal0/devterm_keyboard in order to optimize the trackball

Code/devterm_keyboard/ratemeter.ino

@@ -0,0 +1,40 @@
+#include <Arduino.h>
+#include <cstdint>
+
+#include "ratemeter.h"
+#include "math.h"
+
+RateMeter::RateMeter()
+: lastTime(0)
+{}
+
+void RateMeter::onInterrupt() {
+  const auto now = millis();
+  if (cutoff.get()) {
+    averageDelta = CUTOFF_MS;
+  } else {
+    const auto delta = getDelta(lastTime, now, CUTOFF_MS);
+    averageDelta = (averageDelta + delta) / 2;
+  }
+  lastTime = now;
+  cutoff.reset();
+}
+
+void RateMeter::tick(millis_t delta) {
+  cutoff.updateTime(delta);
+}
+
+uint16_t RateMeter::delta() const {
+  return averageDelta;
+}
+
+float RateMeter::rate() const {
+  if (cutoff.get()) {
+    return 0.0f;
+  } else if (averageDelta == 0) {
+    // to ensure range 0 ~ 1000.0
+    return 1000.0f;
+  } else {
+    return 1000.0f / (float)averageDelta;
+  }
+}