SDL: Test mixer time and volume conversions

Share SDL3_mixer gain and 10-millisecond time conversions across the
PCM, BGM, CD, and MIDI backends. Add SDL3 unit coverage for volume
clamping, combined gain, time units, and tick counter wraparound.

src/CMakeLists.txt

@@ -109,6 +109,7 @@ elseif (XSYSTEM35_SDL_VERSION STREQUAL "3")
   target_sources(xsystem35 PRIVATE
     pcm.sdl3mixer.c
     sdl3_mixer_backend.c
+    sdl3_mixer_utils.c
     bgm.sdl3mixer.c)
   target_compile_definitions(xsystem35 PRIVATE HAVE_SDL3_MIXER_BACKEND)
 else()
@@ -226,6 +227,11 @@ else()  # non-emscripten, non-android
   target_compile_definitions(src_tests PRIVATE
       TEST_FONT_DIR="${PROJECT_SOURCE_DIR}/fonts")
   target_link_libraries(src_tests PRIVATE src_lib xsystem35_sdl)
+  if (XSYSTEM35_SDL_VERSION STREQUAL "3")
+    target_sources(src_tests PRIVATE
+      sdl3_mixer_utils.c
+      sdl3_mixer_utils_test.c)
+  endif()
   add_test(NAME src_tests COMMAND src_tests)
   file(COPY testdata DESTINATION ${CMAKE_CURRENT_BINARY_DIR})
 endif()

src/bgm.sdl3mixer.c

@@ -18,6 +18,7 @@
 #include "music_private.h"
 #include "nact.h"
 #include "sdl3_mixer_backend.h"
+#include "sdl3_mixer_utils.h"
 #include "system.h"
 
 static DRIFILETYPE dri_type;
@@ -26,19 +27,9 @@ static int current_no;
 static int current_vol = 100;
 static uint32_t start_time;
 
-static int clamp_volume(int volume)
-{
-	if (volume < 0)
-		return 0;
-	if (volume > 100)
-		return 100;
-	return volume;
-}
-
 static float music_gain(int volume)
 {
-	float gain = clamp_volume(volume) / 100.0f;
-	return gain * clamp_volume(prv.volval[BGM_VOLVAL_CH]) / 100.0f;
+	return sdl3_mixer_gain(volume, prv.volval[BGM_VOLVAL_CH]);
 }
 
 static void apply_music_volume(int volume)
@@ -105,7 +96,7 @@ bool musbgm_play(int no, int time, int volume, int loop_count)
 	int loops = loop_count == 0 ? -1 : loop_count - 1;
 	current_vol = volume;
 	if (!sdl3_mixer_play_music(SDL3_MIXER_MUSIC_BGM, audio,
-		loops, time * 10, music_gain(current_vol)))
+		loops, sdl3_mixer_10ms_to_ms(time), music_gain(current_vol)))
 		return false;
 	current_no = no;
 	start_time = sys_get_ticks();
@@ -115,7 +106,8 @@ bool musbgm_play(int no, int time, int volume, int loop_count)
 void musbgm_stop(int no, int time)
 {
 	if (no == current_no)
-		sdl3_mixer_stop_music(SDL3_MIXER_MUSIC_BGM, time * 10);
+		sdl3_mixer_stop_music(SDL3_MIXER_MUSIC_BGM,
+			sdl3_mixer_10ms_to_ms(time));
 }
 
 void musbgm_fade(int no, int time, int volume)
@@ -129,7 +121,7 @@ int musbgm_getpos(int no)
 {
 	if (!musbgm_isplaying(no))
 		return 0;
-	return (sys_get_ticks() - start_time) / 10;
+	return sdl3_mixer_elapsed_10ms(start_time, sys_get_ticks());
 }
 
 int musbgm_getlen(int no)

src/cdrom.sdl3mixer.c

@@ -21,6 +21,7 @@
 #include "cdrom.h"
 #include "music_private.h"
 #include "sdl3_mixer_backend.h"
+#include "sdl3_mixer_utils.h"
 #include "system.h"
 
 #define PLAYLIST_MAX 256
@@ -32,18 +33,9 @@ static uint32_t start_time;
 
 static void cdrom_stop(void);
 
-static int clamp_volume(int volume)
-{
-	if (volume < 0)
-		return 0;
-	if (volume > 100)
-		return 100;
-	return volume;
-}
-
 static float music_gain(void)
 {
-	return clamp_volume(prv.volval[BGM_VOLVAL_CH]) / 100.0f;
+	return sdl3_mixer_gain(100, prv.volval[BGM_VOLVAL_CH]);
 }
 
 static void apply_volume(void)

src/midi.sdl3mixer.c

@@ -15,6 +15,7 @@
 #include "midi.h"
 #include "music_private.h"
 #include "sdl3_mixer_backend.h"
+#include "sdl3_mixer_utils.h"
 #include "system.h"
 
 #define FLUIDSYNTH_SOUNDFONT_PATH \
@@ -33,19 +34,9 @@ static bool has_decoder(const char *name)
 	return false;
 }
 
-static int clamp_volume(int volume)
-{
-	if (volume < 0)
-		return 0;
-	if (volume > 100)
-		return 100;
-	return volume;
-}
-
 static float music_gain(void)
 {
-	float gain = clamp_volume(current_vol) / 100.0f;
-	return gain * clamp_volume(prv.volval[BGM_VOLVAL_CH]) / 100.0f;
+	return sdl3_mixer_gain(current_vol, prv.volval[BGM_VOLVAL_CH]);
 }
 
 static void apply_volume(void)

src/pcm.sdl3mixer.c

@@ -21,6 +21,7 @@
 #include "music_private.h"
 #include "nact.h"
 #include "sdl3_mixer_backend.h"
+#include "sdl3_mixer_utils.h"
 #include "system.h"
 
 #define SAMPLE_RATE 44100
@@ -40,15 +41,6 @@ struct decoded_audio {
 	size_t length;
 };
 
-static int clamp_volume(int volume)
-{
-	if (volume < 0)
-		return 0;
-	if (volume > 100)
-		return 100;
-	return volume;
-}
-
 static void apply_volume(int slot)
 {
 	if (!slots[slot].track)
@@ -56,9 +48,8 @@ static void apply_volume(int slot)
 	int channel = prv.vol_pcm_sub[slot];
 	if ((unsigned)channel >= 16)
 		channel = 0;
-	float gain = clamp_volume(slots[slot].volume) / 100.0f;
-	gain *= clamp_volume(prv.volval[channel]) / 100.0f;
-	MIX_SetTrackGain(slots[slot].track, gain);
+	MIX_SetTrackGain(slots[slot].track,
+		sdl3_mixer_gain(slots[slot].volume, prv.volval[channel]));
 }
 
 static MIX_Audio *load_audio(const void *data, size_t size)

src/sdl3_mixer_utils.c

@@ -0,0 +1,25 @@
+#include "sdl3_mixer_utils.h"
+
+static int clamp_volume(int volume)
+{
+	if (volume < 0)
+		return 0;
+	if (volume > 100)
+		return 100;
+	return volume;
+}
+
+float sdl3_mixer_gain(int volume, int balance)
+{
+	return (float)(clamp_volume(volume) * clamp_volume(balance)) / 10000.0f;
+}
+
+int sdl3_mixer_10ms_to_ms(int time)
+{
+	return time * 10;
+}
+
+int sdl3_mixer_elapsed_10ms(uint32_t start, uint32_t now)
+{
+	return (now - start) / 10;
+}

src/sdl3_mixer_utils.h

@@ -0,0 +1,10 @@
+#ifndef XSYSTEM35_SDL3_MIXER_UTILS_H
+#define XSYSTEM35_SDL3_MIXER_UTILS_H
+
+#include <stdint.h>
+
+float sdl3_mixer_gain(int volume, int balance);
+int sdl3_mixer_10ms_to_ms(int time);
+int sdl3_mixer_elapsed_10ms(uint32_t start, uint32_t now);
+
+#endif /* XSYSTEM35_SDL3_MIXER_UTILS_H */

src/sdl3_mixer_utils_test.c

@@ -0,0 +1,38 @@
+/*
+ * Copyright (C) 2026 <KichikuouChrome@gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+#include <stdint.h>
+
+#include "sdl3_mixer_utils.h"
+#include "unittest.h"
+
+static int gain_basis_points(int volume, int balance)
+{
+	return sdl3_mixer_gain(volume, balance) * 10000.0f + 0.5f;
+}
+
+void sdl3_mixer_utils_test(void)
+{
+	ASSERT_EQUAL(gain_basis_points(100, 100), 10000);
+	ASSERT_EQUAL(gain_basis_points(50, 50), 2500);
+	ASSERT_EQUAL(gain_basis_points(33, 77), 2541);
+	ASSERT_EQUAL(gain_basis_points(-1, 100), 0);
+	ASSERT_EQUAL(gain_basis_points(100, -1), 0);
+	ASSERT_EQUAL(gain_basis_points(101, 200), 10000);
+
+	ASSERT_EQUAL(sdl3_mixer_10ms_to_ms(0), 0);
+	ASSERT_EQUAL(sdl3_mixer_10ms_to_ms(1), 10);
+	ASSERT_EQUAL(sdl3_mixer_10ms_to_ms(123), 1230);
+
+	ASSERT_EQUAL(sdl3_mixer_elapsed_10ms(1000, 1000), 0);
+	ASSERT_EQUAL(sdl3_mixer_elapsed_10ms(1000, 1009), 0);
+	ASSERT_EQUAL(sdl3_mixer_elapsed_10ms(1000, 1010), 1);
+	ASSERT_EQUAL(sdl3_mixer_elapsed_10ms(1000, 1123), 12);
+	ASSERT_EQUAL(sdl3_mixer_elapsed_10ms(UINT32_MAX - 4, 5), 1);
+}

src/src_tests.c

@@ -26,6 +26,9 @@ void gameresource_test(void);
 void hankaku_test(void);
 void qnt_test(void);
 void sdl_compat_test(void);
+#if XSYSTEM35_SDL_VERSION == 3
+void sdl3_mixer_utils_test(void);
+#endif
 void variable_test(void);
 
 void sys_error(char *format, ...) {
@@ -51,5 +54,8 @@ int main() {
 	hankaku_test();
 	qnt_test();
 	sdl_compat_test();
+#if XSYSTEM35_SDL_VERSION == 3
+	sdl3_mixer_utils_test();
+#endif
 	return 0;
 }