From d3830870c03aa8340c337d2e0f208d174105184c Mon Sep 17 00:00:00 2001
From: Chris Cannam <cannam@all-day-breakfast.com>
Date: Wed, 15 Mar 2023 18:01:21 +0000
Subject: [PATCH] Experiments with mid-side in R3 - in this code always used
 when channels=2

---
 src/finer/R3Stretcher.cpp | 47 +++++++++++++++++++++++++++++++++++++--
 src/finer/R3Stretcher.h   |  5 ++++-
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/src/finer/R3Stretcher.cpp b/src/finer/R3Stretcher.cpp
index 6cb6ae9..a4e0945 100644
--- a/src/finer/R3Stretcher.cpp
+++ b/src/finer/R3Stretcher.cpp
@@ -752,11 +752,13 @@ R3Stretcher::process(const float *const *input, size_t samples, bool final)
             int maxResampleInput = int(floor(maxResampleOutput * m_pitchScale));
             int resampleInput = std::min(remaining, maxResampleInput);
             if (resampleInput == 0) resampleInput = 1;
+
+            prepareInput(input, inputIx, resampleInput);
             
             int resampleOutput = m_resampler->resample
                 (m_channelAssembly.resampled.data(),
                  maxResampleOutput,
-                 input,
+                 m_channelAssembly.input.data(),
                  resampleInput,
                  1.0 / m_pitchScale,
                  final);
@@ -776,8 +778,11 @@ R3Stretcher::process(const float *const *input, size_t samples, bool final)
 
             m_log.log(2, "process: resamplingBefore is false, writing to inbuf from supplied data, former read space and samples being added", m_channelData[0]->inbuf->getReadSpace(), toWrite);
 
+            prepareInput(input, inputIx, toWrite);
+            
             for (int c = 0; c < m_parameters.channels; ++c) {
-                m_channelData[c]->inbuf->write(input[c] + inputIx, toWrite);
+                m_channelData[c]->inbuf->write
+                    (m_channelAssembly.input[c], toWrite);
             }
             inputIx += toWrite;
         }
@@ -814,9 +819,47 @@ R3Stretcher::retrieve(float *const *output, size_t samples) const
         }
     }
 
+    bool useMidSide = (m_parameters.channels == 2); //!!!
+
+    if (useMidSide) {
+        for (int i = 0; i < got; ++i) {
+            float m = output[0][i];
+            float s = output[1][i];
+            float l = m + s;
+            float r = m - s;
+            output[0][i] = l;
+            output[1][i] = r;
+        }
+    }
+    
     return got;
 }
 
+void
+R3Stretcher::prepareInput(const float *const *input, int ix, int n)
+{
+    bool useMidSide = (m_parameters.channels == 2); //!!!
+
+    if (useMidSide) {
+        auto &c0 = m_channelData.at(0)->mixdown;
+        auto &c1 = m_channelData.at(1)->mixdown;
+        for (int i = 0; i < n; ++i) {
+            float l = input[0][i + ix];
+            float r = input[1][i + ix];
+            float m = (l + r) / 2.f;
+            float s = (l - r) / 2.f;
+            c0[i] = m;
+            c1[i] = s;
+        }
+        m_channelAssembly.input[0] = m_channelData.at(0)->mixdown.data();
+        m_channelAssembly.input[1] = m_channelData.at(1)->mixdown.data();
+    } else {
+        for (int c = 0; c < m_parameters.channels; ++c) {
+            m_channelAssembly.input[c] = input[c] + ix;
+        }
+    }
+}
+
 void
 R3Stretcher::consume()
 {
diff --git a/src/finer/R3Stretcher.h b/src/finer/R3Stretcher.h
index d1b8308..57a3187 100644
--- a/src/finer/R3Stretcher.h
+++ b/src/finer/R3Stretcher.h
@@ -241,7 +241,7 @@ protected:
                                BinClassifier::Classification::Residual),
             segmenter(new BinSegmenter(segmenterParameters)),
             segmentation(), prevSegmentation(), nextSegmentation(),
-            mixdown(longestFftSize, 0.f),
+            mixdown(inRingBufferSize, 0.f),
             resampled(outRingBufferSize, 0.f),
             inbuf(new RingBuffer<float>(inRingBufferSize)),
             outbuf(new RingBuffer<float>(outRingBufferSize)),
@@ -263,6 +263,7 @@ protected:
     struct ChannelAssembly {
         // Vectors of bare pointers, used to package container data
         // from different channels into arguments for PhaseAdvance
+        FixedVector<const float *> input;
         FixedVector<process_t *> mag;
         FixedVector<process_t *> phase;
         FixedVector<process_t *> prevMag;
@@ -271,6 +272,7 @@ protected:
         FixedVector<float *> mixdown;
         FixedVector<float *> resampled;
         ChannelAssembly(int channels) :
+            input(channels, nullptr),
             mag(channels, nullptr), phase(channels, nullptr),
             prevMag(channels, nullptr), guidance(channels, nullptr),
             outPhase(channels, nullptr), mixdown(channels, nullptr),
@@ -350,6 +352,7 @@ protected:
     };
     ProcessMode m_mode;
 
+    void prepareInput(const float *const *input, int ix, int n);
     void consume();
     void createResampler();
     void calculateHop();