feat(fx): add effect rack and effect units

Source/EffectUnits/Delay.cpp

@@ -0,0 +1,62 @@
+#include "Delay.h"
+
+namespace serum
+{
+
+void DelayUnit::prepare (double sampleRate, int)
+{
+    sr = sampleRate;
+    const int maxDelay = (int) (sr * 2.1);   // 2.1 seconds
+    delayL.assign ((size_t) maxDelay, 0.0f);
+    delayR.assign ((size_t) maxDelay, 0.0f);
+    reset();
+}
+
+void DelayUnit::reset()
+{
+    std::fill (delayL.begin(), delayL.end(), 0.0f);
+    std::fill (delayR.begin(), delayR.end(), 0.0f);
+    writePos = 0;
+    dampL = dampR = 0.0f;
+}
+
+void DelayUnit::process (float* l, float* r, int numSamples, const float p[4])
+{
+    const int len = (int) delayL.size();
+    if (len < 4)
+        return;
+
+    const double delaySamples = juce::jlimit (2.0, (double) len - 2.0, maps::delayToMs (p[0]) / 1000.0 * sr);
+    const float  feedback = p[1] * 0.92f;
+    const float  dampCoef = 1.0f - p[2] * 0.95f;      // 1 = no damping
+    const float  pingpong = p[3];
+
+    for (int i = 0; i < numSamples; ++i)
+    {
+        double readPos = writePos - delaySamples;
+        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));
+
+        float dl = delayL[(size_t) i0] * (1.0f - frac) + delayL[(size_t) i1] * frac;
+        float dr = delayR[(size_t) i0] * (1.0f - frac) + delayR[(size_t) i1] * frac;
+
+        // Damping lowpass in the feedback path.
+        dampL = dl * (1.0f - dampCoef) + dampL * dampCoef;
+        dampR = dr * (1.0f - dampCoef) + dampR * dampCoef;
+
+        // Ping-pong: cross-feed between channels.
+        const float feedL = dampR * pingpong + dampL * (1.0f - pingpong);
+        const float feedR = dampL * pingpong + dampR * (1.0f - pingpong);
+
+        delayL[(size_t) writePos] = l[i] + feedL * feedback;
+        delayR[(size_t) writePos] = r[i] + feedR * feedback;
+
+        l[i] = dl;
+        r[i] = dr;
+        writePos = (writePos + 1) % len;
+    }
+}
+
+} // namespace serum