From 1f5835c985056efe67a724829305189a34df56ab Mon Sep 17 00:00:00 2001
From: Misa <infoteddy@infoteddy.info>
Date: Tue, 27 Apr 2021 16:37:16 -0700
Subject: [PATCH] Fix fade volume durations being incorrect

The previous fade system used only one variable, the amount of volume to
fade per frame. However, this variable was an integer, meaning any
decimal portion would be truncated, and would lead to a longer fade
duration than intended.

The fade per volume is calculated by doing MIX_MAX_VOLUME / (fade_ms /
game.get_timestep()). MIX_MAX_VOLUME is 128, and game.get_timestep() is
usually 34, so a 3000 millisecond fade would be calculated as 128 /
(3000 / 34). 3000 / 34 is 88.235..., but that gets truncated to 88, and
then 128 / 88 becomes 1.454545..., which then gets truncated to 1. This
essentially means 1 is added to or subtracted from the volume every
frame, and given that the max volume is 128, this means that the fade
lasts for 128 frames. Now, instead of the fade duration lasting 3
seconds, the fade now lasts for 128 frames, which is 128 * 34 / 1000 =
4.352 seconds long.

This could be fixed using floats, but when you introduce floats, you now
have 1.9999998 problems. For instance, I'm concerned about
floating-point determinism issues.

What I've done instead is switch the system to use four different
variables instead: the start volume, the end volume, the total duration,
and the duration completed so far (called the "step"). For every frame,
the game interpolates which value should be used based on the step, the
total duration, and the start and end volumes, and then adds the
timestep to the step. This way, fades will be correctly timed, and we
don't have potential determinism issues.

Doing this also fixes inaccuracies with the game timestep changing
during the fade, since the timestep is only used in the calculation
once at the beginning in the previous system.
---
 desktop_version/src/Music.cpp | 59 ++++++++++++++++++++++++++++-------
 desktop_version/src/Music.h   |  2 --
 2 files changed, 47 insertions(+), 14 deletions(-)

diff --git a/desktop_version/src/Music.cpp b/desktop_version/src/Music.cpp
index 05e5c644..df493baf 100644
--- a/desktop_version/src/Music.cpp
+++ b/desktop_version/src/Music.cpp
@@ -15,7 +15,6 @@ musicclass::musicclass(void)
 	m_doFadeInVol = false;
 	m_doFadeOutVol = false;
 	musicVolume = 0;
-	FadeVolAmountPerFrame = 0;
 
 	user_music_volume = USER_VOLUME_MAX;
 	user_sound_volume = USER_VOLUME_MAX;
@@ -277,14 +276,45 @@ void musicclass::silencedasmusik(void)
 	musicVolume = 0;
 }
 
-void musicclass::setfadeamount(const int fade_ms)
+struct FadeState
 {
-	if (fade_ms == 0)
+	int start_volume;
+	int end_volume;
+	int duration_ms;
+	int step_ms;
+};
+
+static struct FadeState fade;
+
+enum FadeCode
+{
+	Fade_continue,
+	Fade_finished
+};
+
+static enum FadeCode processmusicfade(struct FadeState* state, int* volume)
+{
+	int range;
+	int new_volume;
+
+	if (state->duration_ms == 0 /* Fast path. */
+	|| state->start_volume == state->end_volume /* Fast path. */
+	|| state->step_ms >= state->duration_ms /* We're finished. */)
 	{
-		FadeVolAmountPerFrame = MIX_MAX_VOLUME;
-		return;
+		*volume = state->end_volume;
+		state->step_ms = 0;
+		return Fade_finished;
 	}
-	FadeVolAmountPerFrame = MIX_MAX_VOLUME / (fade_ms / game.get_timestep());
+
+	range = state->end_volume - state->start_volume;
+	new_volume = range * state->step_ms / state->duration_ms;
+	new_volume += state->start_volume;
+
+	*volume = new_volume;
+
+	state->step_ms += game.get_timestep();
+
+	return Fade_continue;
 }
 
 void musicclass::fadeMusicVolumeIn(int ms)
@@ -298,14 +328,19 @@ void musicclass::fadeMusicVolumeIn(int ms)
 	/* Fix 1-frame glitch */
 	Mix_VolumeMusic(0);
 
-	setfadeamount(ms);
+	fade.duration_ms = ms;
+	fade.start_volume = 0;
+	fade.end_volume = MIX_MAX_VOLUME;
 }
 
 void musicclass::fadeMusicVolumeOut(const int fadeout_ms)
 {
 	m_doFadeInVol = false;
 	m_doFadeOutVol = true;
-	setfadeamount(fadeout_ms);
+
+	fade.duration_ms = fadeout_ms;
+	fade.start_volume = musicVolume;
+	fade.end_volume = 0;
 }
 
 void musicclass::fadeout(const bool quick_fade_ /*= true*/)
@@ -316,8 +351,8 @@ void musicclass::fadeout(const bool quick_fade_ /*= true*/)
 
 void musicclass::processmusicfadein(void)
 {
-	musicVolume += FadeVolAmountPerFrame;
-	if (musicVolume >= MIX_MAX_VOLUME)
+	enum FadeCode fade_code = processmusicfade(&fade, &musicVolume);
+	if (fade_code == Fade_finished)
 	{
 		m_doFadeInVol = false;
 	}
@@ -325,8 +360,8 @@ void musicclass::processmusicfadein(void)
 
 void musicclass::processmusicfadeout(void)
 {
-	musicVolume -= FadeVolAmountPerFrame;
-	if (musicVolume < 0)
+	enum FadeCode fade_code = processmusicfade(&fade, &musicVolume);
+	if (fade_code == Fade_finished)
 	{
 		musicVolume = 0;
 		m_doFadeOutVol = false;
diff --git a/desktop_version/src/Music.h b/desktop_version/src/Music.h
index 593b339e..e987656c 100644
--- a/desktop_version/src/Music.h
+++ b/desktop_version/src/Music.h
@@ -27,7 +27,6 @@ public:
 	void pause(void);
 	void haltdasmusik(void);
 	void silencedasmusik(void);
-	void setfadeamount(const int fade_ms);
 	void fadeMusicVolumeIn(int ms);
 	void fadeMusicVolumeOut(const int fadeout_ms);
 	void fadeout(const bool quick_fade_ = true);
@@ -55,7 +54,6 @@ public:
 
 	bool m_doFadeInVol;
 	bool m_doFadeOutVol;
-	int FadeVolAmountPerFrame;
 	int musicVolume;
 
 	/* 0..USER_VOLUME_MAX */