From b1f1fc40423d013d1430325b064edfa8de278fea Mon Sep 17 00:00:00 2001
From: Jean Pierre Cimalando <jp-dev@inbox.ru>
Date: Thu, 27 Aug 2020 09:12:42 +0200
Subject: [PATCH] Power follower using a fixed block size

---
 src/sfizz/Config.h          |  1 +
 src/sfizz/PowerFollower.cpp | 49 ++++++++++++++++++++++++++++---------
 src/sfizz/PowerFollower.h   |  6 +++--
 3 files changed, 42 insertions(+), 14 deletions(-)

diff --git a/src/sfizz/Config.h b/src/sfizz/Config.h
index fc48aab5..67821b02 100644
--- a/src/sfizz/Config.h
+++ b/src/sfizz/Config.h
@@ -57,6 +57,7 @@ namespace config {
     constexpr Oversampling defaultOversamplingFactor { Oversampling::x1 };
     constexpr float A440 { 440.0 };
     constexpr size_t powerHistoryLength { 16 };
+    constexpr size_t powerFollowerStep { 512 };
     constexpr float powerFollowerAttackFactor { 100 };
     constexpr float powerFollowerReleaseFactor { 10 };
     constexpr uint16_t numCCs { 512 };
diff --git a/src/sfizz/PowerFollower.cpp b/src/sfizz/PowerFollower.cpp
index 56ccb86e..06807c7a 100644
--- a/src/sfizz/PowerFollower.cpp
+++ b/src/sfizz/PowerFollower.cpp
@@ -42,26 +42,51 @@ void PowerFollower::process(AudioSpan<float> buffer) noexcept
     if (numFrames == 0)
         return;
 
-    absl::Span<float> tempBuffer(tempBuffer_.get(), numFrames);
+    ///
+    constexpr size_t step = config::powerFollowerStep;
+    float currentPower = currentPower_;
+    float currentSum = currentSum_;
+    size_t currentCount = currentCount_;
 
-    copy(buffer.getConstSpan(0), tempBuffer);
-    for (unsigned i = 1; i < buffer.getNumChannels(); ++i)
-        add(buffer.getConstSpan(i), tempBuffer);
+    const float attackFactor = static_cast<float>(numFrames) * attackTrackingFactor_;
+    const float releaseFactor = static_cast<float>(numFrames) * releaseTrackingFactor_;
 
-    const float meanPower = meanSquared<float>(tempBuffer);
+    ///
+    size_t index = 0;
+    while (index < numFrames) {
+        size_t blockSize = std::min(step - currentCount, numFrames - index);
+        absl::Span<float> tempBuffer(tempBuffer_.get(), blockSize);
 
-    const float attackFactor = static_cast<float>(buffer.getNumFrames()) * attackTrackingFactor_;
-    const float releaseFactor = static_cast<float>(buffer.getNumFrames()) * releaseTrackingFactor_;
+        copy(buffer.getConstSpan(0).subspan(index, blockSize), tempBuffer);
+        for (unsigned i = 1, n = buffer.getNumChannels(); i < n; ++i)
+            add(buffer.getConstSpan(i).subspan(index, blockSize), tempBuffer);
 
-    meanChannelPower_ = max(
-        meanChannelPower_ * (1 - attackFactor) + meanPower * attackFactor,
-        meanChannelPower_ * (1 - releaseFactor) + meanPower * releaseFactor
-    );
+        currentSum += sumSquares<float>(tempBuffer);
+        currentCount += blockSize;
+
+        if (currentCount == step) {
+            const float meanPower = currentSum / step;
+            currentPower = max(
+                currentPower * (1 - attackFactor) + meanPower * attackFactor,
+                currentPower * (1 - releaseFactor) + meanPower * releaseFactor);
+            currentSum = 0;
+            currentCount = 0;
+        }
+
+        index += blockSize;
+    }
+
+    ///
+    currentPower_ = currentPower;
+    currentSum_ = currentSum;
+    currentCount_ = currentCount;
 }
 
 void PowerFollower::clear() noexcept
 {
-    meanChannelPower_ = 0;
+    currentPower_ = 0;
+    currentSum_ = 0;
+    currentCount_ = 0;
 }
 
 void PowerFollower::updateTrackingFactor() noexcept
diff --git a/src/sfizz/PowerFollower.h b/src/sfizz/PowerFollower.h
index 2deff3d5..1febd9f1 100644
--- a/src/sfizz/PowerFollower.h
+++ b/src/sfizz/PowerFollower.h
@@ -17,7 +17,7 @@ public:
     void setSamplesPerBlock(unsigned samplesPerBlock);
     void process(AudioSpan<float> buffer) noexcept;
     void clear() noexcept;
-    float getAveragePower() const noexcept { return meanChannelPower_; }
+    float getAveragePower() const noexcept { return currentPower_; }
 
 private:
     void updateTrackingFactor() noexcept;
@@ -31,7 +31,9 @@ private:
     float attackTrackingFactor_ {};
     float releaseTrackingFactor_ {};
 
-    float meanChannelPower_ {};
+    float currentPower_ {};
+    float currentSum_ = 0;
+    size_t currentCount_ = 0;
 };
 
 } // namespace sfz