Add mcutie MQTT client implementation and improve library structure

- Integrated `mcutie` library as a core MQTT client for device communication.
- Added support for Home Assistant entities (binary sensor, button) via MQTT.
- Implemented buffer management, async operations, and packet encoding/decoding.
- Introduced structured error handling and device registration features.
- Updated `Cargo.toml` with new dependencies and enabled feature flags for `serde` and `log`.
- Enhanced logging macros with configurable options (`defmt` or `log`).
- Organized codebase into modules (buffer, components, IO, publish, etc.) for better maintainability.

Software/MainBoard/rust/src/mcutie_3_0_0/homeassistant/binary_sensor.rs

@@ -0,0 +1,120 @@
+//! Tools for publishing a [Home Assistant binary sensor](https://www.home-assistant.io/integrations/binary_sensor.mqtt/).
+use core::ops::Deref;
+
+use serde::{Deserialize, Serialize};
+
+use crate::{homeassistant::Component, Error, Publishable, Topic};
+
+/// The state of the sensor. Can be easily converted to or from a [`bool`].
+#[derive(Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(from = "&str", into = "&'static str")]
+#[allow(missing_docs)]
+pub enum BinarySensorState {
+    On,
+    Off,
+}
+
+impl From<BinarySensorState> for &'static str {
+    fn from(state: BinarySensorState) -> Self {
+        match state {
+            BinarySensorState::On => "ON",
+            BinarySensorState::Off => "OFF",
+        }
+    }
+}
+
+impl<'a> From<&'a str> for BinarySensorState {
+    fn from(st: &'a str) -> Self {
+        if st == "ON" {
+            Self::On
+        } else {
+            Self::Off
+        }
+    }
+}
+
+impl From<bool> for BinarySensorState {
+    fn from(val: bool) -> Self {
+        if val {
+            BinarySensorState::On
+        } else {
+            BinarySensorState::Off
+        }
+    }
+}
+
+impl From<BinarySensorState> for bool {
+    fn from(val: BinarySensorState) -> Self {
+        match val {
+            BinarySensorState::On => true,
+            BinarySensorState::Off => true,
+        }
+    }
+}
+
+impl AsRef<[u8]> for BinarySensorState {
+    fn as_ref(&self) -> &'static [u8] {
+        match self {
+            Self::On => "ON".as_bytes(),
+            Self::Off => "OFF".as_bytes(),
+        }
+    }
+}
+
+/// The type of sensor.
+#[derive(Serialize)]
+#[serde(rename_all = "snake_case")]
+#[allow(missing_docs)]
+pub enum BinarySensorClass {
+    Battery,
+    BatteryCharging,
+    CarbonMonoxide,
+    Cold,
+    Connectivity,
+    Door,
+    GarageDoor,
+    Gas,
+    Heat,
+    Light,
+    Lock,
+    Moisture,
+    Motion,
+    Moving,
+    Occupancy,
+    Opening,
+    Plug,
+    Power,
+    Presence,
+    Problem,
+    Running,
+    Safety,
+    Smoke,
+    Sound,
+    Tamper,
+    Update,
+    Vibration,
+    Window,
+}
+
+/// A binary sensor that can publish a [`BinarySensorState`] status.
+#[derive(Serialize)]
+pub struct BinarySensor {
+    /// The type of sensor
+    pub device_class: Option<BinarySensorClass>,
+}
+
+impl Component for BinarySensor {
+    type State = BinarySensorState;
+
+    fn platform() -> &'static str {
+        "binary_sensor"
+    }
+
+    async fn publish_state<T: Deref<Target = str>>(
+        &self,
+        topic: &Topic<T>,
+        state: Self::State,
+    ) -> Result<(), Error> {
+        topic.with_bytes(state).publish().await
+    }
+}

Software/MainBoard/rust/src/mcutie_3_0_0/homeassistant/button.rs

@@ -0,0 +1,40 @@
+//! Tools for publishing a [Home Assistant button](https://www.home-assistant.io/integrations/button.mqtt/).
+use core::ops::Deref;
+
+use serde::Serialize;
+
+use crate::{homeassistant::Component, Error, Topic};
+
+/// The type of button.
+#[derive(Serialize)]
+#[serde(rename_all = "snake_case")]
+#[allow(missing_docs)]
+pub enum ButtonClass {
+    Identify,
+    Restart,
+    Update,
+}
+
+/// A button that can be pressed.
+#[derive(Serialize)]
+pub struct Button {
+    /// The type of button.
+    pub device_class: Option<ButtonClass>,
+}
+
+impl Component for Button {
+    type State = ();
+
+    fn platform() -> &'static str {
+        "button"
+    }
+
+    async fn publish_state<T: Deref<Target = str>>(
+        &self,
+        _topic: &Topic<T>,
+        _state: Self::State,
+    ) -> Result<(), Error> {
+        // Buttons don't have a state
+        Err(Error::Invalid)
+    }
+}

Software/MainBoard/rust/src/mcutie_3_0_0/homeassistant/light.rs

@@ -0,0 +1,384 @@
+//! Tools for publishing a [Home Assistant light](https://www.home-assistant.io/integrations/light.mqtt/).
+use core::{ops::Deref, str};
+
+use serde::{ser::SerializeStruct, Deserialize, Serialize, Serializer};
+
+use crate::{
+    fmt::Debug2Format,
+    homeassistant::{binary_sensor::BinarySensorState, ser::List, Component},
+    Error, Payload, Publishable, Topic,
+};
+
+#[derive(Serialize)]
+#[serde(rename_all = "lowercase")]
+#[allow(missing_docs)]
+pub enum SupportedColorMode {
+    OnOff,
+    Brightness,
+    #[serde(rename = "color_temp")]
+    ColorTemp,
+    Hs,
+    Xy,
+    Rgb,
+    Rgbw,
+    Rgbww,
+    White,
+}
+
+#[derive(Serialize, Deserialize, Default)]
+struct SerializedColor {
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    h: Option<f32>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    s: Option<f32>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    x: Option<f32>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    y: Option<f32>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    r: Option<u8>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    g: Option<u8>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    b: Option<u8>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    w: Option<u8>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    c: Option<u8>,
+}
+
+#[derive(Deserialize)]
+struct LedPayload<'a> {
+    state: BinarySensorState,
+    #[serde(default)]
+    brightness: Option<u8>,
+    #[serde(default)]
+    color_temp: Option<u32>,
+    #[serde(default)]
+    color: Option<SerializedColor>,
+    #[serde(default)]
+    effect: Option<&'a str>,
+}
+
+/// The color of the light in various forms.
+#[derive(Serialize)]
+#[serde(rename_all = "lowercase", tag = "color_mode", content = "color")]
+#[allow(missing_docs)]
+pub enum Color {
+    None,
+    Brightness(u8),
+    ColorTemp(u32),
+    Hs {
+        #[serde(rename = "h")]
+        hue: f32,
+        #[serde(rename = "s")]
+        saturation: f32,
+    },
+    Xy {
+        x: f32,
+        y: f32,
+    },
+    Rgb {
+        #[serde(rename = "r")]
+        red: u8,
+        #[serde(rename = "g")]
+        green: u8,
+        #[serde(rename = "b")]
+        blue: u8,
+    },
+    Rgbw {
+        #[serde(rename = "r")]
+        red: u8,
+        #[serde(rename = "g")]
+        green: u8,
+        #[serde(rename = "b")]
+        blue: u8,
+        #[serde(rename = "w")]
+        white: u8,
+    },
+    Rgbww {
+        #[serde(rename = "r")]
+        red: u8,
+        #[serde(rename = "g")]
+        green: u8,
+        #[serde(rename = "b")]
+        blue: u8,
+        #[serde(rename = "c")]
+        cool_white: u8,
+        #[serde(rename = "w")]
+        warm_white: u8,
+    },
+}
+
+/// The state of the light. This can be sent to the broker and received as a
+/// command from Home Assistant.
+pub struct LightState<'a> {
+    /// Whether the light is on or off.
+    pub state: BinarySensorState,
+    /// The color of the light.
+    pub color: Color,
+    /// Any effect that is applied.
+    pub effect: Option<&'a str>,
+}
+
+impl<'a> LightState<'a> {
+    /// Parses the state from a command payload.
+    pub fn from_payload(payload: &'a Payload) -> Result<Self, Error> {
+        let parsed: LedPayload<'a> = match payload.deserialize_json() {
+            Ok(p) => p,
+            Err(e) => {
+                warn!("Failed to deserialize packet: {:?}", Debug2Format(&e));
+                if let Ok(s) = str::from_utf8(payload) {
+                    trace!("{}", s);
+                }
+                return Err(Error::PacketError);
+            }
+        };
+
+        let color = if let Some(color) = parsed.color {
+            if let Some(x) = color.x {
+                Color::Xy {
+                    x,
+                    y: color.y.unwrap_or_default(),
+                }
+            } else if let Some(h) = color.h {
+                Color::Hs {
+                    hue: h,
+                    saturation: color.s.unwrap_or_default(),
+                }
+            } else if let Some(c) = color.c {
+                Color::Rgbww {
+                    red: color.r.unwrap_or_default(),
+                    green: color.g.unwrap_or_default(),
+                    blue: color.b.unwrap_or_default(),
+                    cool_white: c,
+                    warm_white: color.w.unwrap_or_default(),
+                }
+            } else if let Some(w) = color.w {
+                Color::Rgbw {
+                    red: color.r.unwrap_or_default(),
+                    green: color.g.unwrap_or_default(),
+                    blue: color.b.unwrap_or_default(),
+                    white: w,
+                }
+            } else {
+                Color::Rgb {
+                    red: color.r.unwrap_or_default(),
+                    green: color.g.unwrap_or_default(),
+                    blue: color.b.unwrap_or_default(),
+                }
+            }
+        } else if let Some(color_temp) = parsed.color_temp {
+            Color::ColorTemp(color_temp)
+        } else if let Some(brightness) = parsed.brightness {
+            Color::Brightness(brightness)
+        } else {
+            Color::None
+        };
+
+        Ok(LightState {
+            state: parsed.state,
+            color,
+            effect: parsed.effect,
+        })
+    }
+}
+
+impl Serialize for LightState<'_> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let mut len = 1;
+
+        if self.effect.is_some() {
+            len += 1;
+        }
+
+        match self.color {
+            Color::None => {}
+            Color::Brightness(_) | Color::ColorTemp(_) => len += 1,
+            _ => len += 2,
+        }
+
+        let mut serializer = serializer.serialize_struct("LightState", len)?;
+
+        serializer.serialize_field("state", &self.state)?;
+
+        if let Some(effect) = self.effect {
+            serializer.serialize_field("effect", effect)?;
+        } else {
+            serializer.skip_field("effect")?;
+        }
+
+        match self.color {
+            Color::None => {
+                serializer.skip_field("brightness")?;
+                serializer.skip_field("color_temp")?;
+                serializer.skip_field("color")?;
+            }
+            Color::Brightness(b) => {
+                serializer.skip_field("color_temp")?;
+                serializer.skip_field("color")?;
+
+                serializer.serialize_field("brightness", &b)?
+            }
+            Color::ColorTemp(c) => {
+                serializer.skip_field("brightness")?;
+                serializer.skip_field("color")?;
+
+                serializer.serialize_field("color_temp", &c)?
+            }
+            Color::Hs { hue, saturation } => {
+                serializer.skip_field("brightness")?;
+                serializer.skip_field("color_temp")?;
+
+                serializer.serialize_field("color_mode", "hs")?;
+
+                let color = SerializedColor {
+                    h: Some(hue),
+                    s: Some(saturation),
+                    ..Default::default()
+                };
+
+                serializer.serialize_field("color", &color)?
+            }
+            Color::Xy { x, y } => {
+                serializer.skip_field("brightness")?;
+                serializer.skip_field("color_temp")?;
+
+                serializer.serialize_field("color_mode", "xy")?;
+
+                let color = SerializedColor {
+                    x: Some(x),
+                    y: Some(y),
+                    ..Default::default()
+                };
+
+                serializer.serialize_field("color", &color)?
+            }
+            Color::Rgb { red, green, blue } => {
+                serializer.skip_field("brightness")?;
+                serializer.skip_field("color_temp")?;
+
+                serializer.serialize_field("color_mode", "rgb")?;
+
+                let color = SerializedColor {
+                    r: Some(red),
+                    g: Some(green),
+                    b: Some(blue),
+                    ..Default::default()
+                };
+
+                serializer.serialize_field("color", &color)?
+            }
+            Color::Rgbw {
+                red,
+                green,
+                blue,
+                white,
+            } => {
+                serializer.skip_field("brightness")?;
+                serializer.skip_field("color_temp")?;
+
+                serializer.serialize_field("color_mode", "rgbw")?;
+
+                let color = SerializedColor {
+                    r: Some(red),
+                    g: Some(green),
+                    b: Some(blue),
+                    w: Some(white),
+                    ..Default::default()
+                };
+
+                serializer.serialize_field("color", &color)?
+            }
+            Color::Rgbww {
+                red,
+                green,
+                blue,
+                cool_white,
+                warm_white,
+            } => {
+                serializer.skip_field("brightness")?;
+                serializer.skip_field("color_temp")?;
+
+                serializer.serialize_field("color_mode", "rgbww")?;
+
+                let color = SerializedColor {
+                    r: Some(red),
+                    g: Some(green),
+                    b: Some(blue),
+                    c: Some(cool_white),
+                    w: Some(warm_white),
+                    ..Default::default()
+                };
+
+                serializer.serialize_field("color", &color)?
+            }
+        }
+
+        serializer.end()
+    }
+}
+
+/// A light entity
+pub struct Light<'a, const C: usize, const E: usize> {
+    /// The color modes supported by the light.
+    pub supported_color_modes: [SupportedColorMode; C],
+    /// Any effects that can be used.
+    pub effects: [&'a str; E],
+}
+
+impl<const C: usize, const E: usize> Serialize for Light<'_, C, E> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let mut len = 2;
+
+        if C > 0 {
+            len += 1;
+        }
+
+        if E > 0 {
+            len += 2;
+        }
+
+        let mut serializer = serializer.serialize_struct("Light", len)?;
+
+        serializer.serialize_field("schema", "json")?;
+
+        if C > 0 {
+            serializer.serialize_field("sup_clrm", &List::new(&self.supported_color_modes))?;
+        } else {
+            serializer.skip_field("sup_clrm")?;
+        }
+
+        if E > 0 {
+            serializer.serialize_field("effect", &true)?;
+            serializer.serialize_field("fx_list", &List::new(&self.effects))?;
+        } else {
+            serializer.skip_field("effect")?;
+            serializer.skip_field("fx_list")?;
+        }
+
+        serializer.end()
+    }
+}
+
+impl<const C: usize, const E: usize> Component for Light<'_, C, E> {
+    type State = LightState<'static>;
+
+    fn platform() -> &'static str {
+        "light"
+    }
+
+    async fn publish_state<T: Deref<Target = str>>(
+        &self,
+        topic: &Topic<T>,
+        state: Self::State,
+    ) -> Result<(), Error> {
+        topic.with_json(state).publish().await
+    }
+}

Software/MainBoard/rust/src/mcutie_3_0_0/homeassistant/mod.rs

@@ -0,0 +1,295 @@
+//! Home Assistant auto-discovery and related messages.
+//!
+//! Normally you would declare your entities statically in your binary. It is
+//! then trivial to send out discovery messages or state changes.
+//!
+//! ```
+//! # use mcutie::{Publishable, Topic};
+//! # use mcutie::homeassistant::{Entity, Device, Origin, AvailabilityState, AvailabilityTopics};
+//! # use mcutie::homeassistant::binary_sensor::{BinarySensor, BinarySensorClass, BinarySensorState};
+//! const DEVICE_AVAILABILITY_TOPIC: Topic<&'static str> = Topic::Device("status");
+//! const MOTION_STATE_TOPIC: Topic<&'static str> = Topic::Device("motion/status");
+//!
+//! const DEVICE: Device<'static> = Device::new();
+//! const ORIGIN: Origin<'static> = Origin::new();
+//!
+//! const MOTION_SENSOR: Entity<'static, 1, BinarySensor> = Entity {
+//!     device: DEVICE,
+//!     origin: ORIGIN,
+//!     object_id: "motion",
+//!     unique_id: Some("motion"),
+//!     name: "Motion",
+//!     availability: AvailabilityTopics::All([DEVICE_AVAILABILITY_TOPIC]),
+//!     state_topic: Some(MOTION_STATE_TOPIC),
+//!     command_topic: None,
+//!     component: BinarySensor {
+//!         device_class: Some(BinarySensorClass::Motion),
+//!     },
+//! };
+//!
+//! async fn send_discovery_messages() {
+//!     MOTION_SENSOR.publish_discovery().await.unwrap();
+//!     DEVICE_AVAILABILITY_TOPIC.with_bytes(AvailabilityState::Online).publish().await.unwrap();
+//! }
+//!
+//! async fn send_state(state: BinarySensorState) {
+//!     MOTION_SENSOR.publish_state(state).await.unwrap();
+//! }
+//! ```
+use core::{future::Future, ops::Deref};
+
+use mqttrs::QoS;
+use serde::{
+    ser::{Error as _, SerializeStruct},
+    Serialize, Serializer,
+};
+
+use crate::{
+    device_id, device_type, homeassistant::ser::DiscoverySerializer, io::publish, Error,
+    McutieTask, MqttMessage, Payload, Publishable, Topic, TopicString, DATA_CHANNEL,
+};
+
+pub mod binary_sensor;
+pub mod button;
+pub mod light;
+pub mod sensor;
+mod ser;
+
+const HA_STATUS_TOPIC: Topic<&'static str> = Topic::General("homeassistant/status");
+const STATE_ONLINE: &str = "online";
+const STATE_OFFLINE: &str = "offline";
+
+/// A trait representing a specific type of entity in Home Assistant
+pub trait Component: Serialize {
+    /// The state to publish.
+    type State;
+
+    /// The platform identifier for this entity. Internal.
+    fn platform() -> &'static str;
+
+    /// Publishes this entity's state to the MQTT broker.
+    fn publish_state<T: Deref<Target = str>>(
+        &self,
+        topic: &Topic<T>,
+        state: Self::State,
+    ) -> impl Future<Output = Result<(), Error>>;
+}
+
+impl<'t, T, L, const S: usize> McutieTask<'t, T, L, S>
+where
+    T: Deref<Target = str> + 't,
+    L: Publishable + 't,
+{
+    pub(super) async fn ha_after_connected(&self) {
+        let _ = HA_STATUS_TOPIC.subscribe(false).await;
+    }
+
+    pub(super) async fn ha_handle_update(
+        &self,
+        topic: &Topic<TopicString>,
+        payload: &Payload,
+    ) -> bool {
+        if topic == &HA_STATUS_TOPIC {
+            if payload.as_ref() == STATE_ONLINE.as_bytes() {
+                DATA_CHANNEL.send(MqttMessage::HomeAssistantOnline).await;
+            }
+
+            true
+        } else {
+            false
+        }
+    }
+}
+
+impl<T: Deref<Target = str>> Serialize for Topic<T> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        let mut topic = TopicString::new();
+        self.to_string(&mut topic)
+            .map_err(|_| S::Error::custom("topic was too large to serialize"))?;
+        serializer.serialize_str(&topic)
+    }
+}
+
+fn name_or_device<S>(name: &Option<&str>, serializer: S) -> Result<S::Ok, S::Error>
+where
+    S: Serializer,
+{
+    serializer.serialize_str(name.unwrap_or_else(|| device_type()))
+}
+
+/// Represents the device in Home Assistant.
+///
+/// Can just be the default in which case useful properties such as the ID are
+/// automatically included.
+#[derive(Clone, Copy, Default)]
+pub struct Device<'a> {
+    /// A name to identify the device. If not provided the default device type is
+    /// used.
+    pub name: Option<&'a str>,
+    /// An optional configuration URL for the device.
+    pub configuration_url: Option<&'a str>,
+}
+
+impl Device<'_> {
+    /// Creates a new default device.
+    pub const fn new() -> Self {
+        Self {
+            name: None,
+            configuration_url: None,
+        }
+    }
+}
+
+impl Serialize for Device<'_> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let mut len = 2;
+        if self.configuration_url.is_some() {
+            len += 1;
+        }
+
+        let mut serializer = serializer.serialize_struct("Device", len)?;
+
+        serializer.serialize_field("name", self.name.unwrap_or_else(|| device_type()))?;
+        serializer.serialize_field("ids", device_id())?;
+
+        if let Some(cu) = self.configuration_url {
+            serializer.serialize_field("cu", cu)?;
+        } else {
+            serializer.skip_field("cu")?;
+        }
+
+        serializer.end()
+    }
+}
+
+/// Represents the device's origin in Home Assistant.
+///
+/// Can just be the default in which case useful properties are automatically
+/// included.
+#[derive(Clone, Copy, Default, Serialize)]
+pub struct Origin<'a> {
+    /// A name to identify the device's origin. If not provided the default
+    /// device type is used.
+    #[serde(serialize_with = "name_or_device")]
+    pub name: Option<&'a str>,
+}
+
+impl Origin<'_> {
+    /// Creates a new default origin.
+    pub const fn new() -> Self {
+        Self { name: None }
+    }
+}
+
+/// A single entity for Home Assistant.
+///
+/// Calling [`Entity::publish_discovery`] will publish the discovery message to
+/// allow Home Assistant to detect this entity. Read the
+/// [Home Assistant MQTT docs](https://www.home-assistant.io/integrations/mqtt/)
+/// for information on what some of these properties mean.
+pub struct Entity<'a, const A: usize, C: Component> {
+    /// The device this entity is a part of.
+    pub device: Device<'a>,
+    /// The origin of the device.
+    pub origin: Origin<'a>,
+    /// An object identifier to allow for entity ID customisation in Home Assistant.
+    pub object_id: &'a str,
+    /// An optional unique identifier for the entity.
+    pub unique_id: Option<&'a str>,
+    /// A friendly name for the entity.
+    pub name: &'a str,
+    /// Specifies the availability topics that Home Assistant will listen to to
+    /// determine this entity's availability.
+    pub availability: AvailabilityTopics<'a, A>,
+    /// The state topic that this entity's state is published to.
+    pub state_topic: Option<Topic<&'a str>>,
+    /// The command topic that this entity receives commands from.
+    pub command_topic: Option<Topic<&'a str>>,
+    /// The specific entity.
+    pub component: C,
+}
+
+impl<const A: usize, C: Component> Entity<'_, A, C> {
+    /// Publishes the discovery message for this entity to the broker.
+    pub async fn publish_discovery(&self) -> Result<(), Error> {
+        let mut topic = TopicString::new();
+        topic
+            .push_str(option_env!("HA_DISCOVERY_PREFIX").unwrap_or("homeassistant"))
+            .map_err(|_| Error::TooLarge)?;
+        topic.push('/').map_err(|_| Error::TooLarge)?;
+        topic.push_str(C::platform()).map_err(|_| Error::TooLarge)?;
+        topic.push('/').map_err(|_| Error::TooLarge)?;
+        topic
+            .push_str(self.object_id)
+            .map_err(|_| Error::TooLarge)?;
+        topic.push_str("/config").map_err(|_| Error::TooLarge)?;
+
+        let mut payload = Payload::new();
+        payload.serialize_json(self).map_err(|_| Error::TooLarge)?;
+
+        publish(&topic, &payload, QoS::AtMostOnce, false).await
+    }
+
+    /// Publishes this entity's state to the broker.
+    ///
+    /// # Errors
+    ///
+    /// - [`Error::Invalid`] if the entity doesn't have a state topic.
+    pub async fn publish_state(&self, state: C::State) -> Result<(), Error> {
+        if let Some(topic) = self.state_topic {
+            self.component.publish_state(&topic, state).await
+        } else {
+            Err(Error::Invalid)
+        }
+    }
+}
+
+/// A payload representing a device or entity's availability.
+#[allow(missing_docs)]
+pub enum AvailabilityState {
+    Online,
+    Offline,
+}
+
+impl AsRef<[u8]> for AvailabilityState {
+    fn as_ref(&self) -> &'static [u8] {
+        match self {
+            Self::Online => STATE_ONLINE.as_bytes(),
+            Self::Offline => STATE_OFFLINE.as_bytes(),
+        }
+    }
+}
+
+/// The availiabity topics that home assistant will use to determine an entity's
+/// availability.
+pub enum AvailabilityTopics<'a, const A: usize> {
+    /// The entity is always available.
+    None,
+    /// The entity is available if all of the topics are publishes as online.
+    All([Topic<&'a str>; A]),
+    /// The entity is available if any of the topics are publishes as online.
+    Any([Topic<&'a str>; A]),
+    /// The entity is available based on the most recent of the topics to
+    /// publish state.
+    Latest([Topic<&'a str>; A]),
+}
+
+impl<const A: usize, C: Component> Serialize for Entity<'_, A, C> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let outer = DiscoverySerializer {
+            discovery: self,
+            inner: serializer,
+        };
+
+        self.component.serialize(outer)
+    }
+}

Software/MainBoard/rust/src/mcutie_3_0_0/homeassistant/sensor.rs

@@ -0,0 +1,103 @@
+//! Tools for publishing a [Home Assistant sensor](https://www.home-assistant.io/integrations/sensor.mqtt/).
+use core::ops::Deref;
+
+use serde::Serialize;
+
+use crate::{homeassistant::Component, Error, Publishable, Topic};
+
+/// The type of sensor.
+#[derive(Serialize)]
+#[serde(rename_all = "snake_case")]
+#[allow(missing_docs)]
+pub enum SensorClass {
+    ApparentPower,
+    Aqi,
+    AtmosphericPressure,
+    Battery,
+    CarbonDioxide,
+    CarbonMonoxide,
+    Current,
+    DataRate,
+    DataSize,
+    Date,
+    Distance,
+    Duration,
+    Energy,
+    EnergyStorage,
+    Enum,
+    Frequency,
+    Gas,
+    Humidity,
+    Illuminance,
+    Irradiance,
+    Moisture,
+    Monetary,
+    NitrogenDioxide,
+    NitrogenMonoxide,
+    NitrousOxide,
+    Ozone,
+    Ph,
+    Pm1,
+    Pm25,
+    Pm10,
+    PowerFactor,
+    Power,
+    Precipitation,
+    PrecipitationIntensity,
+    Pressure,
+    ReactivePower,
+    SignalStrength,
+    SoundPressure,
+    Speed,
+    SulphurDioxide,
+    Temperature,
+    Timestamp,
+    VolatileOrganicCompounds,
+    VolatileOrganicCompoundsParts,
+    Voltage,
+    Volume,
+    VolumeFlowRate,
+    VolumeStorage,
+    Water,
+    Weight,
+    WindSpeed,
+}
+
+/// The type of measurement that this entity publishes.
+#[derive(Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum SensorStateClass {
+    /// A measurement at a singe point in time.
+    Measurement,
+    /// A cumulative total that can increase or decrease over time.
+    Total,
+    /// A cumulative total that can only increase.
+    TotalIncreasing,
+}
+
+/// A binary sensor that can publish a [`f32`] value.
+#[derive(Serialize)]
+pub struct Sensor<'u> {
+    /// The type of sensor.
+    pub device_class: Option<SensorClass>,
+    /// The type of measurement that this sensor reports.
+    pub state_class: Option<SensorStateClass>,
+    /// The unit of measurement for this sensor.
+    pub unit_of_measurement: Option<&'u str>,
+}
+
+impl Component for Sensor<'_> {
+    type State = f32;
+
+    fn platform() -> &'static str {
+        "sensor"
+    }
+
+    async fn publish_state<T: Deref<Target = str>>(
+        &self,
+        topic: &Topic<T>,
+        state: Self::State,
+    ) -> Result<(), Error> {
+        topic.with_display(state).publish().await
+    }
+}

Software/MainBoard/rust/src/mcutie_3_0_0/homeassistant/ser.rs

@@ -0,0 +1,333 @@
+use core::ops::Deref;
+
+use serde::{
+    ser::{SerializeSeq, SerializeStruct},
+    Serialize, Serializer,
+};
+
+use crate::{
+    homeassistant::{AvailabilityTopics, Component, Entity},
+    Topic,
+};
+
+#[derive(Serialize)]
+pub(super) struct AvailabilityTopicItem<'a> {
+    topic: Topic<&'a str>,
+}
+
+struct AvailabilityTopicList<'a, T: Deref<Target = str>, const N: usize> {
+    list: &'a [Topic<T>; N],
+}
+
+impl<'a, const N: usize, T: Deref<Target = str>> AvailabilityTopicList<'a, T, N> {
+    pub(super) fn new(list: &'a [Topic<T>; N]) -> Self {
+        Self { list }
+    }
+}
+
+impl<T: Deref<Target = str>, const N: usize> Serialize for AvailabilityTopicList<'_, T, N> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let mut serializer = serializer.serialize_seq(Some(N))?;
+
+        for topic in self.list {
+            serializer.serialize_element(&AvailabilityTopicItem {
+                topic: topic.as_ref(),
+            })?;
+        }
+
+        serializer.end()
+    }
+}
+
+pub(super) struct List<'a, T: Serialize, const N: usize> {
+    list: &'a [T; N],
+}
+
+impl<'a, T: Serialize, const N: usize> List<'a, T, N> {
+    pub(super) fn new(list: &'a [T; N]) -> Self {
+        Self { list }
+    }
+}
+
+impl<T: Serialize, const N: usize> Serialize for List<'_, T, N> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let mut serializer = serializer.serialize_seq(Some(N))?;
+
+        for item in self.list {
+            serializer.serialize_element(item)?;
+        }
+
+        serializer.end()
+    }
+}
+
+pub(super) struct DiscoverySerializer<'a, const A: usize, C: Component, S: Serializer> {
+    pub(super) discovery: &'a Entity<'a, A, C>,
+    pub(super) inner: S,
+}
+
+impl<const A: usize, C: Component, S: Serializer> Serializer for DiscoverySerializer<'_, A, C, S> {
+    type Ok = S::Ok;
+    type Error = S::Error;
+    type SerializeSeq = S::SerializeSeq;
+    type SerializeTuple = S::SerializeTuple;
+    type SerializeTupleStruct = S::SerializeTupleStruct;
+    type SerializeTupleVariant = S::SerializeTupleVariant;
+    type SerializeMap = S::SerializeMap;
+    type SerializeStruct = S::SerializeStruct;
+    type SerializeStructVariant = S::SerializeStructVariant;
+
+    fn serialize_struct(
+        self,
+        name: &'static str,
+        mut len: usize,
+    ) -> Result<Self::SerializeStruct, Self::Error> {
+        len += 5;
+        if self.discovery.state_topic.is_some() {
+            len += 1;
+        }
+        if self.discovery.command_topic.is_some() {
+            len += 1;
+        }
+        if self.discovery.unique_id.is_some() {
+            len += 1;
+        }
+        if !matches!(self.discovery.availability, AvailabilityTopics::None) {
+            len += 2;
+        }
+
+        let mut serializer = self.inner.serialize_struct(name, len)?;
+
+        serializer.serialize_field("dev", &self.discovery.device)?;
+        serializer.serialize_field("o", &self.discovery.origin)?;
+        serializer.serialize_field("p", C::platform())?;
+        serializer.serialize_field("obj_id", self.discovery.object_id)?;
+
+        serializer.serialize_field("name", self.discovery.name)?;
+
+        if let Some(t) = self.discovery.state_topic {
+            serializer.serialize_field("stat_t", &t)?;
+        } else {
+            serializer.skip_field("stat_t")?;
+        }
+
+        if let Some(t) = self.discovery.command_topic {
+            serializer.serialize_field("cmd_t", &t)?;
+        } else {
+            serializer.skip_field("cmd_t")?;
+        }
+
+        match &self.discovery.availability {
+            AvailabilityTopics::None => {
+                serializer.skip_field("avty")?;
+                serializer.skip_field("avty_mode")?;
+            }
+            AvailabilityTopics::All(topics) => {
+                serializer.serialize_field("avty_mode", "all")?;
+                serializer.serialize_field("avty", &AvailabilityTopicList::new(topics))?;
+            }
+            AvailabilityTopics::Any(topics) => {
+                serializer.serialize_field("avty_mode", "any")?;
+                serializer.serialize_field("avty", &AvailabilityTopicList::new(topics))?;
+            }
+            AvailabilityTopics::Latest(topics) => {
+                serializer.serialize_field("avty_mode", "latest")?;
+                serializer.serialize_field("avty", &AvailabilityTopicList::new(topics))?;
+            }
+        }
+
+        if let Some(v) = self.discovery.unique_id {
+            serializer.serialize_field("uniq_id", v)?;
+        } else {
+            serializer.skip_field("uniq_id")?;
+        }
+
+        Ok(serializer)
+    }
+
+    fn serialize_bool(self, _: bool) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_i8(self, _: i8) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_i16(self, _: i16) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_i32(self, _: i32) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_i64(self, _: i64) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_u8(self, _: u8) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_u16(self, _: u16) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_u32(self, _: u32) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_u64(self, _: u64) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_f32(self, _: f32) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_f64(self, _: f64) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_char(self, _: char) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_str(self, _: &str) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_bytes(self, _: &[u8]) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_none(self) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_some<T>(self, _: &T) -> Result<Self::Ok, Self::Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        unimplemented!()
+    }
+
+    fn serialize_unit(self) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_unit_struct(self, _: &'static str) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_unit_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        _: &'static str,
+    ) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_newtype_struct<T>(self, _: &'static str, _: &T) -> Result<Self::Ok, Self::Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        unimplemented!()
+    }
+
+    fn serialize_newtype_variant<T>(
+        self,
+        _: &'static str,
+        _: u32,
+        _: &'static str,
+        _: &T,
+    ) -> Result<Self::Ok, Self::Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        unimplemented!()
+    }
+
+    fn serialize_seq(self, _: Option<usize>) -> Result<Self::SerializeSeq, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_tuple(self, _: usize) -> Result<Self::SerializeTuple, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_tuple_struct(
+        self,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeTupleStruct, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_tuple_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeTupleVariant, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_map(self, _: Option<usize>) -> Result<Self::SerializeMap, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_struct_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeStructVariant, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_i128(self, _: i128) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn serialize_u128(self, _: u128) -> Result<Self::Ok, Self::Error> {
+        unimplemented!()
+    }
+
+    fn collect_seq<I>(self, _: I) -> Result<Self::Ok, Self::Error>
+    where
+        I: IntoIterator,
+        <I as IntoIterator>::Item: Serialize,
+    {
+        unimplemented!()
+    }
+
+    fn collect_map<K, V, I>(self, _: I) -> Result<Self::Ok, Self::Error>
+    where
+        K: Serialize,
+        V: Serialize,
+        I: IntoIterator<Item = (K, V)>,
+    {
+        unimplemented!()
+    }
+
+    fn collect_str<T>(self, _: &T) -> Result<Self::Ok, Self::Error>
+    where
+        T: ?Sized + core::fmt::Display,
+    {
+        unimplemented!()
+    }
+
+    fn is_human_readable(&self) -> bool {
+        unimplemented!()
+    }
+}