IIR filter sample implementation

iir.c

#define P 3
#define Q 3

const float feedforward_coeffs[P] = { 0.0316, -0.06319, 0.0316 };
const float feedback_coeffs[Q] = { 0.99835, -1.99835, 1.0 };

float feedforward_buf[P] = { 0.0 };
float feedback_buf[Q] = { 0.0 };

float pass(float inp) {
    float out = 0.0;

    // shift value
    for (int i = P-1; i > 0; i--) {
        feedforward_buf[i-1] = feedforward_buf[i];
    }

    for (int i = Q-1; i > 0; --i) {
        feedback_buf[i-1] = feedback_buf[i];
    }

    // add new input value
    feedforward_buf[0] = inp;

    // feedfoward
    for (int i = 0; i < P; ++i) {
        out += feedforward_buf[i] * feedforward_coeffs[i];
    }

    // feedback
    for (int i = 1; i < Q; ++i) {
        out -= feedback_buf[i] * feedback_coeffs[i];
    }

    // add new output value
    feedback_buf[0] = out;

    return out;
}

int main() {
    for (;;) {
        // Read without from ADC
        float inp = adc_read_value();

        // Filter
        float out = pass(inp);

        // out is filtered with coeffs stored in feedforward_coeffs and
        // feedback_coeffs

        // Do something with out ...
    }
}