MeshCoreTel-firmware/examples/simple_sensor/SensorMesh.cpp

#include "SensorMesh.h"

/* ------------------------------ Config -------------------------------- */

#ifndef LORA_FREQ
  #define LORA_FREQ   915.0
#endif
#ifndef LORA_BW
  #define LORA_BW     250
#endif
#ifndef LORA_SF
  #define LORA_SF     10
#endif
#ifndef LORA_CR
  #define LORA_CR      5
#endif
#ifndef LORA_TX_POWER
  #define LORA_TX_POWER  20
#endif

#ifndef ADVERT_NAME
  #define  ADVERT_NAME   "sensor"
#endif
#ifndef ADVERT_LAT
  #define  ADVERT_LAT  0.0
#endif
#ifndef ADVERT_LON
  #define  ADVERT_LON  0.0
#endif

#ifndef ADMIN_PASSWORD
  #define  ADMIN_PASSWORD  "password"
#endif

#ifndef SERVER_RESPONSE_DELAY
  #define SERVER_RESPONSE_DELAY   300
#endif

#ifndef TXT_ACK_DELAY
  #define TXT_ACK_DELAY     200
#endif

#ifndef SENSOR_READ_INTERVAL_SECS
  #define SENSOR_READ_INTERVAL_SECS  60
#endif

/* ------------------------------ Code -------------------------------- */

#define REQ_TYPE_LOGIN               0x00
#define REQ_TYPE_GET_STATUS          0x01
#define REQ_TYPE_KEEP_ALIVE          0x02
#define REQ_TYPE_GET_TELEMETRY_DATA  0x03
#define REQ_TYPE_GET_AVG_MIN_MAX     0x04

#define RESP_SERVER_LOGIN_OK      0   // response to ANON_REQ

#define CLI_REPLY_DELAY_MILLIS  1000

#define LAZY_CONTACTS_WRITE_DELAY       5000

#define ALERT_ACK_EXPIRY_MILLIS         6000   // wait 6 secs for ACKs to alert messages

static File openAppend(FILESYSTEM* _fs, const char* fname) {
  #if defined(NRF52_PLATFORM) || defined(STM32_PLATFORM)
    return _fs->open(fname, FILE_O_WRITE);
  #elif defined(RP2040_PLATFORM)
    return _fs->open(fname, "a");
  #else
    return _fs->open(fname, "a", true);
  #endif
}

static File openWrite(FILESYSTEM* _fs, const char* filename) {
  #if defined(NRF52_PLATFORM) || defined(STM32_PLATFORM)
    _fs->remove(filename);
    return _fs->open(filename, FILE_O_WRITE);
  #elif defined(RP2040_PLATFORM)
    return _fs->open(filename, "w");
  #else
    return _fs->open(filename, "w", true);
  #endif
}

void SensorMesh::loadContacts() {
  num_contacts = 0;
  if (_fs->exists("/s_contacts")) {
  #if defined(RP2040_PLATFORM)
    File file = _fs->open("/s_contacts", "r");
  #else
    File file = _fs->open("/s_contacts");
  #endif
    if (file) {
      bool full = false;
      while (!full) {
        ContactInfo c;
        uint8_t pub_key[32];
        uint8_t unused[5];

        bool success = (file.read(pub_key, 32) == 32);
        success = success && (file.read((uint8_t *) &c.permissions, 2) == 2);
        success = success && (file.read(unused, 5) == 5);
        success = success && (file.read((uint8_t *)&c.out_path_len, 1) == 1);
        success = success && (file.read(c.out_path, 64) == 64);
        success = success && (file.read(c.shared_secret, PUB_KEY_SIZE) == PUB_KEY_SIZE);
        c.last_timestamp = 0;  // transient
        c.last_activity = 0;

        if (!success) break; // EOF

        c.id = mesh::Identity(pub_key);
        if (num_contacts < MAX_CONTACTS) {
          contacts[num_contacts++] = c;
        } else {
          full = true;
        }
      }
      file.close();
    }
  }
}

void SensorMesh::saveContacts() {
  File file = openWrite(_fs, "/s_contacts");
  if (file) {
    uint8_t unused[5];
    memset(unused, 0, sizeof(unused));

    for (int i = 0; i < num_contacts; i++) {
      auto c = &contacts[i];
      if (c->permissions == 0) continue;    // skip deleted entries

      bool success = (file.write(c->id.pub_key, 32) == 32);
      success = success && (file.write((uint8_t *) &c->permissions, 2) == 2);
      success = success && (file.write(unused, 5) == 5);
      success = success && (file.write((uint8_t *)&c->out_path_len, 1) == 1);
      success = success && (file.write(c->out_path, 64) == 64);
      success = success && (file.write(c->shared_secret, PUB_KEY_SIZE) == PUB_KEY_SIZE);

      if (!success) break; // write failed
    }
    file.close();
  }
}

static uint8_t getDataSize(uint8_t type) {
    switch (type) {
      case LPP_GPS:
        return 9;
      case LPP_POLYLINE:
        return 8;  // TODO: this is MINIMIUM
      case LPP_GYROMETER:
      case LPP_ACCELEROMETER:
        return 6;
      case LPP_GENERIC_SENSOR:
      case LPP_FREQUENCY:
      case LPP_DISTANCE:
      case LPP_ENERGY:
      case LPP_UNIXTIME:
        return 4;
      case LPP_COLOUR:
        return 3;
      case LPP_ANALOG_INPUT:
      case LPP_ANALOG_OUTPUT:
      case LPP_LUMINOSITY:
      case LPP_TEMPERATURE:
      case LPP_CONCENTRATION:
      case LPP_BAROMETRIC_PRESSURE:
      case LPP_RELATIVE_HUMIDITY:
      case LPP_ALTITUDE:
      case LPP_VOLTAGE:
      case LPP_CURRENT:
      case LPP_DIRECTION:
      case LPP_POWER:
        return 2;
    }
    return 1;
}

static uint32_t getMultiplier(uint8_t type) {
    switch (type) {
      case LPP_CURRENT:
      case LPP_DISTANCE:
      case LPP_ENERGY:
        return 1000;
      case LPP_VOLTAGE:
      case LPP_ANALOG_INPUT:
      case LPP_ANALOG_OUTPUT:
        return 100;
      case LPP_TEMPERATURE:
      case LPP_BAROMETRIC_PRESSURE:
      case LPP_RELATIVE_HUMIDITY:
        return 10;
    }
    return 1;
}

static bool isSigned(uint8_t type) {
  return type == LPP_ALTITUDE || type == LPP_TEMPERATURE || type == LPP_GYROMETER ||
      type == LPP_ANALOG_INPUT || type == LPP_ANALOG_OUTPUT || type == LPP_GPS || type == LPP_ACCELEROMETER;
}

static float getFloat(const uint8_t * buffer, uint8_t size, uint32_t multiplier, bool is_signed) {
  uint32_t value = 0;
  for (uint8_t i = 0; i < size; i++) {
    value = (value << 8) + buffer[i];
  }

  int sign = 1;
  if (is_signed) {
    uint32_t bit = 1ul << ((size * 8) - 1);
    if ((value & bit) == bit) {
      value = (bit << 1) - value;
      sign = -1;
    }
  }
  return sign * ((float) value / multiplier);
}

static uint8_t putFloat(uint8_t * dest, float value, uint8_t size, uint32_t multiplier, bool is_signed) {
  // check sign
  bool sign = value < 0;
  if (sign) value = -value;

  // get value to store
  uint32_t v = value * multiplier;

  // format an uint32_t as if it was an int32_t
  if (is_signed & sign) {
    uint32_t mask = (1 << (size * 8)) - 1;
    v = v & mask;
    if (sign) v = mask - v + 1;
  }

  // add bytes (MSB first)
  for (uint8_t i=1; i<=size; i++) {
    dest[size - i] = (v & 0xFF);
    v >>= 8;
  }
  return size;
}

uint8_t SensorMesh::handleRequest(uint16_t perms, uint32_t sender_timestamp, uint8_t req_type, uint8_t* payload, size_t payload_len) {
  memcpy(reply_data, &sender_timestamp, 4);   // reflect sender_timestamp back in response packet (kind of like a 'tag')

  if (req_type == REQ_TYPE_GET_TELEMETRY_DATA && (perms & PERM_GET_TELEMETRY) != 0) {
    telemetry.reset();
    telemetry.addVoltage(TELEM_CHANNEL_SELF, (float)board.getBattMilliVolts() / 1000.0f);
    // query other sensors -- target specific
    sensors.querySensors(0xFF, telemetry);  // allow all telemetry permissions for admin or guest

    uint8_t tlen = telemetry.getSize();
    memcpy(&reply_data[4], telemetry.getBuffer(), tlen);
    return 4 + tlen;  // reply_len
  }
  if (req_type == REQ_TYPE_GET_AVG_MIN_MAX && (perms & PERM_GET_MIN_MAX_AVG) != 0) {
    uint32_t start_secs_ago, end_secs_ago;
    memcpy(&start_secs_ago, &payload[0], 4);
    memcpy(&end_secs_ago, &payload[4], 4);
    uint8_t res1 = payload[8];   // reserved for future  (extra query params)
    uint8_t res2 = payload[8];

    MinMaxAvg data[8];
    int n;
    if (res1 == 0 && res2 == 0) {
      n = querySeriesData(start_secs_ago, end_secs_ago, data, 8);
    } else {
      n = 0;
    }

    uint8_t ofs = 4;
    {
      uint32_t now = getRTCClock()->getCurrentTime();
      memcpy(&reply_data[ofs], &now, 4); ofs += 4;
    }

    for (int i = 0; i < n; i++) {
      auto d = &data[i];
      reply_data[ofs++] = d->_channel;
      reply_data[ofs++] = d->_lpp_type;
      uint8_t sz = getDataSize(d->_lpp_type);
      uint32_t mult = getMultiplier(d->_lpp_type);
      bool is_signed = isSigned(d->_lpp_type);
      ofs += putFloat(&reply_data[ofs], d->_min, sz, mult, is_signed);
      ofs += putFloat(&reply_data[ofs], d->_max, sz, mult, is_signed);
      ofs += putFloat(&reply_data[ofs], d->_avg, sz, mult, is_signed);
    }
    return ofs;
  }
  return 0;  // unknown command
}

mesh::Packet* SensorMesh::createSelfAdvert() {
  uint8_t app_data[MAX_ADVERT_DATA_SIZE];
  uint8_t app_data_len;
  {
    AdvertDataBuilder builder(ADV_TYPE_SENSOR, _prefs.node_name, _prefs.node_lat, _prefs.node_lon);
    app_data_len = builder.encodeTo(app_data);
  }

  return createAdvert(self_id, app_data, app_data_len);
}

ContactInfo* SensorMesh::putContact(const mesh::Identity& id) {
  uint32_t min_time = 0xFFFFFFFF;
  ContactInfo* oldest = &contacts[MAX_CONTACTS - 1];
  for (int i = 0; i < num_contacts; i++) {
    if (id.matches(contacts[i].id)) return &contacts[i];  // already known
    if (!contacts[i].isAdmin() && contacts[i].last_activity < min_time) {
      oldest = &contacts[i];
      min_time = oldest->last_activity;
    }
  }

  ContactInfo* c;
  if (num_contacts < MAX_CONTACTS) {
    c = &contacts[num_contacts++];
  } else {
    c = oldest;  // evict least active contact
  }
  memset(c, 0, sizeof(*c));
  c->id = id;
  c->out_path_len = -1;  // initially out_path is unknown
  return c;
}

void SensorMesh::applyContactPermissions(const uint8_t* pubkey, uint16_t perms) {
  mesh::Identity id(pubkey);
  auto c = putContact(id);

  if (perms == 0) {  // no permissions, remove from contacts
    memset(c, 0, sizeof(*c));
  } else {
    c->permissions = perms;  // update their permissions
    self_id.calcSharedSecret(c->shared_secret, pubkey);
  }
  dirty_contacts_expiry = futureMillis(LAZY_CONTACTS_WRITE_DELAY);   // trigger saveContacts()
}

void SensorMesh::sendAlert(ContactInfo* c, Trigger* t) {
  int text_len = strlen(t->text);

  uint8_t data[MAX_PACKET_PAYLOAD];
  memcpy(data, &t->timestamp, 4);
  data[4] = (TXT_TYPE_PLAIN << 2) | t->attempt;  // attempt and flags
  memcpy(&data[5], t->text, text_len);

  // calc expected ACK reply
  mesh::Utils::sha256((uint8_t *)&t->expected_acks[t->attempt], 4, data, 5 + text_len, self_id.pub_key, PUB_KEY_SIZE);
  t->attempt++;

  auto pkt = createDatagram(PAYLOAD_TYPE_TXT_MSG, c->id, c->shared_secret, data, 5 + text_len);
  if (pkt) {
    if (c->out_path_len >= 0) {  // we have an out_path, so send DIRECT
      sendDirect(pkt, c->out_path, c->out_path_len);
    } else {
      sendFlood(pkt);
    }
  }
  t->send_expiry = futureMillis(ALERT_ACK_EXPIRY_MILLIS);
}

void SensorMesh::alertIf(bool condition, Trigger& t, AlertPriority pri, const char* text) {
  if (condition) {
    if (!t.isTriggered() && num_alert_tasks < MAX_CONCURRENT_ALERTS) {
      StrHelper::strncpy(t.text, text, sizeof(t.text));
      t.pri = pri;
      t.send_expiry = 0;  // signal that initial send is needed
      t.attempt = 4;
      t.curr_contact_idx = -1;  // start iterating thru contacts[]

      alert_tasks[num_alert_tasks++] = &t;  // add to queue
    }
  } else {
    if (t.isTriggered()) {
      t.text[0] = 0;
      // remove 't' from alert queue
      int i = 0;
      while (i < num_alert_tasks && alert_tasks[i] != &t) i++;

      if (i < num_alert_tasks) {  // found,  now delete from array
        num_alert_tasks--;
        while (i < num_alert_tasks) {
          alert_tasks[i] = alert_tasks[i + 1];
          i++;
        }
      }
    }
  }
}

float SensorMesh::getAirtimeBudgetFactor() const {
  return _prefs.airtime_factor;
}

bool SensorMesh::allowPacketForward(const mesh::Packet* packet) {
  if (_prefs.disable_fwd) return false;
  if (packet->isRouteFlood() && packet->path_len >= _prefs.flood_max) return false;
  return true;
}

int SensorMesh::calcRxDelay(float score, uint32_t air_time) const {
  if (_prefs.rx_delay_base <= 0.0f) return 0;
  return (int) ((pow(_prefs.rx_delay_base, 0.85f - score) - 1.0) * air_time);
}

uint32_t SensorMesh::getRetransmitDelay(const mesh::Packet* packet) {
  uint32_t t = (_radio->getEstAirtimeFor(packet->path_len + packet->payload_len + 2) * _prefs.tx_delay_factor);
  return getRNG()->nextInt(0, 6)*t;
}
uint32_t SensorMesh::getDirectRetransmitDelay(const mesh::Packet* packet) {
  uint32_t t = (_radio->getEstAirtimeFor(packet->path_len + packet->payload_len + 2) * _prefs.direct_tx_delay_factor);
  return getRNG()->nextInt(0, 6)*t;
}
int SensorMesh::getInterferenceThreshold() const {
  return _prefs.interference_threshold;
}
int SensorMesh::getAGCResetInterval() const {
  return ((int)_prefs.agc_reset_interval) * 4000;   // milliseconds
}

uint8_t SensorMesh::handleLoginReq(const mesh::Identity& sender, const uint8_t* secret, uint32_t sender_timestamp, const uint8_t* data) {
  if (strcmp((char *) data, _prefs.password) != 0) {  // check for valid password
  #if MESH_DEBUG
    MESH_DEBUG_PRINTLN("Invalid password: %s", &data[4]);
  #endif
    return 0;
  }

  auto client = putContact(sender);  // add to contacts (if not already known)
  if (sender_timestamp <= client->last_timestamp) {
    MESH_DEBUG_PRINTLN("Possible login replay attack!");
    return 0;  // FATAL: client table is full -OR- replay attack
  }

  MESH_DEBUG_PRINTLN("Login success!");
  client->last_timestamp = sender_timestamp;
  client->last_activity = getRTCClock()->getCurrentTime();
  client->permissions = PERM_IS_ADMIN | PERM_RECV_ALERTS_HI | PERM_RECV_ALERTS_LO;  // initially opt-in to receive alerts (can opt out)
  memcpy(client->shared_secret, secret, PUB_KEY_SIZE);

  dirty_contacts_expiry = futureMillis(LAZY_CONTACTS_WRITE_DELAY);

  uint32_t now = getRTCClock()->getCurrentTimeUnique();
  memcpy(reply_data, &now, 4);   // response packets always prefixed with timestamp
  reply_data[4] = RESP_SERVER_LOGIN_OK;
  reply_data[5] = 0;  // NEW: recommended keep-alive interval (secs / 16)
  reply_data[6] = 1;  // 1 = is admin
  reply_data[7] = 0;  // FUTURE: reserved
  getRNG()->random(&reply_data[8], 4);   // random blob to help packet-hash uniqueness

  return 12;  // reply length
}

void SensorMesh::handleCommand(uint32_t sender_timestamp, char* command, char* reply) {
  while (*command == ' ') command++;   // skip leading spaces

  if (strlen(command) > 4 && command[2] == '|') {  // optional prefix (for companion radio CLI)
    memcpy(reply, command, 3);  // reflect the prefix back
    reply += 3;
    command += 3;
  }

  // handle sensor-specific CLI commands
  if (memcmp(command, "setperm ", 8) == 0) {   // format:  setperm {pubkey-hex} {permissions-int16}
    char* hex = &command[8];
    char* sp = strchr(hex, ' ');   // look for separator char
    if (sp == NULL || sp - hex != PUB_KEY_SIZE*2) {
      strcpy(reply, "Err - bad pubkey len");
    } else {
      *sp++ = 0;   // replace space with null terminator

      uint8_t pubkey[PUB_KEY_SIZE];
      if (mesh::Utils::fromHex(pubkey, PUB_KEY_SIZE, hex)) {
        uint16_t perms = atoi(sp);
        applyContactPermissions(pubkey, perms);
        strcpy(reply, "OK");
      } else {
        strcpy(reply, "Err - bad pubkey");
      }
    }
  } else if (sender_timestamp == 0 && strcmp(command, "getperm") == 0) {
    Serial.println("Permissions:");
    for (int i = 0; i < num_contacts; i++) {
      auto c = &contacts[i];

      mesh::Utils::printHex(Serial, c->id.pub_key, PUB_KEY_SIZE);
      Serial.printf(" %04X\n", c->permissions);
    }
    reply[0] = 0;
  } else {
    _cli.handleCommand(sender_timestamp, command, reply);  // common CLI commands
  }
}

void SensorMesh::onAnonDataRecv(mesh::Packet* packet, const uint8_t* secret, const mesh::Identity& sender, uint8_t* data, size_t len) {
  if (packet->getPayloadType() == PAYLOAD_TYPE_ANON_REQ) {  // received an initial request by a possible admin client (unknown at this stage)
    uint32_t timestamp;
    memcpy(&timestamp, data, 4);

    data[len] = 0;  // ensure null terminator
    uint8_t reply_len = handleLoginReq(sender, secret, timestamp, &data[4]);

    if (reply_len == 0) return;   // invalid request

    if (packet->isRouteFlood()) {
      // let this sender know path TO here, so they can use sendDirect(), and ALSO encode the response
      mesh::Packet* path = createPathReturn(sender, secret, packet->path, packet->path_len,
                                            PAYLOAD_TYPE_RESPONSE, reply_data, reply_len);
      if (path) sendFlood(path, SERVER_RESPONSE_DELAY);
    } else {
      mesh::Packet* reply = createDatagram(PAYLOAD_TYPE_RESPONSE, sender, secret, reply_data, reply_len);
      if (reply) sendFlood(reply, SERVER_RESPONSE_DELAY);
    }
  }
}

int SensorMesh::searchPeersByHash(const uint8_t* hash) {
  int n = 0;
  for (int i = 0; i < num_contacts && n < MAX_SEARCH_RESULTS; i++) {
    if (contacts[i].id.isHashMatch(hash)) {
      matching_peer_indexes[n++] = i;  // store the INDEXES of matching contacts (for subsequent 'peer' methods)
    }
  }
  return n;
}

void SensorMesh::getPeerSharedSecret(uint8_t* dest_secret, int peer_idx) {
  int i = matching_peer_indexes[peer_idx];
  if (i >= 0 && i < num_contacts) {
    // lookup pre-calculated shared_secret
    memcpy(dest_secret, contacts[i].shared_secret, PUB_KEY_SIZE);
  } else {
    MESH_DEBUG_PRINTLN("getPeerSharedSecret: Invalid peer idx: %d", i);
  }
}

void SensorMesh::onPeerDataRecv(mesh::Packet* packet, uint8_t type, int sender_idx, const uint8_t* secret, uint8_t* data, size_t len) {
  int i = matching_peer_indexes[sender_idx];
  if (i < 0 || i >= num_contacts) {
    MESH_DEBUG_PRINTLN("onPeerDataRecv: Invalid sender idx: %d", i);
    return;
  }

  ContactInfo& from = contacts[i];

  if (type == PAYLOAD_TYPE_REQ) {  // request (from a known contact)
    uint32_t timestamp;
    memcpy(&timestamp, data, 4);

    if (timestamp > from.last_timestamp) {  // prevent replay attacks
      uint8_t reply_len = handleRequest(from.isAdmin() ? 0xFFFF : from.permissions, timestamp, data[4], &data[5], len - 5);
      if (reply_len == 0) return;  // invalid command

      from.last_timestamp = timestamp;
      from.last_activity = getRTCClock()->getCurrentTime();

      if (packet->isRouteFlood()) {
        // let this sender know path TO here, so they can use sendDirect(), and ALSO encode the response
        mesh::Packet* path = createPathReturn(from.id, secret, packet->path, packet->path_len,
                                              PAYLOAD_TYPE_RESPONSE, reply_data, reply_len);
        if (path) sendFlood(path, SERVER_RESPONSE_DELAY);
      } else {
        mesh::Packet* reply = createDatagram(PAYLOAD_TYPE_RESPONSE, from.id, secret, reply_data, reply_len);
        if (reply) {
          if (from.out_path_len >= 0) {  // we have an out_path, so send DIRECT
            sendDirect(reply, from.out_path, from.out_path_len, SERVER_RESPONSE_DELAY);
          } else {
            sendFlood(reply, SERVER_RESPONSE_DELAY);
          }
        }
      }
    } else {
      MESH_DEBUG_PRINTLN("onPeerDataRecv: possible replay attack detected");
    }
  } else if (type == PAYLOAD_TYPE_TXT_MSG && len > 5 && from.isAdmin()) {   // a CLI command
    uint32_t sender_timestamp;
    memcpy(&sender_timestamp, data, 4);  // timestamp (by sender's RTC clock - which could be wrong)
    uint flags = (data[4] >> 2);   // message attempt number, and other flags

    if (!(flags == TXT_TYPE_CLI_DATA)) {
      MESH_DEBUG_PRINTLN("onPeerDataRecv: unsupported text type received: flags=%02x", (uint32_t)flags);
    } else if (sender_timestamp > from.last_timestamp) {  // prevent replay attacks
      from.last_timestamp = sender_timestamp;
      from.last_activity = getRTCClock()->getCurrentTime();

      // len can be > original length, but 'text' will be padded with zeroes
      data[len] = 0; // need to make a C string again, with null terminator

      uint8_t temp[166];
      char *command = (char *) &data[5];
      char *reply = (char *) &temp[5];
      handleCommand(sender_timestamp, command, reply);

      int text_len = strlen(reply);
      if (text_len > 0) {
        uint32_t timestamp = getRTCClock()->getCurrentTimeUnique();
        if (timestamp == sender_timestamp) {
          // WORKAROUND: the two timestamps need to be different, in the CLI view
          timestamp++;
        }
        memcpy(temp, &timestamp, 4);   // mostly an extra blob to help make packet_hash unique
        temp[4] = (TXT_TYPE_CLI_DATA << 2);

        auto reply = createDatagram(PAYLOAD_TYPE_TXT_MSG, from.id, secret, temp, 5 + text_len);
        if (reply) {
          if (from.out_path_len < 0) {
            sendFlood(reply, CLI_REPLY_DELAY_MILLIS);
          } else {
            sendDirect(reply, from.out_path, from.out_path_len, CLI_REPLY_DELAY_MILLIS);
          }
        }
      }
    } else {
      MESH_DEBUG_PRINTLN("onPeerDataRecv: possible replay attack detected");
    }
  }
}

bool SensorMesh::onPeerPathRecv(mesh::Packet* packet, int sender_idx, const uint8_t* secret, uint8_t* path, uint8_t path_len, uint8_t extra_type, uint8_t* extra, uint8_t extra_len) {
  int i = matching_peer_indexes[sender_idx];
  if (i < 0 || i >= num_contacts) {
    MESH_DEBUG_PRINTLN("onPeerPathRecv: Invalid sender idx: %d", i);
    return false;
  }

  ContactInfo& from = contacts[i];

  MESH_DEBUG_PRINTLN("PATH to contact, path_len=%d", (uint32_t) path_len);
  // NOTE: for this impl, we just replace the current 'out_path' regardless, whenever sender sends us a new out_path.
  // FUTURE: could store multiple out_paths per contact, and try to find which is the 'best'(?)
  memcpy(from.out_path, path, from.out_path_len = path_len);  // store a copy of path, for sendDirect()
  from.last_activity = getRTCClock()->getCurrentTime();

  // REVISIT: maybe make ALL out_paths non-persisted to minimise flash writes??
  if (from.isAdmin()) {
    // only do saveContacts() (of this out_path change) if this is an admin
    dirty_contacts_expiry = futureMillis(LAZY_CONTACTS_WRITE_DELAY);
  }

  // NOTE: no reciprocal path send!!
  return false;
}

void SensorMesh::onAckRecv(mesh::Packet* packet, uint32_t ack_crc) {
  if (num_alert_tasks > 0) {
    auto t = alert_tasks[0];   // check current alert task
    for (int i = 0; i < t->attempt; i++) {
      if (ack_crc == t->expected_acks[i]) {   // matching ACK!
        t->attempt = 4;  // signal to move to next contact
        t->send_expiry = 0;
        packet->markDoNotRetransmit();   // ACK was for this node, so don't retransmit
        return;
      }
    }
  }
}

SensorMesh::SensorMesh(mesh::MainBoard& board, mesh::Radio& radio, mesh::MillisecondClock& ms, mesh::RNG& rng, mesh::RTCClock& rtc, mesh::MeshTables& tables)
     : mesh::Mesh(radio, ms, rng, rtc, *new StaticPoolPacketManager(32), tables),
      _cli(board, rtc, &_prefs, this), telemetry(MAX_PACKET_PAYLOAD - 4)
{
  num_contacts = 0;
  next_local_advert = next_flood_advert = 0;
  dirty_contacts_expiry = 0;
  last_read_time = 0;
  num_alert_tasks = 0;

  // defaults
  memset(&_prefs, 0, sizeof(_prefs));
  _prefs.airtime_factor = 1.0;    // one half
  _prefs.rx_delay_base =   0.0f;  // turn off by default, was 10.0;
  _prefs.tx_delay_factor = 0.5f;   // was 0.25f
  StrHelper::strncpy(_prefs.node_name, ADVERT_NAME, sizeof(_prefs.node_name));
  _prefs.node_lat = ADVERT_LAT;
  _prefs.node_lon = ADVERT_LON;
  StrHelper::strncpy(_prefs.password, ADMIN_PASSWORD, sizeof(_prefs.password));
  _prefs.freq = LORA_FREQ;
  _prefs.sf = LORA_SF;
  _prefs.bw = LORA_BW;
  _prefs.cr = LORA_CR;
  _prefs.tx_power_dbm = LORA_TX_POWER;
  _prefs.advert_interval = 1;  // default to 2 minutes for NEW installs
  _prefs.flood_advert_interval = 0;   // disabled
  _prefs.disable_fwd = true;
  _prefs.flood_max = 64;
  _prefs.interference_threshold = 0;  // disabled
}

void SensorMesh::begin(FILESYSTEM* fs) {
  mesh::Mesh::begin();
  _fs = fs;
  // load persisted prefs
  _cli.loadPrefs(_fs);

  loadContacts();

  radio_set_params(_prefs.freq, _prefs.bw, _prefs.sf, _prefs.cr);
  radio_set_tx_power(_prefs.tx_power_dbm);

  updateAdvertTimer();
  updateFloodAdvertTimer();
}

bool SensorMesh::formatFileSystem() {
#if defined(NRF52_PLATFORM) || defined(STM32_PLATFORM)
    return InternalFS.format();
#elif defined(RP2040_PLATFORM)
    return LittleFS.format();
#elif defined(ESP32)
    return SPIFFS.format();
#else
    #error "need to implement file system erase"
    return false;
#endif
}

void SensorMesh::sendSelfAdvertisement(int delay_millis) {
  mesh::Packet* pkt = createSelfAdvert();
  if (pkt) {
    sendFlood(pkt, delay_millis);
  } else {
    MESH_DEBUG_PRINTLN("ERROR: unable to create advertisement packet!");
  }
}

void SensorMesh::updateAdvertTimer() {
  if (_prefs.advert_interval > 0) {  // schedule local advert timer
    next_local_advert = futureMillis( ((uint32_t)_prefs.advert_interval) * 2 * 60 * 1000);
  } else {
    next_local_advert = 0;  // stop the timer
  }
}
void SensorMesh::updateFloodAdvertTimer() {
  if (_prefs.flood_advert_interval > 0) {  // schedule flood advert timer
    next_flood_advert = futureMillis( ((uint32_t)_prefs.flood_advert_interval) * 60 * 60 * 1000);
  } else {
    next_flood_advert = 0;  // stop the timer
  }
}

void SensorMesh::setTxPower(uint8_t power_dbm) {
  radio_set_tx_power(power_dbm);
}

float SensorMesh::getTelemValue(uint8_t channel, uint8_t type) {
  auto buf = telemetry.getBuffer();
  uint8_t size = telemetry.getSize();
  uint8_t i = 0;

  while (i + 2 < size) {
    // Get channel #
    uint8_t ch = buf[i++];
    // Get data type
    uint8_t t = buf[i++];
    uint8_t sz = getDataSize(t);

    if (ch == channel && t == type) {
      return getFloat(&buf[i], sz, getMultiplier(t), isSigned(t));
    }
    i += sz;  // skip
  }
  return 0.0f;   // not found
}

bool  SensorMesh::getGPS(uint8_t channel, float& lat, float& lon, float& alt) {
  if (channel == TELEM_CHANNEL_SELF) {
    lat = sensors.node_lat;
    lon = sensors.node_lon;
    alt = sensors.node_altitude;
    return true;
  }
  // REVISIT: custom GPS channels??
  return false;
}

void SensorMesh::loop() {
  mesh::Mesh::loop();

  if (next_flood_advert && millisHasNowPassed(next_flood_advert)) {
    mesh::Packet* pkt = createSelfAdvert();
    if (pkt) sendFlood(pkt);

    updateFloodAdvertTimer();   // schedule next flood advert
    updateAdvertTimer();   // also schedule local advert (so they don't overlap)
  } else if (next_local_advert && millisHasNowPassed(next_local_advert)) {
    mesh::Packet* pkt = createSelfAdvert();
    if (pkt) sendZeroHop(pkt);

    updateAdvertTimer();   // schedule next local advert
  }

  uint32_t curr = getRTCClock()->getCurrentTime();
  if (curr >= last_read_time + SENSOR_READ_INTERVAL_SECS) {
    telemetry.reset();
    telemetry.addVoltage(TELEM_CHANNEL_SELF, (float)board.getBattMilliVolts() / 1000.0f);
    // query other sensors -- target specific
    sensors.querySensors(0xFF, telemetry);  // allow all telemetry permissions

    onSensorDataRead();

    last_read_time = curr;
  }

  // check the alert send queue
  if (num_alert_tasks > 0) {
    auto t = alert_tasks[0];   // process head of queue

    if (millisHasNowPassed(t->send_expiry)) {  // next send needed?
      if (t->attempt >= 4) {  // max attempts reached, try next contact
        t->curr_contact_idx++;
        if (t->curr_contact_idx >= num_contacts) {  // no more contacts to try?
          num_alert_tasks--;   // remove t from queue
          for (int i = 0; i < num_alert_tasks; i++) {
            alert_tasks[i] = alert_tasks[i + 1];
          }
        } else {
          auto c = &contacts[t->curr_contact_idx];
          uint16_t pri_mask = (t->pri == HIGH_PRI_ALERT) ? PERM_RECV_ALERTS_HI : PERM_RECV_ALERTS_LO;

          if (c->permissions & pri_mask) {   // contact wants alert
            // reset attempts
            t->attempt = (t->pri == LOW_PRI_ALERT) ? 3 : 0;   // Low pri alerts, start at attempt #3 (ie. only make ONE attempt)
            t->timestamp = getRTCClock()->getCurrentTimeUnique();   // need unique timestamp per contact

            sendAlert(c, t);  // NOTE: modifies attempt, expected_acks[] and send_expiry
          } else {
            // next contact tested in next ::loop()
          }
        }
      } else if (t->curr_contact_idx < num_contacts) {
        auto c = &contacts[t->curr_contact_idx];   // send next attempt
        sendAlert(c, t);  // NOTE: modifies attempt, expected_acks[] and send_expiry
      } else {
        // contact list has likely been modified while waiting for alert ACK, cancel this task
        t->attempt = 4;   // next ::loop() will remove t from queue
      }
    }
  }

  // is there are pending dirty contacts write needed?
  if (dirty_contacts_expiry && millisHasNowPassed(dirty_contacts_expiry)) {
    saveContacts();
    dirty_contacts_expiry = 0;
  }
}