FIRFilter.hpp

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#pragma once
 
#include <AH/Containers/Array.hpp>
 
 
template <uint8_t N, class T = float>
class FIRFilter {
  public:
    FIRFilter(const AH::Array<T, N> &coefficients) {
        for (uint8_t i = 0; i < 2 * N - 1; ++i)
            this->coefficients[i] = coefficients[(2 * N - 1 - i) % N];
    }
 
    T operator()(T input) {
        // Save the new value to the ring buffer.
        x[index_b] = input;
 
        // Calculate the offset to the shifted coefficients.
        T *coeff_shift = coefficients.end() - N - index_b;
 
        // Multiply and accumulate the inputs and their respective coefficients.
        T acc = {};
        for (uint8_t i = 0; i < N; i++)
            acc += x[i] * coeff_shift[i];
 
        // Increment and wrap around the index of the ring buffer.
        index_b++;
        if (index_b == N)
            index_b = 0;
 
        return acc;
    }
 
  private:
    uint8_t index_b = 0;
    AH::Array<T, N> x = {};
    AH::Array<T, 2 * N - 1> coefficients;
};