feat(fx): add effect rack and effect units

Source/EffectUnits/Compressor.cpp

@@ -0,0 +1,52 @@
+#include "Compressor.h"
+
+namespace serum
+{
+
+void CompressorUnit::prepare (double sampleRate, int)
+{
+    sr = sampleRate;
+    reset();
+}
+
+void CompressorUnit::reset()
+{
+    envL = envR = 0.0f;
+    gainL = gainR = 1.0f;
+}
+
+void CompressorUnit::process (float* l, float* r, int numSamples, const float p[4])
+{
+    const double thresholdDb = -40.0 + p[0] * 40.0;   // -40..0 dB
+    const double ratio       = 2.0 + p[1] * 18.0;     // 2..20
+    const double attackMs    = 2.0 + p[2] * 98.0;
+    const double releaseMs   = 30.0 + p[3] * 470.0;
+    const double attackCoef  = std::exp (-1.0 / (attackMs / 1000.0 * sr));
+    const double releaseCoef = std::exp (-1.0 / (releaseMs / 1000.0 * sr));
+    const double threshold   = std::pow (10.0, thresholdDb / 20.0);
+    const double makeup      = std::pow (10.0, -thresholdDb * 0.5 / 20.0);
+
+    for (int i = 0; i < numSamples; ++i)
+    {
+        const float aL = std::abs (l[i]);
+        const float aR = std::abs (r[i]);
+
+        envL = aL > envL ? (float) (attackCoef * envL + (1.0 - attackCoef) * aL)
+                         : (float) (releaseCoef * envL + (1.0 - releaseCoef) * aL);
+        envR = aR > envR ? (float) (attackCoef * envR + (1.0 - attackCoef) * aR)
+                         : (float) (releaseCoef * envR + (1.0 - releaseCoef) * aR);
+
+        const double targetL = (envL > threshold) ? threshold * std::pow (envL / threshold, 1.0 / ratio) : envL;
+        const double targetR = (envR > threshold) ? threshold * std::pow (envR / threshold, 1.0 / ratio) : envR;
+        const float desiredL = (float) juce::jlimit (0.05, 1.0, (envL > 1e-6) ? targetL / envL : 1.0);
+        const float desiredR = (float) juce::jlimit (0.05, 1.0, (envR > 1e-6) ? targetR / envR : 1.0);
+
+        gainL += 0.01f * (desiredL - gainL);
+        gainR += 0.01f * (desiredR - gainR);
+
+        l[i] = (float) (l[i] * gainL * makeup);
+        r[i] = (float) (r[i] * gainR * makeup);
+    }
+}
+
+} // namespace serum