Codechange: track _realtime_tick more accurate

_realtime_tick was reset every time the diff was calculated. This
means if it would trigger, say, every N.9 milliseconds, it would
after two iterations already drift a millisecond. This adds up
pretty quick.

7 files changed, 55 insertions, 18 deletions

diff --git a/src/video/allegro_v.cpp b/src/video/allegro_v.cpp
index 20f0e65ed..493e5a81d 100644
--- a/src/video/allegro_v.cpp
+++ b/src/video/allegro_v.cpp
@@ -448,7 +448,7 @@ void VideoDriver_Allegro::Stop()
 void VideoDriver_Allegro::MainLoop()
 {
 	auto cur_ticks = std::chrono::steady_clock::now();
-	auto last_cur_ticks = cur_ticks;
+	auto last_realtime_tick = cur_ticks;
 	auto next_tick = cur_ticks;
 
 	CheckPaletteAnim();
@@ -473,9 +473,15 @@ void VideoDriver_Allegro::MainLoop()
 		}
 
 		cur_ticks = std::chrono::steady_clock::now();
+
+		/* If more than a millisecond has passed, increase the _realtime_tick. */
+		if (cur_ticks - last_realtime_tick > std::chrono::milliseconds(1)) {
+			auto delta = std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_realtime_tick);
+			_realtime_tick += delta.count();
+			last_realtime_tick += delta;
+		}
+
 		if (cur_ticks >= next_tick || (_fast_forward && !_pause_mode)) {
-			_realtime_tick += std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_cur_ticks).count();
-			last_cur_ticks = cur_ticks;
 			next_tick = cur_ticks + std::chrono::milliseconds(MILLISECONDS_PER_TICK);
 
 			bool old_ctrl_pressed = _ctrl_pressed;
diff --git a/src/video/cocoa/cocoa_v.mm b/src/video/cocoa/cocoa_v.mm
index 122a38be5..c9e6768a8 100644
--- a/src/video/cocoa/cocoa_v.mm
+++ b/src/video/cocoa/cocoa_v.mm
@@ -635,7 +635,7 @@ bool VideoDriver_Cocoa::PollEvent()
 void VideoDriver_Cocoa::GameLoop()
 {
 	auto cur_ticks = std::chrono::steady_clock::now();
-	auto last_cur_ticks = cur_ticks;
+	auto last_realtime_tick = cur_ticks;
 	auto next_tick = cur_ticks;
 
 	for (;;) {
@@ -664,9 +664,15 @@ void VideoDriver_Cocoa::GameLoop()
 			}
 
 			cur_ticks = std::chrono::steady_clock::now();
+
+			/* If more than a millisecond has passed, increase the _realtime_tick. */
+			if (cur_ticks - last_realtime_tick > std::chrono::milliseconds(1)) {
+				auto delta = std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_realtime_tick);
+				_realtime_tick += delta.count();
+				last_realtime_tick += delta;
+			}
+
 			if (cur_ticks >= next_tick || (_fast_forward && !_pause_mode)) {
-				_realtime_tick += std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_cur_ticks).count();
-				last_cur_ticks = cur_ticks;
 				next_tick = cur_ticks + std::chrono::milliseconds(MILLISECONDS_PER_TICK);
 
 				bool old_ctrl_pressed = _ctrl_pressed;
diff --git a/src/video/dedicated_v.cpp b/src/video/dedicated_v.cpp
index 60459099b..06b359880 100644
--- a/src/video/dedicated_v.cpp
+++ b/src/video/dedicated_v.cpp
@@ -236,7 +236,7 @@ static void DedicatedHandleKeyInput()
 void VideoDriver_Dedicated::MainLoop()
 {
 	auto cur_ticks = std::chrono::steady_clock::now();
-	auto last_cur_ticks = cur_ticks;
+	auto last_realtime_tick = cur_ticks;
 	auto next_tick = cur_ticks;
 
 	/* Signal handlers */
@@ -283,7 +283,14 @@ void VideoDriver_Dedicated::MainLoop()
 		if (!_dedicated_forks) DedicatedHandleKeyInput();
 
 		cur_ticks = std::chrono::steady_clock::now();
-		_realtime_tick += std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_cur_ticks).count();
+
+		/* If more than a millisecond has passed, increase the _realtime_tick. */
+		if (cur_ticks - last_realtime_tick > std::chrono::milliseconds(1)) {
+			auto delta = std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_realtime_tick);
+			_realtime_tick += delta.count();
+			last_realtime_tick += delta;
+		}
+
 		if (cur_ticks >= next_tick || _ddc_fastforward) {
 			next_tick = cur_ticks + std::chrono::milliseconds(MILLISECONDS_PER_TICK);
 
diff --git a/src/video/sdl2_v.cpp b/src/video/sdl2_v.cpp
index 0b2e257bd..6db220470 100644
--- a/src/video/sdl2_v.cpp
+++ b/src/video/sdl2_v.cpp
@@ -768,9 +768,15 @@ void VideoDriver_SDL::LoopOnce()
 	}
 
 	cur_ticks = std::chrono::steady_clock::now();
+
+	/* If more than a millisecond has passed, increase the _realtime_tick. */
+	if (cur_ticks - last_realtime_tick > std::chrono::milliseconds(1)) {
+		auto delta = std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_realtime_tick);
+		_realtime_tick += delta.count();
+		last_realtime_tick += delta;
+	}
+
 	if (cur_ticks >= next_tick || (_fast_forward && !_pause_mode)) {
-		_realtime_tick += std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_cur_ticks).count();
-		last_cur_ticks = cur_ticks;
 		next_tick = cur_ticks + std::chrono::milliseconds(MILLISECONDS_PER_TICK);
 
 		bool old_ctrl_pressed = _ctrl_pressed;
@@ -826,7 +832,7 @@ void VideoDriver_SDL::LoopOnce()
 void VideoDriver_SDL::MainLoop()
 {
 	cur_ticks = std::chrono::steady_clock::now();
-	last_cur_ticks = cur_ticks;
+	last_realtime_tick = cur_ticks;
 	next_tick = cur_ticks;
 
 	this->CheckPaletteAnim();
diff --git a/src/video/sdl2_v.h b/src/video/sdl2_v.h
index 61d607d56..6b5d3dd14 100644
--- a/src/video/sdl2_v.h
+++ b/src/video/sdl2_v.h
@@ -63,7 +63,7 @@ private:
 	bool edit_box_focused;
 
 	std::chrono::steady_clock::time_point cur_ticks;
-	std::chrono::steady_clock::time_point last_cur_ticks;
+	std::chrono::steady_clock::time_point last_realtime_tick;
 	std::chrono::steady_clock::time_point next_tick;
 
 	int startup_display;
diff --git a/src/video/sdl_v.cpp b/src/video/sdl_v.cpp
index cc6555461..fa0294200 100644
--- a/src/video/sdl_v.cpp
+++ b/src/video/sdl_v.cpp
@@ -649,7 +649,7 @@ void VideoDriver_SDL::Stop()
 void VideoDriver_SDL::MainLoop()
 {
 	auto cur_ticks = std::chrono::steady_clock::now();
-	auto last_cur_ticks = cur_ticks;
+	auto last_realtime_tick = cur_ticks;
 	auto next_tick = cur_ticks;
 	uint32 mod;
 	int numkeys;
@@ -719,9 +719,15 @@ void VideoDriver_SDL::MainLoop()
 		}
 
 		cur_ticks = std::chrono::steady_clock::now();
+
+		/* If more than a millisecond has passed, increase the _realtime_tick. */
+		if (cur_ticks - last_realtime_tick > std::chrono::milliseconds(1)) {
+			auto delta = std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_realtime_tick);
+			_realtime_tick += delta.count();
+			last_realtime_tick += delta;
+		}
+
 		if (cur_ticks >= next_tick || (_fast_forward && !_pause_mode)) {
-			_realtime_tick += std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_cur_ticks).count();
-			last_cur_ticks = cur_ticks;
 			next_tick = cur_ticks + std::chrono::milliseconds(MILLISECONDS_PER_TICK);
 
 			bool old_ctrl_pressed = _ctrl_pressed;
diff --git a/src/video/win32_v.cpp b/src/video/win32_v.cpp
index 714e50942..57f146e5e 100644
--- a/src/video/win32_v.cpp
+++ b/src/video/win32_v.cpp
@@ -1134,7 +1134,7 @@ void VideoDriver_Win32::MainLoop()
 {
 	MSG mesg;
 	auto cur_ticks = std::chrono::steady_clock::now();
-	auto last_cur_ticks = cur_ticks;
+	auto last_realtime_tick = cur_ticks;
 	auto next_tick = cur_ticks;
 
 	std::thread draw_thread;
@@ -1197,9 +1197,15 @@ void VideoDriver_Win32::MainLoop()
 		}
 
 		cur_ticks = std::chrono::steady_clock::now();
+
+		/* If more than a millisecond has passed, increase the _realtime_tick. */
+		if (cur_ticks - last_realtime_tick > std::chrono::milliseconds(1)) {
+			auto delta = std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_realtime_tick);
+			_realtime_tick += delta.count();
+			last_realtime_tick += delta;
+		}
+
 		if (cur_ticks >= next_tick || (_fast_forward && !_pause_mode)) {
-			_realtime_tick += std::chrono::duration_cast<std::chrono::milliseconds>(cur_ticks - last_cur_ticks).count();
-			last_cur_ticks = cur_ticks;
 			next_tick = cur_ticks + std::chrono::milliseconds(MILLISECONDS_PER_TICK);
 
 			bool old_ctrl_pressed = _ctrl_pressed;