From 49836386304c7e18c14e3078060bc2fd773cb8b7 Mon Sep 17 00:00:00 2001
From: Brick <6098371+0x1F9F1@users.noreply.github.com>
Date: Mon, 21 Aug 2023 20:45:52 +0100
Subject: [PATCH] Misc audio tweaks/cleanup

---
 src/audio/SDL_audiocvt.c    | 16 ++++++++++------
 test/testautomation_audio.c |  2 +-
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/src/audio/SDL_audiocvt.c b/src/audio/SDL_audiocvt.c
index 0cb85008d..5496d238f 100644
--- a/src/audio/SDL_audiocvt.c
+++ b/src/audio/SDL_audiocvt.c
@@ -32,8 +32,12 @@
 
 #include "SDL_audio_resampler_filter.h"
 
+/* For a given srcpos, `srcpos + frame` are sampled, where `-RESAMPLER_ZERO_CROSSINGS < frame <= RESAMPLER_ZERO_CROSSINGS`.
+ * Note, when upsampling, it is also possible to start sampling from `srcpos = -1`. */
 #define RESAMPLER_MAX_PADDING_FRAMES (RESAMPLER_ZERO_CROSSINGS + 1)
 
+/* The source position is tracked using 32:32 fixed-point arithmetic.
+ * This gives high precision and avoids lots of divides in ResampleAudio. */
 static Sint64 GetResampleRate(const int src_rate, const int dst_rate)
 {
     SDL_assert(src_rate > 0);
@@ -47,16 +51,16 @@ static Sint64 GetResampleRate(const int src_rate, const int dst_rate)
 
 static size_t GetResamplerAvailableOutputFrames(const size_t input_frames, const Sint64 resample_rate, const Sint64 resample_offset)
 {
-    const Sint64 frames = ((((Sint64)input_frames << 32) - resample_offset - 1) / resample_rate) + 1;
+    const Sint64 output_frames = ((((Sint64)input_frames << 32) - resample_offset - 1) / resample_rate) + 1;
 
-    return (size_t) SDL_max(frames, 0);
+    return (size_t) SDL_max(output_frames, 0);
 }
 
 static int GetResamplerNeededInputFrames(const int output_frames, const Sint64 resample_rate, const Sint64 resample_offset)
 {
-    const Sint32 frames = (Sint32)((((output_frames - 1) * resample_rate) + resample_offset) >> 32) + 1;
+    const Sint32 input_frames = (Sint32)((((output_frames - 1) * resample_rate) + resample_offset) >> 32) + 1;
 
-    return (int) SDL_max(frames, 0);
+    return (int) SDL_max(input_frames, 0);
 }
 
 static int GetResamplerPaddingFrames(const Sint64 resample_rate)
@@ -68,6 +72,7 @@ static int GetHistoryBufferSampleFrames(const int required_resampler_frames)
 {
     SDL_assert(required_resampler_frames <= RESAMPLER_MAX_PADDING_FRAMES);
 
+    // Even if we aren't currently resampling, make sure to keep enough history in case we need to later.
     return RESAMPLER_MAX_PADDING_FRAMES;
 }
 
@@ -88,8 +93,7 @@ static void ResampleAudio(const int chans, const float *lpadding, const float *r
         Uint32 srcfraction = (Uint32)(srcpos & 0xFFFFFFFF);
         srcpos += resample_rate;
 
-        SDL_assert(srcindex >= -1);
-        SDL_assert(srcindex < inframes);
+        SDL_assert(srcindex >= -1 && srcindex < inframes);
 
         const int filterindex = (int)(srcfraction >> (32 - RESAMPLER_BITS_PER_ZERO_CROSSING)) * RESAMPLER_ZERO_CROSSINGS;
 
diff --git a/test/testautomation_audio.c b/test/testautomation_audio.c
index 0454d986e..6785b5e06 100644
--- a/test/testautomation_audio.c
+++ b/test/testautomation_audio.c
@@ -796,7 +796,7 @@ static int audio_resampleLoss(void *arg)
   } test_specs[] = {
     { 50, 440, 0, 44100, 48000, 79, 0.0008 },
     { 50, 5000, SDL_PI_D / 2, 20000, 10000, 999, 0.0001 },
-    { 50, 440, 0, 22050, 96000, 76, 0.0120 }, /* I have no idea how to tune these values */
+    { 50, 440, 0, 22050, 96000, 76, 0.0120 },
     { 50, 440, 0, 96000, 22050, 80, 0.0014 },
     { 0 }
   };