From e38413a4fbb0b4f85950c929141856a767993f4f Mon Sep 17 00:00:00 2001
From: Chris Robinson <chris.kcat@gmail.com>
Date: Wed, 4 Jan 2023 01:44:40 -0800
Subject: Handle negative offset starts for callback buffers

---
 core/voice.cpp | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/core/voice.cpp b/core/voice.cpp
index 0199d948..fc3a38cc 100644
--- a/core/voice.cpp
+++ b/core/voice.cpp
@@ -297,14 +297,15 @@ void LoadBufferStatic(VoiceBufferItem *buffer, VoiceBufferItem *&bufferLoopItem,
 
 void LoadBufferCallback(VoiceBufferItem *buffer, const size_t numCallbackSamples,
     const FmtType sampleType, const FmtChannels sampleChannels, const size_t srcStep,
-    const size_t samplesToLoad, const al::span<float*> voiceSamples)
+    size_t samplesLoaded, const size_t samplesToLoad, const al::span<float*> voiceSamples)
 {
     /* Load what's left to play from the buffer */
-    const size_t remaining{minz(samplesToLoad, numCallbackSamples)};
-    LoadSamples(voiceSamples, 0, buffer->mSamples, 0, sampleType, sampleChannels, srcStep,
-        remaining);
+    const size_t remaining{minz(samplesToLoad-samplesLoaded, numCallbackSamples)};
+    LoadSamples(voiceSamples, samplesLoaded, buffer->mSamples, 0, sampleType, sampleChannels,
+        srcStep, remaining);
+    samplesLoaded += remaining;
 
-    if(const size_t toFill{samplesToLoad - remaining})
+    if(const size_t toFill{samplesToLoad - samplesLoaded})
     {
         for(auto *chanbuffer : voiceSamples)
         {
@@ -649,10 +650,11 @@ void Voice::mix(const State vstate, ContextBase *Context, const nanoseconds devi
                     mFmtChannels, mFrameStep, samplesLoaded, SrcBufferSize, MixingSamples);
             else if(mFlags.test(VoiceIsCallback))
             {
-                if(!mFlags.test(VoiceCallbackStopped) && SrcBufferSize > mNumCallbackSamples)
+                const size_t remaining{SrcBufferSize - samplesLoaded};
+                if(!mFlags.test(VoiceCallbackStopped) && remaining > mNumCallbackSamples)
                 {
                     const size_t byteOffset{mNumCallbackSamples*mFrameSize};
-                    const size_t needBytes{SrcBufferSize*mFrameSize - byteOffset};
+                    const size_t needBytes{remaining*mFrameSize - byteOffset};
 
                     const int gotBytes{BufferListItem->mCallback(BufferListItem->mUserData,
                         &BufferListItem->mSamples[byteOffset], static_cast<int>(needBytes))};
@@ -664,10 +666,10 @@ void Voice::mix(const State vstate, ContextBase *Context, const nanoseconds devi
                         mNumCallbackSamples += static_cast<uint>(gotBytes) / mFrameSize;
                     }
                     else
-                        mNumCallbackSamples = SrcBufferSize;
+                        mNumCallbackSamples = static_cast<uint>(remaining);
                 }
                 LoadBufferCallback(BufferListItem, mNumCallbackSamples, mFmtType, mFmtChannels,
-                    mFrameStep, SrcBufferSize, MixingSamples);
+                    mFrameStep, samplesLoaded, SrcBufferSize, MixingSamples);
             }
             else
                 LoadBufferQueue(BufferListItem, BufferLoopItem, DataPosUInt, mFmtType, mFmtChannels,
-- 
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