archy/core/archipelago/src/mesh/meshtastic.rs

//! Async serial driver for Meshtastic devices.
//!
//! Meshtastic uses protobuf payloads over a SLIP-like serial stream. This
//! module implements only the small subset Archipelago needs: connect,
//! discover the local node, send/receive text packets, and provide synthetic
//! contacts to the existing mesh listener.

use super::protocol::{InboundFrame, ParsedContact};
use super::types::{DeviceInfo, DeviceType};
use anyhow::{Context, Result};
use std::collections::HashMap;
use std::time::Duration;
use tracing::{debug, info, warn};

const BAUD_RATE: u32 = 115200;
const READ_TIMEOUT: Duration = Duration::from_secs(5);
const WRITE_TIMEOUT: Duration = Duration::from_secs(2);
const READ_BUF_SIZE: usize = 512;

const START1: u8 = 0x94;
const START2: u8 = 0xc3;
const TO_RADIO_MAX: usize = 512;
const BROADCAST_NUM: u32 = 0xffff_ffff;
const TEXT_MESSAGE_APP: u32 = 1;

const TO_RADIO_PACKET: u64 = 1;
const TO_RADIO_WANT_CONFIG_ID: u64 = 3;
const TO_RADIO_HEARTBEAT: u64 = 7;

const FROM_RADIO_PACKET: u64 = 2;
const FROM_RADIO_MY_INFO: u64 = 3;
const FROM_RADIO_NODE_INFO: u64 = 4;
const FROM_RADIO_CONFIG_COMPLETE_ID: u64 = 7;
const FROM_RADIO_REBOOTED: u64 = 8;

/// Async Meshtastic device handle.
pub struct MeshtasticDevice {
    port: serial2_tokio::SerialPort,
    read_buf: Vec<u8>,
    node_num: Option<u32>,
    user_id: Option<String>,
    long_name: Option<String>,
    short_name: Option<String>,
    contacts: HashMap<u32, ParsedContact>,
    /// Real Curve25519 public keys, keyed by node-num, as learned from NodeInfo
    /// (`User.public_key`) or PKC-encrypted inbound packets (`MeshPacket
    /// .public_key`). Kept SEPARATE from `contacts[*].public_key_hex`, which is
    /// the synthetic node-num-derived routing key that `send_text_msg` relies
    /// on — we must not overwrite that or unicast routing breaks. This map only
    /// records which peers are PKC-capable, so we can tell a true end-to-end
    /// (PKI) DM from a channel-PSK fallback.
    peer_pubkeys: HashMap<u32, Vec<u8>>,
    device_path: String,
}

impl MeshtasticDevice {
    pub async fn open(path: &str) -> Result<Self> {
        match tokio::fs::metadata(path).await {
            Ok(meta) => {
                debug!(path = %path, permissions = ?meta.permissions(), "Device node exists")
            }
            Err(e) => anyhow::bail!("Serial device {} not accessible: {}", path, e),
        }

        let port = serial2_tokio::SerialPort::open(path, BAUD_RATE).context(format!(
            "Failed to open serial port {} (permission denied? device busy?)",
            path
        ))?;
        info!(path = %path, baud = BAUD_RATE, "Opened Meshtastic serial port");

        Ok(Self {
            port,
            read_buf: Vec::with_capacity(READ_BUF_SIZE),
            node_num: None,
            user_id: None,
            long_name: None,
            short_name: None,
            contacts: HashMap::new(),
            peer_pubkeys: HashMap::new(),
            device_path: path.to_string(),
        })
    }

    pub async fn initialize(&mut self) -> Result<DeviceInfo> {
        info!(path = %self.device_path, "Starting Meshtastic handshake");
        self.send_to_radio(&encode_want_config()).await?;

        let deadline = tokio::time::Instant::now() + READ_TIMEOUT;
        let mut saw_meshtastic_frame = false;
        let mut saw_config_complete = false;

        loop {
            let remaining = deadline.saturating_duration_since(tokio::time::Instant::now());
            if remaining.is_zero() {
                break;
            }
            match tokio::time::timeout(
                remaining.min(Duration::from_millis(250)),
                self.read_from_radio(),
            )
            .await
            {
                Ok(Ok(Some(frame))) => {
                    saw_meshtastic_frame = true;
                    if matches!(
                        decode_top_level_variant(&frame),
                        Some((FROM_RADIO_CONFIG_COMPLETE_ID, _))
                    ) {
                        saw_config_complete = true;
                    }
                    self.handle_from_radio(&frame);
                    if saw_config_complete && self.node_num.is_some() {
                        break;
                    }
                }
                Ok(Ok(None)) | Err(_) => {}
                Ok(Err(e)) => return Err(e),
            }
        }

        if !saw_meshtastic_frame {
            anyhow::bail!("No Meshtastic serial API response");
        }

        let node_id = self
            .node_num
            .ok_or_else(|| anyhow::anyhow!("Meshtastic serial API did not provide MyInfo"))?;
        if self.user_id.is_none() && self.long_name.is_none() && self.short_name.is_none() {
            anyhow::bail!("Meshtastic serial API did not provide node identity");
        }
        let firmware_version = self
            .long_name
            .clone()
            .or_else(|| self.user_id.clone())
            .unwrap_or_else(|| "Meshtastic".to_string());

        info!(node_id, name = %firmware_version, "Meshtastic identity");
        Ok(DeviceInfo {
            firmware_version,
            node_id,
            max_contacts: 200,
            device_type: DeviceType::Meshtastic,
        })
    }

    pub async fn set_advert_name(&mut self, name: &str) -> Result<()> {
        self.long_name = Some(name.to_string());
        Ok(())
    }

    pub async fn send_self_advert(&mut self) -> Result<()> {
        self.send_to_radio(&encode_heartbeat()).await
    }

    pub async fn send_channel_text(&mut self, _channel: u8, msg: &[u8]) -> Result<()> {
        let text = String::from_utf8_lossy(msg);
        let packet = encode_mesh_packet(BROADCAST_NUM, TEXT_MESSAGE_APP, text.as_bytes());
        self.send_to_radio(&encode_to_radio_variant(TO_RADIO_PACKET, &packet))
            .await
    }

    /// Native Meshtastic unicast DM. Our synthetic Meshtastic pubkeys carry the
    /// numeric node-id in their first 4 bytes (little-endian, see
    /// `synthetic_pubkey`), so `dest_pubkey_prefix` directly yields the
    /// destination node number. We send a directed MeshPacket (`to` = node num)
    /// rather than a `BROADCAST_NUM` channel blast — this is the Meshtastic
    /// analog of the meshcore `CMD_SEND_TXT_MSG` fix: the message is delivered
    /// as a real DM (only the recipient's client surfaces it) instead of
    /// polluting the shared primary channel where every node would see it.
    ///
    /// If the prefix decodes to node 0 / broadcast (e.g. a non-Meshtastic
    /// synthetic key routed here by mistake), fall back to a channel send so the
    /// device interface stays uniform and the message still goes out.
    pub async fn send_text_msg(&mut self, dest_pubkey_prefix: &[u8; 6], msg: &[u8]) -> Result<()> {
        let node_num = u32::from_le_bytes([
            dest_pubkey_prefix[0],
            dest_pubkey_prefix[1],
            dest_pubkey_prefix[2],
            dest_pubkey_prefix[3],
        ]);
        if node_num == 0 || node_num == BROADCAST_NUM {
            return self.send_channel_text(0, msg).await;
        }
        let text = String::from_utf8_lossy(msg);
        let packet = encode_mesh_packet(node_num, TEXT_MESSAGE_APP, text.as_bytes());
        self.send_to_radio(&encode_to_radio_variant(TO_RADIO_PACKET, &packet))
            .await
    }

    /// Meshtastic has no meshcore-style contact table; these are no-ops so the
    /// device interface stays uniform.
    pub async fn remove_contact(&mut self, _pubkey: &[u8; 32]) -> Result<()> {
        Ok(())
    }

    pub async fn add_contact(
        &mut self,
        _pubkey: &[u8; 32],
        _contact_type: u8,
        _flags: u8,
        _out_path_len: u8,
        _name: &str,
        _last_advert: u32,
    ) -> Result<()> {
        Ok(())
    }

    pub async fn get_contacts(&mut self) -> Result<Vec<ParsedContact>> {
        if self.contacts.is_empty() {
            self.send_to_radio(&encode_want_config()).await?;
            let deadline = tokio::time::Instant::now() + Duration::from_secs(2);
            while tokio::time::Instant::now() < deadline {
                match self.read_from_radio().await? {
                    Some(frame) => {
                        let config_complete = matches!(
                            decode_top_level_variant(&frame),
                            Some((FROM_RADIO_CONFIG_COMPLETE_ID, _))
                        );
                        self.handle_from_radio(&frame);
                        if config_complete || !self.contacts.is_empty() {
                            break;
                        }
                    }
                    None => tokio::time::sleep(Duration::from_millis(50)).await,
                }
            }
        }
        Ok(self.contacts.values().cloned().collect())
    }

    pub async fn reset_contact_path(&mut self, _pubkey: &[u8; 32]) -> Result<()> {
        Ok(())
    }

    pub async fn sync_messages(&mut self) -> Result<Vec<InboundFrame>> {
        Ok(Vec::new())
    }

    pub async fn try_recv_frame(&mut self) -> Result<Option<InboundFrame>> {
        let Some(frame) = self.read_from_radio().await? else {
            return Ok(None);
        };
        Ok(self.handle_from_radio(&frame))
    }

    /// Whether we've learned `node_num`'s real PKI (Curve25519) key — from a
    /// NodeInfo `public_key` or an inbound PKC DM — meaning the firmware can
    /// deliver DMs to/from it end-to-end encrypted instead of falling back to
    /// the channel PSK. Driver-internal for now; lets a future mesh-tab badge
    /// distinguish a true E2E DM from a channel-encrypted one without changing
    /// the shared device interface (which would break meshcore hot-swap).
    #[allow(dead_code)] // seam: consumed when the mesh-tab E2E badge lands
    pub fn peer_is_pkc_capable(&self, node_num: u32) -> bool {
        self.peer_pubkeys
            .get(&node_num)
            .is_some_and(|k| !k.is_empty())
    }

    pub fn advert_name(&self) -> Option<String> {
        self.long_name
            .clone()
            .or_else(|| self.short_name.clone())
            .or_else(|| self.user_id.clone())
    }

    async fn send_to_radio(&mut self, payload: &[u8]) -> Result<()> {
        if payload.len() > TO_RADIO_MAX {
            anyhow::bail!("Meshtastic payload too large: {} bytes", payload.len());
        }
        let mut frame = Vec::with_capacity(4 + payload.len());
        frame.push(START1);
        frame.push(START2);
        frame.extend_from_slice(&(payload.len() as u16).to_be_bytes());
        frame.extend_from_slice(payload);
        tokio::time::timeout(WRITE_TIMEOUT, self.port.write_all(&frame))
            .await
            .context("Meshtastic serial write timed out")?
            .context("Meshtastic serial write failed")?;
        Ok(())
    }

    async fn read_from_radio(&mut self) -> Result<Option<Vec<u8>>> {
        if let Some(frame) = decode_serial_frame(&mut self.read_buf) {
            return Ok(Some(frame));
        }

        let mut tmp = [0u8; READ_BUF_SIZE];
        match tokio::time::timeout(Duration::from_millis(50), self.port.read(&mut tmp)).await {
            Ok(Ok(0)) => anyhow::bail!("Meshtastic serial port closed"),
            Ok(Ok(n)) => self.read_buf.extend_from_slice(&tmp[..n]),
            Ok(Err(e)) => return Err(e).context("Meshtastic serial read error"),
            Err(_) => return Ok(None),
        }

        Ok(decode_serial_frame(&mut self.read_buf))
    }

    fn handle_from_radio(&mut self, frame: &[u8]) -> Option<InboundFrame> {
        let Some((field, value)) = decode_top_level_variant(frame) else {
            return None;
        };
        match field {
            FROM_RADIO_MY_INFO => {
                if let Some((node_num, user_id)) = parse_my_info(value) {
                    self.node_num = Some(node_num);
                    if let Some(user_id) = user_id {
                        self.user_id = Some(user_id);
                    }
                }
                None
            }
            FROM_RADIO_NODE_INFO => {
                self.update_node_info(value);
                None
            }
            FROM_RADIO_PACKET => self.packet_to_inbound_frame(value),
            FROM_RADIO_CONFIG_COMPLETE_ID | FROM_RADIO_REBOOTED => None,
            other => {
                debug!(
                    field = other,
                    len = value.len(),
                    "Unhandled Meshtastic FromRadio field"
                );
                None
            }
        }
    }

    fn update_node_info(&mut self, data: &[u8]) {
        if let Some(node) = parse_node_info(data) {
            if let Some(pk) = node.public_key.as_ref() {
                if self.peer_pubkeys.insert(node.num, pk.clone()).is_none() {
                    debug!(
                        node = node.num,
                        key_len = pk.len(),
                        "Meshtastic peer is PKC-capable (NodeInfo public_key)"
                    );
                }
            }
            let key = synthetic_pubkey(node.num);
            let name = node
                .long_name
                .or(node.short_name)
                .or(node.id)
                .unwrap_or_else(|| format!("Meshtastic !{:08x}", node.num));
            if Some(node.num) == self.node_num {
                self.long_name = Some(name.clone());
            }
            self.contacts.insert(
                node.num,
                ParsedContact {
                    public_key_hex: hex::encode(key),
                    advert_name: name,
                    last_advert: node.last_heard.unwrap_or_default(),
                    contact_type: 1,
                    path_len: 0xff,
                    flags: 0,
                },
            );
        }
    }

    fn packet_to_inbound_frame(&mut self, data: &[u8]) -> Option<InboundFrame> {
        let packet = parse_mesh_packet(data)?;
        if packet.portnum != TEXT_MESSAGE_APP || packet.payload.is_empty() {
            return None;
        }
        let from = packet.from.unwrap_or(0);
        if Some(from) == self.node_num {
            return None;
        }
        // Record E2E status: a `pki_encrypted` packet (or one carrying the
        // sender's `public_key`) proves this DM arrived end-to-end encrypted via
        // the PKI, not the shared channel PSK. We learn the sender's key here too
        // — but keep it OUT of the routing `public_key_hex` (synthetic) so the
        // device interface stays identical to meshcore's and the two remain
        // hot-swappable behind the mesh listener.
        if let Some(pk) = packet.public_key.as_ref() {
            self.peer_pubkeys.entry(from).or_insert_with(|| pk.clone());
        }
        if packet.pki_encrypted {
            debug!(node = from, "Meshtastic DM received end-to-end encrypted (PKI)");
        }
        let from_key = synthetic_pubkey(from);
        self.contacts.entry(from).or_insert_with(|| ParsedContact {
            public_key_hex: hex::encode(synthetic_pubkey(from)),
            advert_name: format!("Meshtastic !{:08x}", from),
            last_advert: 0,
            contact_type: 1,
            path_len: 0xff,
            flags: 0,
        });

        let mut payload = Vec::with_capacity(15 + packet.payload.len());
        payload.push(0); // SNR unknown
        payload.extend_from_slice(&[0, 0]); // reserved
        payload.extend_from_slice(&from_key[..6]);
        payload.push(0xff); // unknown/flood path
        payload.push(0); // text type
        payload.extend_from_slice(&0u32.to_le_bytes());
        payload.extend_from_slice(&packet.payload);
        Some(InboundFrame {
            code: super::protocol::RESP_CONTACT_MSG_V3,
            data: payload,
            bytes_consumed: 0,
        })
    }
}

fn decode_serial_frame(buf: &mut Vec<u8>) -> Option<Vec<u8>> {
    let start = buf.windows(2).position(|w| w == [START1, START2])?;
    if start > 0 {
        buf.drain(..start);
    }
    if buf.len() < 4 {
        return None;
    }
    let len = u16::from_be_bytes([buf[2], buf[3]]) as usize;
    if buf.len() < 4 + len {
        return None;
    }
    let payload = buf[4..4 + len].to_vec();
    buf.drain(..4 + len);
    Some(payload)
}

fn encode_want_config() -> Vec<u8> {
    encode_varint_field(TO_RADIO_WANT_CONFIG_ID, 1)
}

fn encode_heartbeat() -> Vec<u8> {
    encode_to_radio_variant(TO_RADIO_HEARTBEAT, &[])
}

fn encode_to_radio_variant(field: u64, bytes: &[u8]) -> Vec<u8> {
    let mut out = Vec::new();
    encode_len_field(field, bytes, &mut out);
    out
}

fn encode_mesh_packet(to: u32, portnum: u32, payload: &[u8]) -> Vec<u8> {
    let mut decoded = Vec::new();
    encode_varint_field_into(1, portnum as u64, &mut decoded);
    encode_len_field(2, payload, &mut decoded);

    let mut packet = Vec::new();
    encode_fixed32_field(2, to, &mut packet);
    encode_len_field(4, &decoded, &mut packet);
    packet
}

fn decode_top_level_variant(buf: &[u8]) -> Option<(u64, &[u8])> {
    let mut idx = 0;
    while idx < buf.len() {
        let (key, n) = read_varint(&buf[idx..])?;
        idx += n;
        let field = key >> 3;
        match key & 0x07 {
            0 => {
                let (_, n) = read_varint(&buf[idx..])?;
                idx += n;
                if matches!(field, FROM_RADIO_CONFIG_COMPLETE_ID | FROM_RADIO_REBOOTED) {
                    return Some((field, &[]));
                }
            }
            2 => {
                let (len, n) = read_varint(&buf[idx..])?;
                idx += n;
                let end = idx.checked_add(len as usize)?;
                if end > buf.len() {
                    return None;
                }
                if matches!(
                    field,
                    FROM_RADIO_PACKET | FROM_RADIO_MY_INFO | FROM_RADIO_NODE_INFO
                ) {
                    return Some((field, &buf[idx..end]));
                }
                idx = end;
            }
            _ => return None,
        }
    }
    None
}

fn parse_my_info(data: &[u8]) -> Option<(u32, Option<String>)> {
    let mut idx = 0;
    let mut node_num = None;
    let mut user_id = None;
    while idx < data.len() {
        let (field, value, next) = next_field(data, idx)?;
        idx = next;
        match (field, value) {
            (1, FieldValue::Varint(v)) => node_num = Some(v as u32),
            (1, FieldValue::Fixed32(v)) => node_num = Some(v),
            (3, FieldValue::Bytes(b)) => user_id = parse_user(b).and_then(|u| u.id),
            _ => {}
        }
    }
    node_num.map(|n| (n, user_id))
}

struct ParsedNode {
    num: u32,
    id: Option<String>,
    long_name: Option<String>,
    short_name: Option<String>,
    last_heard: Option<u32>,
    public_key: Option<Vec<u8>>,
}

fn parse_node_info(data: &[u8]) -> Option<ParsedNode> {
    let mut idx = 0;
    let mut node = ParsedNode {
        num: 0,
        id: None,
        long_name: None,
        short_name: None,
        last_heard: None,
        public_key: None,
    };
    while idx < data.len() {
        let (field, value, next) = next_field(data, idx)?;
        idx = next;
        match (field, value) {
            (1, FieldValue::Varint(v)) => node.num = v as u32,
            (1, FieldValue::Fixed32(v)) => node.num = v,
            (2, FieldValue::Bytes(b)) => {
                if let Some(user) = parse_user(b) {
                    node.id = user.id;
                    node.long_name = user.long_name;
                    node.short_name = user.short_name;
                    node.public_key = user.public_key;
                }
            }
            (5, FieldValue::Fixed32(v)) => node.last_heard = Some(v),
            _ => {}
        }
    }
    if node.num == 0 {
        None
    } else {
        Some(node)
    }
}

struct ParsedUser {
    id: Option<String>,
    long_name: Option<String>,
    short_name: Option<String>,
    public_key: Option<Vec<u8>>,
}

fn parse_user(data: &[u8]) -> Option<ParsedUser> {
    let mut idx = 0;
    let mut user = ParsedUser {
        id: None,
        long_name: None,
        short_name: None,
        public_key: None,
    };
    while idx < data.len() {
        let (field, value, next) = next_field(data, idx)?;
        idx = next;
        match (field, value) {
            (1, FieldValue::Bytes(b)) => user.id = string_field(b),
            (2, FieldValue::Bytes(b)) => user.long_name = string_field(b),
            (3, FieldValue::Bytes(b)) => user.short_name = string_field(b),
            // User.public_key (field 8): the peer's Curve25519 key. Its presence
            // means the radio can PKC-encrypt DMs to this node end-to-end.
            (8, FieldValue::Bytes(b)) if !b.is_empty() => user.public_key = Some(b.to_vec()),
            _ => {}
        }
    }
    Some(user)
}

struct ParsedPacket {
    from: Option<u32>,
    portnum: u32,
    payload: Vec<u8>,
    /// MeshPacket.pki_encrypted (field 17): the firmware decrypted this packet
    /// with the PKI (Curve25519) key, i.e. it arrived end-to-end encrypted
    /// rather than via the shared channel PSK.
    pki_encrypted: bool,
    /// MeshPacket.public_key (field 16): the sender's key, carried on PKC DMs.
    public_key: Option<Vec<u8>>,
}

fn parse_mesh_packet(data: &[u8]) -> Option<ParsedPacket> {
    let mut idx = 0;
    let mut from = None;
    let mut decoded = None;
    let mut pki_encrypted = false;
    let mut public_key = None;
    while idx < data.len() {
        let (field, value, next) = next_field(data, idx)?;
        idx = next;
        match (field, value) {
            (1, FieldValue::Fixed32(v)) => from = Some(v),
            (4, FieldValue::Bytes(b)) => decoded = Some(b),
            (16, FieldValue::Bytes(b)) if !b.is_empty() => public_key = Some(b.to_vec()),
            (17, FieldValue::Varint(v)) => pki_encrypted = v != 0,
            _ => {}
        }
    }
    let decoded = decoded?;
    let mut didx = 0;
    let mut portnum = 0;
    let mut payload = Vec::new();
    while didx < decoded.len() {
        let (field, value, next) = next_field(decoded, didx)?;
        didx = next;
        match (field, value) {
            (1, FieldValue::Varint(v)) => portnum = v as u32,
            (2, FieldValue::Bytes(b)) => payload = b.to_vec(),
            _ => {}
        }
    }
    Some(ParsedPacket {
        from,
        portnum,
        payload,
        pki_encrypted,
        public_key,
    })
}

enum FieldValue<'a> {
    Varint(u64),
    Fixed32(u32),
    Bytes(&'a [u8]),
}

fn next_field(buf: &[u8], idx: usize) -> Option<(u64, FieldValue<'_>, usize)> {
    let (key, n) = read_varint(&buf[idx..])?;
    let field = key >> 3;
    let mut pos = idx + n;
    match key & 0x07 {
        0 => {
            let (v, n) = read_varint(&buf[pos..])?;
            pos += n;
            Some((field, FieldValue::Varint(v), pos))
        }
        2 => {
            let (len, n) = read_varint(&buf[pos..])?;
            pos += n;
            let end = pos.checked_add(len as usize)?;
            if end > buf.len() {
                return None;
            }
            Some((field, FieldValue::Bytes(&buf[pos..end]), end))
        }
        5 => {
            let end = pos.checked_add(4)?;
            if end > buf.len() {
                None
            } else {
                let value =
                    u32::from_le_bytes([buf[pos], buf[pos + 1], buf[pos + 2], buf[pos + 3]]);
                Some((field, FieldValue::Fixed32(value), end))
            }
        }
        1 => {
            let end = pos.checked_add(8)?;
            if end > buf.len() {
                None
            } else {
                Some((field, FieldValue::Bytes(&buf[pos..end]), end))
            }
        }
        wire => {
            warn!(wire, "Unsupported Meshtastic protobuf wire type");
            None
        }
    }
}

fn read_varint(buf: &[u8]) -> Option<(u64, usize)> {
    let mut out = 0u64;
    for (i, b) in buf.iter().copied().enumerate().take(10) {
        out |= ((b & 0x7f) as u64) << (7 * i);
        if b & 0x80 == 0 {
            return Some((out, i + 1));
        }
    }
    None
}

fn encode_varint_field(field: u64, value: u64) -> Vec<u8> {
    let mut out = Vec::new();
    encode_varint_field_into(field, value, &mut out);
    out
}

fn encode_varint_field_into(field: u64, value: u64, out: &mut Vec<u8>) {
    write_varint((field << 3) | 0, out);
    write_varint(value, out);
}

fn encode_len_field(field: u64, bytes: &[u8], out: &mut Vec<u8>) {
    write_varint((field << 3) | 2, out);
    write_varint(bytes.len() as u64, out);
    out.extend_from_slice(bytes);
}

fn encode_fixed32_field(field: u64, value: u32, out: &mut Vec<u8>) {
    write_varint((field << 3) | 5, out);
    out.extend_from_slice(&value.to_le_bytes());
}

fn write_varint(mut value: u64, out: &mut Vec<u8>) {
    while value >= 0x80 {
        out.push((value as u8 & 0x7f) | 0x80);
        value >>= 7;
    }
    out.push(value as u8);
}

fn string_field(bytes: &[u8]) -> Option<String> {
    std::str::from_utf8(bytes).ok().map(|s| s.to_string())
}

fn synthetic_pubkey(node_num: u32) -> [u8; 32] {
    let mut out = [0u8; 32];
    out[..4].copy_from_slice(&node_num.to_le_bytes());
    out[4..15].copy_from_slice(b"meshtastic:");
    out
}