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+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * This code was translated from the JSyn Java code.
+ * JSyn is Copyright 2009 Phil Burk, Mobileer Inc
+ * JSyn is licensed under the Apache License, Version 2.0
+ */
+
+#ifndef SYNTHMARK_DIFFERENTIATED_PARABOLA_H
+#define SYNTHMARK_DIFFERENTIATED_PARABOLA_H
+
+#include <cstdint>
+#include <math.h>
+#include "SynthTools.h"
+
+namespace marksynth {
+
+constexpr double kDPWVeryLowFrequency  = 2.0 * 0.1 / kSynthmarkSampleRate;
+
+/**
+ * DPW is a tool for generating band-limited waveforms
+ * based on a paper by Antti Huovilainen and Vesa Valimaki:
+ *  "New Approaches to Digital Subtractive Synthesis"
+ */
+class DifferentiatedParabola
+{
+public:
+    DifferentiatedParabola()
+    : mZ1(0)
+    , mZ2(0) {}
+
+    virtual ~DifferentiatedParabola() = default;
+
+    synth_float_t next(synth_float_t phase, synth_float_t phaseIncrement) {
+        synth_float_t dpw;
+        synth_float_t positivePhaseIncrement = (phaseIncrement < 0.0)
+                ? phaseIncrement
+                : 0.0 - phaseIncrement;
+
+        // If the frequency is very low then just use the raw sawtooth.
+        // This avoids divide by zero problems and scaling problems.
+        if (positivePhaseIncrement < kDPWVeryLowFrequency) {
+            dpw = phase;
+        } else {
+            // Calculate the parabola.
+            synth_float_t squared = phase * phase;
+            // Differentiate using a delayed value.
+            synth_float_t diffed = squared - mZ2;
+            // Delay line.
+            // TODO - Why Z2. Vesa's paper says use Z1?
+            mZ2 = mZ1;
+            mZ1 = squared;
+
+            // Calculate scaling
+            dpw = diffed * 0.25f / positivePhaseIncrement; // TODO extract and optimize
+        }
+        return dpw;
+    }
+
+private:
+    synth_float_t mZ1;
+    synth_float_t mZ2;
+};
+
+};
+#endif // SYNTHMARK_DIFFERENTIATED_PARABOLA_H