main: prevent ticks from going backwards

Outside of overflow (which I'm ignoring for now) ticks shouldn't
go backwards.  With the introduction of adding the frame-buffer
delays, which vary, there's the potential for the delay to go
from large to short in quick succession, to such a degree that
the next render's now+delay is in the past relative to the
previous now+delay.

For now this simple fix is to just track the last_ticks and
always use the maximum of the last_ticks and now+delay, ensuring
it never goes backwards.

This was making alphazed exit prematurely at spurious times by
sending the rocket_row into oblivion because (ticks - last_ticks)
was negative w/unsigned arithmetic.  This will all get more work,
and maybe ticks should be allowed to go backwards actually, but
some things are assuming that's not the case as-is.

Regardless, it's not desirable for ticks to go backwards because
of the frame-buffer delay.  In that case just chill on the ticks
advancement for a frame.  This will need revisiting for sure, as
I don't think re-rendering the exact same tick as the last frame
is likely to be what's wanted either.  Probably some little
advancement should still be performed...

1 files changed, 4 insertions, 2 deletions

diff --git a/src/main.c b/src/main.c
index b555bbf..45e8c3b 100644
--- a/src/main.c
+++ b/src/main.c
@@ -324,6 +324,7 @@ static int print_help(void)
 static void * rototiller_thread(void *_rt)
 {
 	rototiller_t	*rt = _rt;
+	unsigned	last_ticks = til_ticks_now();
 
 	while (til_stream_active(rt->stream)) {
 		unsigned	ticks, delay = 0;
@@ -335,9 +336,10 @@ static void * rototiller_thread(void *_rt)
 		}
 
 		til_stream_start_frame(rt->stream);
-		ticks = til_ticks_now();
-		til_module_render(rt->module_context, rt->stream, ticks + delay, &rt->fragment);
+		ticks = MAX(til_ticks_now() + delay, last_ticks);
+		til_module_render(rt->module_context, rt->stream, ticks, &rt->fragment);
 		til_fb_fragment_submit(rt->fragment);
+		last_ticks = ticks;
 
 		if (rt->args.print_module_contexts || rt->args.print_pipes) {
 			/* render threads are idle at this point */