File content as of revision 5:503bd366fd73:

//--------------------------------------------------------------
//  縦続形 IIR フィルタで使う 1D タイプの 2 次のフィルタ
//  Biquad filter of 1D type for IIR filter of cascade structure
//      このクラスでは，係数は実行中に書き換えられることを想定している
//
//      u[n] = x[n] + a1*u[n-1] + a2*u[n-2]
//      y[n] = u[n] + b1*u[n-1] + b2*u[n-2]
//          x[n] :  input signal
//          y[n] :  output signal
//          b0 = 1
//
// 2017/01/26, Copyright (c) 2017 MIKAMI, Naoki
//--------------------------------------------------------------

#ifndef IIR_BIQUAD_HPP
#define IIR_BIQUAD_HPP

#include "mbed.h"

// 2nd order IIR filter
namespace Mikami
{
    class Biquad
    {
    public:
        struct Coefs { float a1, a2, b1, b2; };

        Biquad(const Coefs ck = (Coefs){0, 0, 0, 0})
        {
            SetCoefs(ck);
            Clear();
        }
        
        void SetCoefs(const Coefs cf) { cf_ = cf; }
        
        void GetCoefs(Coefs &cf) { cf = cf_; }

        float Execute(float xn)
        {
            float un = xn + cf_.a1*un1_ + cf_.a2*un2_;
            float yn = un + cf_.b1*un1_ + cf_.b2*un2_;
        
            un2_ = un1_;
            un1_ = un;

            return yn;
        }

        void Clear() { un1_ = un2_ = 0; }

    private:
        Coefs cf_;
        float un1_, un2_;

        // disallow copy constructor and assignment operator
        Biquad(const Biquad&);
        Biquad& operator=(const Biquad&);
    };
}
#endif  // IIR_BIQUAD_HPP