[feat]: Filter (#18)

closes #16

Reviewed-on: #18

inc/Filter.h

@@ -0,0 +1,35 @@
+#pragma once
+#include "Effect.h"
+
+enum FilterType {
+    LowPass,
+    BandPass,
+    HighPass
+};
+
+class Filter : public Effect {
+  protected:
+    float m_freq;  // cutoff frequency
+    float m_q;     // filter quantity (resonance)
+    float m_order; // filter order (peakGain)
+    /* todo: filter adsr */
+    float m_norm, m_v, m_k;
+    float m_a0, m_a1, m_a2, m_b1, m_b2;
+    float m_z1, m_z2;
+
+    void CalculateNormals();
+    virtual void CalculateCoefficients(){};
+
+  public:
+    Filter(/* args */);
+    virtual ~Filter();
+    void Trigger() override;
+    void Release() override;
+    float Process(float in);
+    void Process(std::vector<float>& samples) override;
+    void SetParameters(float freq, float res, float q);
+    float GetFreq() { return m_freq; }
+    float GetRes() { return m_q; }
+    float GetPeakGain() { return m_norm; }
+    virtual bool IsSameFilterType(FilterType type){ return false; };
+};