feat(fx): add effect rack and effect units

Source/EffectUnits/Distortion.cpp

@@ -0,0 +1,44 @@
+#include "Distortion.h"
+
+namespace serum
+{
+
+void DistortionUnit::prepare (double sampleRate, int)
+{
+    sr = sampleRate;
+    lpL.setSampleRate (sr); lpR.setSampleRate (sr);
+    lpL.setLowpass (8000.0, 0.7071); lpR.setLowpass (8000.0, 0.7071);
+    reset();
+}
+
+void DistortionUnit::reset()
+{
+    lpL.clear(); lpR.clear();
+}
+
+float DistortionUnit::shape (float x, float shapeParam) noexcept
+{
+    if (shapeParam < 0.33f)                        return std::tanh (x);
+    if (shapeParam < 0.66f)                        return juce::jlimit (-1.0f, 1.0f, x);
+    return std::sin (x * 1.5707963267948966f);     // sine fold
+}
+
+void DistortionUnit::process (float* l, float* r, int numSamples, const float p[4])
+{
+    const float drive  = 1.0f + p[0] * 24.0f;
+    const float shapeP = p[1];
+    const float output = 0.4f + p[3] * 1.2f;
+
+    // Tone: re-tune the lowpass from 800..16000 Hz.
+    const double toneHz = 800.0 + p[2] * 15200.0;
+    lpL.setLowpass (toneHz, 0.7071);
+    lpR.setLowpass (toneHz, 0.7071);
+
+    for (int i = 0; i < numSamples; ++i)
+    {
+        l[i] = lpL.process (shape (l[i] * drive, shapeP) * output);
+        r[i] = lpR.process (shape (r[i] * drive, shapeP) * output);
+    }
+}
+
+} // namespace serum