feat(fx): add effect rack and effect units

Source/EffectUnits/Flanger.cpp

@@ -0,0 +1,59 @@
+#include "Flanger.h"
+
+namespace serum
+{
+
+void FlangerUnit::prepare (double sampleRate, int)
+{
+    sr = sampleRate;
+    const int maxDelay = (int) (sr * 0.03);   // 30ms
+    delayL.assign ((size_t) maxDelay, 0.0f);
+    delayR.assign ((size_t) maxDelay, 0.0f);
+    reset();
+}
+
+void FlangerUnit::reset()
+{
+    std::fill (delayL.begin(), delayL.end(), 0.0f);
+    std::fill (delayR.begin(), delayR.end(), 0.0f);
+    writePos = 0;
+    lfoPhase = 0.0;
+}
+
+void FlangerUnit::process (float* l, float* r, int numSamples, const float p[4])
+{
+    const int len = (int) delayL.size();
+    if (len < 4)
+        return;
+
+    const double rate     = 0.05 + p[0] * 2.0;
+    const double depth    = (0.3 + p[1] * 4.7) * sr / 1000.0;   // 0.3..5 ms
+    const double base     = 1.5 * sr / 1000.0;
+    const float  feedback = p[2] * 0.9f;
+
+    for (int i = 0; i < numSamples; ++i)
+    {
+        const float inL = l[i], inR = r[i];
+
+        lfoPhase += 6.28318530717958647692 * rate / sr;
+        if (lfoPhase > 6.28318530717958647692) lfoPhase -= 6.28318530717958647692;
+        const double mod = std::sin (lfoPhase) * depth;
+
+        double readPos = writePos - (base + mod);
+        if (readPos < 0.0) readPos += len;
+        const int i0 = (int) readPos;
+        const int i1 = (i0 + 1) % len;
+        const float frac = (float) (readPos - std::floor (readPos));
+        const float outL = delayL[(size_t) i0] * (1.0f - frac) + delayL[(size_t) i1] * frac;
+        const float outR = delayR[(size_t) i0] * (1.0f - frac) + delayR[(size_t) i1] * frac;
+
+        delayL[(size_t) writePos] = inL + outL * feedback;
+        delayR[(size_t) writePos] = inR + outR * feedback;
+
+        l[i] = outL;
+        r[i] = outR;
+        writePos = (writePos + 1) % len;
+    }
+}
+
+} // namespace serum