Fix OPL MIDI tempo calculations.

It turns out that the way that tempo has been calculated in OPL playback
has been broken for a long time. The mysterious "fudge factor" that I
had to apply to tempo calculations is actually completely unnecessary:
the byte-swapping in the MIDI_GetFileTimeDivision() function was being
done wrong, so the time division being used by the OPL MIDI code was
completely wrong. Presumably the multiply by 260 was close enough to an
8-bit bitshift that it worked okayish, but large enough time division
values would overflow a single byte and screw up.

This fixes long-running OPL playback problems in a number of WADs, most
notably Alien Vendetta. This *really* fixes #352.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/midifile.c b/src/midifile.c
index 94ec4441..92ffd803 100644
--- a/src/midifile.c
+++ b/src/midifile.c
@@ -699,14 +699,14 @@ int MIDI_GetNextEvent(midi_track_iter_t *iter, midi_event_t **event)
 
 unsigned int MIDI_GetFileTimeDivision(midi_file_t *file)
 {
-    short result = SHORT(file->header.time_division);
+    short result = SDL_SwapBE16(file->header.time_division);
 
     // Negative time division indicates SMPTE time and must be handled
     // differently.
     if (result < 0)
     {
-        // TODO: Figure this out.
-        return 96;
+        return (signed int)(-(result/256))
+             * (signed int)(result & 0xFF);
     }
     else
     {