text: //-----------------------------------------------------------------------------
  // Project     : VST SDK
  // Version     : 3.6.0
  //
  // Category    : Examples
  // Filename    : public.sdk/samples/vst/note_expression_synth/source/filter.h
  // Created by  : Steinberg, 03/2010
  // Description : 
  //
  //-----------------------------------------------------------------------------
  // LICENSE
  // (c) 2013, Steinberg Media Technologies GmbH, All Rights Reserved
  //-----------------------------------------------------------------------------
  // Redistribution and use in source and binary forms, with or without modification,
  // are permitted provided that the following conditions are met:
  // 
  //   * Redistributions of source code must retain the above copyright notice, 
  //     this list of conditions and the following disclaimer.
  //   * Redistributions in binary form must reproduce the above copyright notice,
  //     this list of conditions and the following disclaimer in the documentation 
  //     and/or other materials provided with the distribution.
  //   * Neither the name of the Steinberg Media Technologies nor the names of its
  //     contributors may be used to endorse or promote products derived from this 
  //     software without specific prior written permission.
  // 
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
  // ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED 
  // WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 
  // IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 
  // INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, 
  // BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 
  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF 
  // LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE 
  // OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
  // OF THE POSSIBILITY OF SUCH DAMAGE.
  //-----------------------------------------------------------------------------
  
  #ifndef __filter__
  #define __filter__
  
  #include <cmath>
  #include <algorithm>
  
  #ifndef M_PI
  #define M_PI       3.14159265358979323846
  #endif
  
  namespace Steinberg {
  namespace Vst {
  namespace NoteExpressionSynth {
  
  //-----------------------------------------------------------------------------
  class Filter
  {
  public:
  	enum Type {
  		kLowpass,
  		kHighpass,
  		kBandpass,
  		
  		kNumTypes
  	};
  
  	Filter (Type type) : type (type), sampleRate (44100.) { reset (); }
  
  	inline void setType (Type t) { type = t; }
  	inline void setSampleRate (double sampleRate);
  	inline void setFreqAndQ (double frequency, double q);
  	
  	inline double process (double sample);
  
  	inline void reset ();
  protected:
  	Type type;
  
  	double sampleRate;
  	double invSampleRate;
  
  	double in1;
  	double in2;
  	double out1;
  	double out2;
  
  	double b0a0;
  	double b1a0;
  	double b2a0;
  	double a1a0;
  	double a2a0;
  };
  
  //-----------------------------------------------------------------------------
  void Filter::reset ()
  {
  	in1 = in2 = out1 = out2 = 0.;
  	b0a0 = 1.;
  	b1a0 = b2a0 = a1a0 = a2a0 = 0.;
  }
  
  //-----------------------------------------------------------------------------
  void Filter::setSampleRate (double sampleRate)
  {
  	this->sampleRate = sampleRate;
  	this->invSampleRate = 1. / sampleRate;
  }
  
  //-----------------------------------------------------------------------------
  void Filter::setFreqAndQ (double freq, double q)
  {
  	static const double M_LOG2 = log (2.0);
  
  	double frequency = std::max<double> (80., freq);
  	bool q_is_bandwidth = true;
  
  	// temp coefficient vars
  	double alpha = 1.;
  	double a0 = 0.;
  	double a1 = 0.;
  	double a2 = 0.;
  	double b0 = 0.;
  	double b1 = 0.;
  	double b2 = 0.;
  
  	double const omega = 2.0 * M_PI * frequency * invSampleRate;
  	double const tsin = sin (omega);
  	double const tcos = cos (omega);
  
  	if (type == kBandpass)
  	{
  		if (q_is_bandwidth)
  			alpha = tsin * sinh (M_LOG2 * 0.5 * q * omega / tsin);
  		else
  			alpha = tsin / (2.0 * q);
  
  		b0=tsin * 0.5;
  		b1=0.0;
  		b2=-tsin * 0.5;
  		a0=1.0+alpha;
  		a1=-2.0*tcos;
  		a2=1.0-alpha;
  	}
  	else
  	{
  		if (q_is_bandwidth)
  			alpha = tsin * sinh (M_LOG2 * 0.5 * q * omega / tsin);
  		else
  			alpha = tsin / (2.0 * q);
  
  		if (type == kLowpass)
  		{
  			b0=(1.0-tcos) * 0.5;
  			b1=1.0-tcos;
  			b2=(1.0-tcos) * 0.5;
  			a0=1.0+alpha;
  			a1=-2.0*tcos;
  			a2=1.0-alpha;
  		}
  		else if (type == kHighpass)
  		{
  			b0=(1.0+tcos) * 0.5;
  			b1=-(1.0+tcos);
  			b2=(1.0+tcos) * 0.5;
  			a0=1.0+ alpha;
  			a1=-2.0*tcos;
  			a2=1.0-alpha;
  		}
  	}
  	
  	double invA0 = 1.0 / a0;
  	b0a0=float(b0 * invA0);
  	b1a0=float(b1 * invA0);
  	b2a0=float(b2 * invA0);
  	a1a0=float(a1 * invA0);
  	a2a0=float(a2 * invA0);
  }
  
  //-----------------------------------------------------------------------------
  double Filter::process (double sample)
  {
  	double output = b0a0 * sample + b1a0 * in1 + b2a0 * in2 - a1a0 * out1 - a2a0 * out2;
  	in2 = in1;
  	in1 = sample;
  	out2 = out1;
  	out1 = output;
  
  	return output;
  }
  
  }}} // namespaces
  
  #endif // __filter__
  