Implement PID thremostat controller

For now only the proportional (Kp) and derivative (Kd) components are
used, the integral term is 0.

barista/test/test_thermostat.c

@@ -0,0 +1,49 @@
+/* Copyright (c) 2025 Rodrigo Arias Mallo <rodarima@gmail.com>
+ * SPDX-License-Identifier: GPL-3.0-or-later */
+
+#include <stdio.h>
+#include "overheat.h"
+#include "thermostat.h"
+#include "heater.h"
+
+/* Read a CSV from the stdin in the format
+ * <time_in_seconds> <state> <temp_in_C>
+ * skipping the first row (header).
+ *
+ * Outputs overheat state. */
+
+#define MAX_LINE 1024
+
+int main(void)
+{
+	char buf[MAX_LINE];
+	fgets(buf, MAX_LINE, stdin);
+
+	float t, temp;
+	int st;
+
+	struct overheat ovh = { 0 };
+	struct thermostat th = { 0 };
+	struct heater heater = { 0 };
+
+	overheat_init(&ovh);
+
+	for (int i = 0; scanf("%f %d %f", &t, &st, &temp) == 3; i++) {
+		if (i == 0)
+			thermostat_set(&th, 60.0);
+
+		unsigned long t_ms = t * 1000;
+
+		overheat_input(&ovh, t_ms, temp);
+		float delta = overheat_delta(&ovh);
+		float speed = overheat_speed(&ovh);
+		float u = thermostat_state(&th, temp, speed);
+		heater_on(&heater, t_ms, u);
+		int h = heater_state(&heater, t_ms);
+		int panic = overheat_panic(&ovh);
+
+		printf("%8.3f %2d %6.2f %6.1f %8.3f %8.3f %d %s\n", t, st, temp, delta, speed, u, h, panic ? "PANIC" : "OK");
+	}
+
+	return 0;
+}