Fir1fixed.h

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/* (C) 2013-2020 Graeme Hattan & Bernd Porr */

#ifndef FIR1FIXED_H
#define FIR1FIXED_H

#include <stdio.h>
#include <cmath>
#include <complex>

/**
 * Fixed point FIR filter with short int coefficients and data processed as int.
 **/
class Fir1fixed {
public:
        /**
         * Coefficients as an array of short ints (and length provided separately)
         * \param coefficients Array of the coefficients scaled up by 2^bitshift
         * \param bitshift Scaling factor of the coefficients as power of 2: 2^bitshift
         * \param number_of_taps Number of taps (= number of coefficients)
         **/
	Fir1fixed(short int *coefficients, 
		  unsigned bitshift,
		  unsigned number_of_taps);

	/**
         * Coefficients as an const array of short ints (and length automatically detected)
         * \param _coefficients Array of the coefficients (const short) scaled up by 2^bitshift
         * \param bitshift Scaling factor of the coefficients as power of 2: 2^bitshift
         **/
        template <unsigned nTaps> Fir1fixed(const short int (&_coefficients)[nTaps],
					    unsigned bitshift) :
		coefficients(new short int[nTaps]),
		buffer(new int[nTaps]),  
		taps(nTaps),
		numberOfBitsToShift(bitshift) {
			for(unsigned i = 0; i < nTaps; i++) {
				coefficients[i] = _coefficients[i];
				buffer[i] = 0;
			}
		}

	/**
	 * Coefficients as a text file (for example from MATLAB).
	 * The number of taps is automatically detected
	 * when the taps are kept zero.
         * \param coeffFile File containing the coefficients
         * \param bitshift Scaling factor of the coefficients as power of 2: 2^bitshift
         * \param number_of_taps Number of taps (= number of coefficients)
         **/
	Fir1fixed(const char* coeffFile,
		  unsigned  bitshift,
		  unsigned number_of_taps = 0);

	/**
	 * The actual filtering operation. Takes one sample and returns
	 * the filtred one with the same type. The input type is usually
	 * short int but can be also int or char if special care it taken against 
	 * overflows.
	 * \param input is the input sample of any integer type (int,short,char)
	 **/
	template <typename Sample> inline Sample filter(Sample input) {
		short int *coeff     = coefficients;
		short int *coeff_end = coefficients + taps;
		
		int *buf_val = buffer + offset;
		
		*buf_val = static_cast<Sample>(input);
		int output_ = 0;
		
		while(buf_val >= buffer)
			output_ += *buf_val-- * *coeff++;
		
		buf_val = buffer + taps-1;
		
		while(coeff < coeff_end)
			output_ += *buf_val-- * *coeff++;
		
		if(++offset >= taps)
			offset = 0;
		
		return static_cast<Sample>(output_ >> numberOfBitsToShift);
	}

	
	/**
         * Frees the buffer and coefficients
         **/
	~Fir1fixed();

	/** 
         * Resets the buffer
         **/
	void reset();

	/**
         * Returns the number of taps
         **/
	unsigned getTaps() {return taps;};
	
private:
	short int        *coefficients;
	int              *buffer;
	unsigned         taps;
	unsigned         offset = 0;
	unsigned         numberOfBitsToShift;
};

#endif