[feat]: Filter LFO (#23)

Reviewed-on: #23
Co-authored-by: HiveBeats <e1lama@protonmail.com>
Co-committed-by: HiveBeats <e1lama@protonmail.com>

src/Filter.cpp

@@ -1,36 +1,50 @@
 #include "Filter.h"
 #include "Settings.h"
+#include <algorithm>
+#include <cmath>
 
 Filter::Filter(/* args */) {}
 
 Filter::~Filter() {}
 
-void Filter::CalculateNormals() {
-    m_v = powf(10, fabs(m_order) / 20.0);
-    m_k = tanf(M_PI * m_freq);
-}
-
 void Filter::Trigger() {}
 
 void Filter::Release() {}
 
-float Filter::Process(float in) {
-    // may move to a compile-time dictionary calculation, if needed
-    CalculateCoefficients();
-    float out = in * m_a0 + m_z1;
-    m_z1 = in * m_a1 + m_z2 - m_b1 * out;
-    m_z2 = in * m_a2 - m_b2 * out;
-    return out;
-}
-
 void Filter::Process(std::vector<float>& samples) {
     for (std::size_t i = 0; i < samples.size(); i++) {
         samples[i] = Process(samples[i]);
     }
 }
 
-void Filter::SetParameters(float freq, float res, float q) {
-    m_freq = freq / SAMPLE_RATE;
-    m_q = res;
-    m_order = q;
+
+float Filter::Process(float in) {
+    m_notcho = in - m_damp * m_bando;
+    m_lowo = m_lowo + m_freq * m_bando;
+    m_higho = m_notcho - m_lowo;
+    m_bando =
+        m_freq * m_higho + m_bando - m_drive * m_bando * m_bando * m_bando;
+    // (m_notcho or m_lowo or m_higho or m_bando or m_peako)
+    float out = 0.5 * GetSampleForFilterType();
+    m_notcho = in - m_damp * m_bando;
+    m_lowo = m_lowo + m_freq * m_bando;
+    m_higho = m_notcho - m_lowo;
+    m_bando =
+        m_freq * m_higho + m_bando - m_drive * m_bando * m_bando * m_bando;
+    out += 0.5 * GetSampleForFilterType();
+
+    return out;
+}
+
+void Filter::SetParameters(float freq, float res, float drive) {
+    m_fc = freq;
+    m_res = res;
+    m_drive = drive;
+
+    // the fs*2 is because it's double sampled
+    m_freq =
+        2.0 * std::sinf(SYNTH_PI * std::min(0.25f, m_fc / (m_fs * 2)));
+
+    m_damp = std::min(2.0f * (1.0f - std::powf(m_res, 0.25f)),
+                            std::min(2.0f, 2.0f / m_freq - m_freq * 0.5f));
 }