Dynamically maintaining Home Assistant MQTT topics for a presence sensor from an ESP32

main.cpp

#include <Arduino.h>
#include <vector>
#include <WiFi.h>
#include <PubSubClient.h>
#include <ArduinoJson.h>

#include "led.h"
#include "secrets.h"

const char *DEVICE_ID = "esp32_presence";
const char *DEVICE_NAME = "ESP32 Presence";

#define MAX_ZONES 3
#define NAME_LEN 16 // "Zone123" fits
#define ID_LEN 12	// "zone123" fits

WiFiClient wifiClient;
PubSubClient mqtt(wifiClient);

struct Zone
{
	bool active;
	char id[ID_LEN];
	char name[NAME_LEN];
	bool state;
};

Zone zones[MAX_ZONES] = {}; // all zero-initialised
uint8_t activeCnt = 0;
uint16_t nameCounter = 0;

void connectWiFi()
{
	WiFi.mode(WIFI_STA);
	WiFi.begin(WIFI_SSID, WIFI_PASS);
	while (WiFi.status() != WL_CONNECTED)
		delay(250);
}

void connectMQTT()
{
	mqtt.setServer(MQTT_HOST, MQTT_PORT);
	mqtt.setBufferSize(1024);
	while (!mqtt.connected())
		mqtt.connect(DEVICE_ID, MQTT_USER, MQTT_PASS, "homeassistant/status", 0, true, "offline");
}

void topicConfig(char *out, size_t len, const Zone &z)
{
	snprintf(out, len, "homeassistant/binary_sensor/%s_%s/config", DEVICE_ID, z.id);
}

void topicState(char *out, size_t len, const Zone &z)
{
	snprintf(out, len, "esp32/%s/%s", DEVICE_ID, z.id);
}

void publishDiscovery(const Zone &z)
{
	JsonDocument doc; // ← heap-backed, auto-grows

	doc["name"] = z.name;
	doc["unique_id"] = String(DEVICE_ID) + "_" + z.id;

	char st[64];
	snprintf(st, sizeof(st), "esp32/%s/%s", DEVICE_ID, z.id);
	doc["state_topic"] = st;

	doc["device_class"] = "occupancy";
	doc["payload_on"] = "ON";
	doc["payload_off"] = "OFF";

	JsonObject dev = doc["device"].to<JsonObject>();
	dev["name"] = DEVICE_NAME;

	JsonArray ids = dev["identifiers"].to<JsonArray>();
	ids.add(DEVICE_ID);

	char topic[128];
	snprintf(topic, sizeof(topic), "homeassistant/binary_sensor/%s_%s/config", DEVICE_ID, z.id);

	size_t len = measureJson(doc); // you can reserve() if you like
	uint8_t *buf = new uint8_t[len + 1];
	serializeJson(doc, buf, len + 1);

	mqtt.publish(topic, buf, len, true);
	delete[] buf;
}

void unpublishDiscovery(const Zone &z)
{
	char topic[128], st[64];
	topicConfig(topic, sizeof(topic), z);
	topicState(st, sizeof(st), z);

	mqtt.publish(topic, nullptr, 0, true); // delete retained config
	mqtt.publish(st, nullptr, 0, true);	   // delete last state
}

void publishState(const Zone &z)
{
	char st[64];
	topicState(st, sizeof(st), z);
	mqtt.publish(st, z.state ? "ON" : "OFF", true);
}

Zone *randomActiveZone()
{
	if (!activeCnt) return nullptr;
	uint8_t pick = random(activeCnt);
	for (uint8_t i = 0; i < MAX_ZONES; ++i)
		if (zones[i].active)
			if (!pick--) return &zones[i];
	return nullptr; // should never hit
}

void addZone()
{
	if (activeCnt >= MAX_ZONES) return;
	for (Zone &z : zones)
		if (!z.active)
		{
			z.active = true;
			++nameCounter;
			snprintf(z.id, ID_LEN, "zone%u", nameCounter);
			snprintf(z.name, NAME_LEN, "Zone%u", nameCounter);
			z.state = false;
			++activeCnt;
			publishDiscovery(z);
			publishState(z);
			Serial.printf("[Add] %s='%s' created\n", z.id, z.name);
			setLED(0, 8, 0);
			return;
		}
}

void removeZone()
{
	Zone *z = randomActiveZone();
	if (!z) return;
	unpublishDiscovery(*z);
	z->active = false;
	--activeCnt;
	Serial.printf("[Remove] %s='%s' removed\n", z->id, z->name);
	setLED(8, 0, 0);
}

void renameZone()
{
	Zone *z = randomActiveZone();
	if (!z) return;
	unpublishDiscovery(*z);
	++nameCounter;
	snprintf(z->name, NAME_LEN, "Zone%u", nameCounter);
	publishDiscovery(*z);
	publishState(*z);
	Serial.printf("[Rename] %s='%s'\n", z->id, z->name);
	setLED(0, 0, 8);
}

void toggleZone()
{
	Zone *z = randomActiveZone();
	if (!z) return;
	z->state = !z->state;
	publishState(*z);
	Serial.printf("[Toggle] %s='%s' ➜ %s\n", z->id, z->name, z->state ? "ON" : "OFF");
	setLED(8, 0, 8);
}

void randomAction()
{
	bool canAdd = activeCnt < MAX_ZONES;
	bool canChange = activeCnt > 0;
	bool canRemove = activeCnt > 1;

	uint8_t choices[4];
	uint8_t n = 0;
	if (canAdd)
		choices[n++] = 0; // add
	if (canRemove)
		choices[n++] = 1; // remove
	if (canChange)
	{
		choices[n++] = 2; // rename
		choices[n++] = 3; // toggle
	}
	if (!n) return;

	switch (choices[random(n)])
	{
	case 0:
		addZone();
		break;
	case 1:
		removeZone();
		break;
	case 2:
		renameZone();
		break;
	case 3:
		toggleZone();
		break;
	}
}

void setup()
{
	Serial.begin(115200);
	setLED(0, 0, 0);
	delay(1000);
	randomSeed(esp_random());

	setLED(0, 8, 0);
	connectWiFi();

	setLED(8, 0, 8);
	connectMQTT();
}

unsigned long lastTick = 0;
void loop()
{
	if (!mqtt.connected()) connectMQTT();
	mqtt.loop();

	if (millis() - lastTick > PUBLISH_INTERVAL)
	{
		randomAction();
		lastTick = millis();
	}
}

Also needs this LED.h file:

#include <Adafruit_NeoPixel.h>

Adafruit_NeoPixel neo_pixels = Adafruit_NeoPixel(1, 12, NEO_GRB + NEO_KHZ800);

void initLED()
{
	neo_pixels.clear();
	neo_pixels.begin();
}

void setLED()
{
	neo_pixels.setPixelColor(0, 0);
	neo_pixels.show();
}

void setLED(uint8_t r, uint8_t g, uint8_t b)
{
	neo_pixels.setPixelColor(0, neo_pixels.Color(r, g, b));
	neo_pixels.show();
}

And for an M5Stack Atom3, this platformio.ini:

[env:atom-s3]
platform = espressif32
board = m5stack-atoms3
framework = arduino
monitor_speed = 115200
build_flags =
	-DARDUINO_USB_CDC_ON_BOOT=1
	-DMQTT_MAX_PACKET_SIZE=1024
lib_deps =
	m5stack/M5Unified@^0.1.17
	knolleary/PubSubClient@^2.8
	bblanchon/ArduinoJson@^7.4.1
	adafruit/Adafruit NeoPixel@^1.12.4

Not to forget this secrets.h file template:

#pragma once
// ---------- Wi-Fi ----------
#define WIFI_SSID ""
#define WIFI_PASS ""

// ---------- MQTT ----------
#define MQTT_HOST "hass.local" // IP or mDNS name of broker
#define MQTT_PORT 1883
#define MQTT_USER ""
#define MQTT_PASS ""

// How often (ms) to publish simulated states
#define PUBLISH_INTERVAL 8000