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/// @file	LowPassFilter.cpp
/// @brief	A class to implement a second order low pass filter 
/// Author: Leonard Hall <LeonardTHall@gmail.com>

#include "LowPassFilter2p.hpp"
#include "math.h"

namespace math
{

void LowPassFilter2p::set_cutoff_frequency(float sample_freq, float cutoff_freq)
{
    _cutoff_freq = cutoff_freq;
    float fr = sample_freq/_cutoff_freq;
    float ohm = tanf(M_PI_F/fr);
    float c = 1.0f+2.0f*cosf(M_PI_F/4.0f)*ohm + ohm*ohm;
    _b0 = ohm*ohm/c;
    _b1 = 2.0f*_b0;
    _b2 = _b0;
    _a1 = 2.0f*(ohm*ohm-1.0f)/c;
    _a2 = (1.0f-2.0f*cosf(M_PI_F/4.0f)*ohm+ohm*ohm)/c;
}

float LowPassFilter2p::apply(float sample)
{
    // do the filtering
    float delay_element_0 = sample - _delay_element_1 * _a1 - _delay_element_2 * _a2;
    if (isnan(delay_element_0) || isinf(delay_element_0)) {
        // don't allow bad values to propogate via the filter
        delay_element_0 = sample;
    }
    float output = delay_element_0 * _b0 + _delay_element_1 * _b1 + _delay_element_2 * _b2;
    
    _delay_element_2 = _delay_element_1;
    _delay_element_1 = delay_element_0;

    // return the value.  Should be no need to check limits
    return output;
}

} // namespace math