/* Copyright (c) 2025 Rodrigo Arias Mallo <rodarima@gmail.com>
 * SPDX-License-Identifier: GPL-3.0-or-later */

#include "thermostat.h"

#define TEMP_MIN	35.0 /* °C */
#define DELTA_LOW	1.0
#define DELTA_HIGH	1.0

#define T_ERR_MIN	35.0 /* °C */

void
thermostat_set(struct thermostat *th, float temp_target)
{
	if (th->st == THERMOSTAT_ON && th->temp_target == temp_target)
		return;

	th->st = THERMOSTAT_ON;
	th->temp_target = temp_target;
	th->temp_min = temp_target - DELTA_LOW;
	th->temp_max = temp_target + DELTA_HIGH;
	th->on = 1;
}

void
thermostat_off(struct thermostat *th)
{
	th->st = THERMOSTAT_OFF;
	th->on = 0;
}

static float
pid(float T0, float T, float dT_dt)
{
	float err_min = 2.0;

	/* The rate of change of error is the same as the temperature, as they
	 * are only offset by a mostly constant value */
	float derr_dt = dT_dt;

	if ((T0 - T) < err_min)
		return 0.0;

	float Kp = 1.0 / 20.0;
	float Kd = - 1.0 / 3.0;
	float u = Kp * (T0 - T) + Kd * dT_dt;

	if (u < 0.0)
		u = 0.0;
	else if (u > 1.0)
		u = 1.0;

	return u;
}

/* Return a value in [0, 1] to set the heater duty cycle */
float
thermostat_state(struct thermostat *th, float T, float dT_dt)
{
	if (th->st == THERMOSTAT_OFF)
		return 0.0;

	return pid(th->temp_target, T, dT_dt);
}