decoupled timers from interpreter tick rate

CountdownTimerSDL.cpp

@@ -0,0 +1,18 @@
+#include "CountdownTimerSDL.hpp"
+
+CountdownTimerSDL::CountdownTimerSDL(unsigned frequency):
+  mDesiredFrequency{frequency},
+  mSDLFrequency{SDL_GetPerformanceFrequency()},
+  mStartTime{0},
+  mStartValue{0} {}
+
+void CountdownTimerSDL::set(unsigned value) {
+  mStartTime = SDL_GetPerformanceCounter();
+  mStartValue = value;
+}
+
+unsigned CountdownTimerSDL::get() const {
+  Uint64 elapsedTime = SDL_GetPerformanceCounter() - mStartTime;
+  Uint64 elapsedTicks = (elapsedTime * mDesiredFrequency) / mSDLFrequency;
+  return elapsedTicks >= mStartValue ? 0 : mStartValue - elapsedTicks;
+}