Dead simple AGC.

agc.c

typedef struct {
    float attack_rate;
    float decay_rate;
    float reference;
    float max_gain;
    float gain;
} agc_t;

void agc_init(agc_t *agc, float attack_rate, float decay_rate, float reference) {
    agc->attack_rate = attack_rate;
    agc->decay_rate = decay_rate;
    agc->gain = 1.;
    agc->max_gain = 65000.;
    agc->reference = reference;
}

void agc_execute(agc_t *agc, const float complex *input, float complex *output, int n) {
    float pow;
    float diff;
    for (int i = 0; i < n; i++) {
        output[i] = agc->gain * input[i]; // apply gain
        pow = crealf(output[i] * conjf(output[i])); // is always real
        diff = agc->reference - pow;
        // adjust gain
        agc->gain += diff * (diff > 0 ? agc->decay_rate : agc->attack_rate);
        // clamp to max gain
        agc->gain = agc->gain > agc->max_gain ? agc->max_gain : agc->gain;
    }
}