From bf3eb50b55f61140d4614b16d70bcc57755b9d64 Mon Sep 17 00:00:00 2001
From: veclavtalica <veclavtalica@tutamail.com>
Date: Sun, 26 Jan 2025 11:08:13 +0300
Subject: [PATCH] twn_audio.c: only apply volume scaling on mixing of different
 streams

---
 src/twn_audio.c | 56 ++++++++++++++++++++++++++++++++-----------------
 1 file changed, 37 insertions(+), 19 deletions(-)

diff --git a/src/twn_audio.c b/src/twn_audio.c
index 7288f5f..f4dd69f 100644
--- a/src/twn_audio.c
+++ b/src/twn_audio.c
@@ -356,7 +356,8 @@ TWN_API void audio_parameter(const char *channel, const char *param, float value
 static void audio_mixin_streams(const AudioChannel *channel,
                                 uint8_t *restrict a,
                                 uint8_t *restrict b,
-                                size_t frames)
+                                size_t frames,
+                                bool last)
 {
     float *const sa = (float *)(void *)a;
     float *const sb = (float *)(void *)b;
@@ -364,14 +365,22 @@ static void audio_mixin_streams(const AudioChannel *channel,
     const float left_panning = fminf(fabsf(channel->panning - 1.0f), 1.0f);
     const float right_panning = fminf(fabsf(channel->panning + 1.0f), 1.0f);
 
-    for (size_t s = 0; s < frames; ++s) {
-        /* left channel */
-        sa[s * 2 + 0] += (float)(sb[s * 2 + 0] * channel->volume * left_panning);
-        sa[s * 2 + 0] *= 1 / (float)M_2_SQRTPI;
-
-        /* right channel */
-        sa[s * 2 + 1] += (float)(sb[s * 2 + 1] * channel->volume * right_panning);
-        sa[s * 2 + 1] *= 1 / (float)M_2_SQRTPI;
+    if (last) {
+        for (size_t s = 0; s < frames; ++s) {
+            /* left channel */
+            sa[s * 2 + 0] += (float)(sb[s * 2 + 0] * channel->volume * left_panning);
+            /* right channel */
+            sa[s * 2 + 1] += (float)(sb[s * 2 + 1] * channel->volume * right_panning);
+        }
+    } else {
+        for (size_t s = 0; s < frames; ++s) {
+            /* left channel */
+            sa[s * 2 + 0] += (float)(sb[s * 2 + 0] * channel->volume * left_panning);
+            sa[s * 2 + 0] *= 1 / (float)M_2_SQRTPI;
+            /* right channel */
+            sa[s * 2 + 1] += (float)(sb[s * 2 + 1] * channel->volume * right_panning);
+            sa[s * 2 + 1] *= 1 / (float)M_2_SQRTPI;
+        }
     }
 }
 
@@ -379,7 +388,7 @@ static void audio_mixin_streams(const AudioChannel *channel,
 /* remember: frame consists of sample * channel_count */
 static void audio_sample_and_mixin_channel(AudioChannel *channel,
                                            uint8_t *stream,
-                                           int len)
+                                           int len, bool last)
 {
     static uint8_t buffer[16384]; /* TODO: better make it a growable scratch instead, which will simplify things */
     const size_t float_buffer_frames = sizeof (buffer) / sizeof (float) / 2;
@@ -414,7 +423,8 @@ static void audio_sample_and_mixin_channel(AudioChannel *channel,
             /* panning and mixing */
             audio_mixin_streams(channel,
                                 &stream[i * sizeof(float) * 2], buffer,
-                                samples_per_channel);
+                                samples_per_channel,
+                                last);
 
             i += samples_per_channel;
         }
@@ -431,7 +441,8 @@ static void audio_sample_and_mixin_channel(AudioChannel *channel,
             audio_mixin_streams(channel,
                                 &stream[i * sizeof(float) * 2],
                                 &((uint8_t *)channel->context.wav.samples)[channel->context.wav.position * sizeof (float) * 2],
-                                limit);
+                                limit,
+                                last);
 
             channel->context.wav.position += limit;
 
@@ -477,7 +488,8 @@ static void audio_sample_and_mixin_channel(AudioChannel *channel,
             audio_mixin_streams(channel,
                                 &stream[i * sizeof(float) * 2],
                                 buffer,
-                                samples_per_channel);
+                                samples_per_channel,
+                                last);
 
             i += samples_per_channel;
         }
@@ -509,19 +521,25 @@ void audio_callback(void *userdata, uint8_t *stream, int len) {
     /* prepare for mixing */
     SDL_memset(stream, 0, len);
 
-    for (int i = 0; i < shlen(ctx.audio_channels); ++i) {
+    size_t const audio_channels_len = shlen(ctx.audio_channels);
+    for (size_t i = 0; i < audio_channels_len; ++i) {
         sanity_check_channel(&ctx.audio_channels[i].value);
-        audio_sample_and_mixin_channel(&ctx.audio_channels[i].value, stream, len);
+        audio_sample_and_mixin_channel(&ctx.audio_channels[i].value,
+                                       stream, len,
+                                       i == audio_channels_len - 1);
     }
 
-    for (int i = 0; i < arrlen(ctx.unnamed_audio_channels); ++i) {
+    size_t const unnamed_audio_channels_len = shlen(ctx.unnamed_audio_channels);
+    for (size_t i = 0; i < unnamed_audio_channels_len; ++i) {
         sanity_check_channel(&ctx.unnamed_audio_channels[i]);
-        audio_sample_and_mixin_channel(&ctx.unnamed_audio_channels[i], stream, len);
+        audio_sample_and_mixin_channel(&ctx.unnamed_audio_channels[i],
+                                       stream, len,
+                                       i == unnamed_audio_channels_len - 1);
     }
 
     /* ditch finished unnamed */
-    int i = 0;
-    while (i < arrlen(ctx.unnamed_audio_channels)) {
+    size_t i = 0;
+    while (i < unnamed_audio_channels_len) {
         if (ctx.unnamed_audio_channels[i].finished) {
             free_audio_channel(ctx.unnamed_audio_channels[i]);
             arrdelswap(ctx.unnamed_audio_channels, i);