#define SAMPLE_RATE 44100
#define PI 3.14159

double* low_pass_filter(double* data_ptr, uint32_t data_length, double cutoff_frequency, double resonance) {
    assert(data_ptr != NULL);
    assert(cutoff_frequency > 0 && cutoff_frequency < sample_rate / 2)
    assert(resonance < sqrt(2) && resonance > 0.1);

    double c = 1 / tan(PI * cutoff_frequency / SAMPLE_RATE);
    double a1 = 1 / (1 + resonance * c + c * c);
    double a2 = 2 * a1;
    double b1 = 2 * (1 - c * c) * a1;
    double b2 = (1 - resonance * c + c * c) * a1;

    double output[data_length];

    for(uint32_t i = 0; i < (data_length - 2); i++) {
        double in = *data_ptr;
        double in1 = 0.0;
        double in2 = 0.0;
        double out1 = 0.0;
        double out2 = 0.0;

        if(i > 0) {
            in1 = *(data_ptr - 1);
            out1 = output[i - 1];

            if(i > 1) {
                in2 = (data_ptr - 2);
                out2 = output[i - 2];
            }
        }

        output[i] = a1 * in + a2 * in1 + a3 * in2 - b1 * out1 - b2 * out2;
    }

    return output;
}