Add: support for emscripten (play-OpenTTD-in-the-browser)

Emscripten compiles to WASM, which can be loaded via
HTML / JavaScript. This allows you to play OpenTTD inside a
browser.

Co-authored-by: milek7 <me@milek7.pl>

2 files changed, 46 insertions, 4 deletions

diff --git a/src/video/sdl2_v.cpp b/src/video/sdl2_v.cpp
index 09d0e8a0b..68b0aa983 100644
--- a/src/video/sdl2_v.cpp
+++ b/src/video/sdl2_v.cpp
@@ -27,6 +27,10 @@
 #include <mutex>
 #include <condition_variable>
 #include <algorithm>
+#ifdef __EMSCRIPTEN__
+#	include <emscripten.h>
+#	include <emscripten/html5.h>
+#endif
 
 #include "../safeguards.h"
 
@@ -673,7 +677,19 @@ void VideoDriver_SDL::LoopOnce()
 	InteractiveRandom(); // randomness
 
 	while (PollEvent() == -1) {}
-	if (_exit_game) return;
+	if (_exit_game) {
+#ifdef __EMSCRIPTEN__
+		/* Emscripten is event-driven, and as such the main loop is inside
+		 * the browser. So if _exit_game goes true, the main loop ends (the
+		 * cancel call), but we still have to call the cleanup that is
+		 * normally done at the end of the main loop for non-Emscripten.
+		 * After that, Emscripten just halts, and the HTML shows a nice
+		 * "bye, see you next time" message. */
+		emscripten_cancel_main_loop();
+		MainLoopCleanup();
+#endif
+		return;
+	}
 
 	mod = SDL_GetModState();
 	keys = SDL_GetKeyboardState(&numkeys);
@@ -722,9 +738,17 @@ void VideoDriver_SDL::LoopOnce()
 		_local_palette = _cur_palette;
 	} else {
 		/* Release the thread while sleeping */
-		if (_draw_mutex != nullptr) draw_lock.unlock();
-		CSleep(1);
-		if (_draw_mutex != nullptr) draw_lock.lock();
+		if (_draw_mutex != nullptr) {
+			draw_lock.unlock();
+			CSleep(1);
+			draw_lock.lock();
+		} else {
+/* Emscripten is running an event-based mainloop; there is already some
+ * downtime between each iteration, so no need to sleep. */
+#ifndef __EMSCRIPTEN__
+			CSleep(1);
+#endif
+		}
 
 		NetworkDrawChatMessage();
 		DrawMouseCursor();
@@ -778,6 +802,10 @@ void VideoDriver_SDL::MainLoop()
 
 	DEBUG(driver, 1, "SDL2: using %sthreads", _draw_threaded ? "" : "no ");
 
+#ifdef __EMSCRIPTEN__
+	/* Run the main loop event-driven, based on RequestAnimationFrame. */
+	emscripten_set_main_loop_arg(&this->EmscriptenLoop, this, 0, 1);
+#else
 	while (!_exit_game) {
 		LoopOnce();
 	}
@@ -803,6 +831,15 @@ void VideoDriver_SDL::MainLoopCleanup()
 		_draw_mutex = nullptr;
 		_draw_signal = nullptr;
 	}
+
+#ifdef __EMSCRIPTEN__
+	emscripten_exit_pointerlock();
+	/* In effect, the game ends here. As emscripten_set_main_loop() caused
+	 * the stack to be unwound, the code after MainLoop() in
+	 * openttd_main() is never executed. */
+	EM_ASM(if (window["openttd_syncfs"]) openttd_syncfs());
+	EM_ASM(if (window["openttd_exit"]) openttd_exit());
+#endif
 }
 
 bool VideoDriver_SDL::ChangeResolution(int w, int h)
diff --git a/src/video/sdl2_v.h b/src/video/sdl2_v.h
index 6318c403f..c2ac87a06 100644
--- a/src/video/sdl2_v.h
+++ b/src/video/sdl2_v.h
@@ -46,6 +46,11 @@ private:
 	void MainLoopCleanup();
 	bool CreateMainSurface(uint w, uint h, bool resize);
 
+#ifdef __EMSCRIPTEN__
+	/* Convert a constant pointer back to a non-constant pointer to a member function. */
+	static void EmscriptenLoop(void *self) { ((VideoDriver_SDL *)self)->LoopOnce(); }
+#endif
+
 	/**
 	 * This is true to indicate that keyboard input is in text input mode, and SDL_TEXTINPUT events are enabled.
 	 */