Navien.cpp

/*
Copyright (c) 2024 Hovhannes Tumanyan (htumanyan)
Copyright (c) 2025 David Carson (dacarson)

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

#include <Arduino.h>
#include <cstring>
#include "Navien.h"

const uint8_t Navien::COMMAND_HEADER[] = { 0xF7, 0x05, 0x0F, 0x50, 0x10, 0x0C, 0x4F };

const uint8_t Navien::ANNOUNCE_PACKET[] = {
    0xF7, 0x05, 0x0F, 0x50, 0x10, 0x03, 0x4A, 0x00, 0x01, 0x55};

bool Navien::seek_to_marker() {
  uint8_t byte;

  int availableBytes = available();
  for (int i = 0; i < availableBytes; i++) {
    byte = peek();
    if (byte == PACKET_MARKER)
      return true;

    read();
  }

  return false;
}

void Navien::parse_water() {
  if (recv_buffer.water.cmd_type != CMD_TYPE_WATER) {
    // Cascading units, have seen F7 13 50 52 10 03 40 00 04
    Serial.println("Unknown water packet");
    return;
  }

  uint8_t device_number = recv_buffer.hdr.packet_type - PACKET_TYPE_WATER_MIN;
  if (device_number > state.max_water_devices_seen)
    state.max_water_devices_seen = device_number;

  state.water[device_number].device_number = device_number;
  state.water[device_number].system_power = (recv_buffer.water.system_power & 0x5) ? 0x1 : 0x0;
  state.water[device_number].flow_state = recv_buffer.water.flow_state;
  state.water[device_number].consumption_active = (recv_buffer.water.flow_state & 0x20) ? 0x1 : 0x0;
  state.water[device_number].recirculation_running = (recv_buffer.water.flow_state & 0x08) ? 0x1 : 0x0;
  state.water[device_number].set_temp = Navien::t2c(recv_buffer.water.set_temp);
  state.water[device_number].outlet_temp = Navien::t2c(recv_buffer.water.outlet_temp);
  state.water[device_number].inlet_temp = Navien::t2c(recv_buffer.water.inlet_temp);
  state.water[device_number].display_metric        = (recv_buffer.water.system_status & 0x08) ? true : false;
  state.water[device_number].internal_recirculation = (recv_buffer.water.system_status & 0x01) ? true : false;
  state.water[device_number].external_recirculation = (recv_buffer.water.system_status & 0x02) ? true : false;
  state.water[device_number].operating_capacity = 0.5 * recv_buffer.water.operating_capacity;  // 0.5 increments
  state.water[device_number].flow_lpm = Navien::flow2lpm(recv_buffer.water.water_flow);
  state.water[device_number].recirculation_active = (recv_buffer.water.recirculation_enabled & 0x2) ? true : false;
  uint8_t stage = recv_buffer.water.system_stage;
  state.water[device_number].system_stage = stage;

  uint8_t upper_nibble = stage & 0xF0;
  state.water[device_number].stage_idle          = (upper_nibble == 0x10);
  state.water[device_number].stage_starting      = (upper_nibble == 0x20);
  state.water[device_number].stage_active        = (upper_nibble == 0x30);
  state.water[device_number].stage_shutting_down = (upper_nibble == 0x40);

  state.water[device_number].stage_standby           = (stage == 0x14);
  state.water[device_number].stage_demand            = (stage == 0x20);
  state.water[device_number].stage_pre_purge         = (stage == 0x29);
  state.water[device_number].stage_ignition          = (stage == 0x2B);
  state.water[device_number].stage_flame_on          = (stage == 0x2C);
  state.water[device_number].stage_ramp_up           = (stage == 0x2D);
  state.water[device_number].stage_active_combustion = (stage == 0x33);
  state.water[device_number].stage_water_adjustment  = (stage == 0x34);
  state.water[device_number].stage_flame_off         = (stage == 0x3C);
  state.water[device_number].stage_post_purge_1     = (stage == 0x46);
  state.water[device_number].stage_post_purge_2     = (stage == 0x47);
  state.water[device_number].stage_dhw_wait         = (stage == 0x49);

  state.water[device_number].system_active   = recv_buffer.water.system_active ? true : false;
  state.water[device_number].operation_time = (uint16_t)recv_buffer.water.operation_time_hi << 8 | recv_buffer.water.operation_time_lo;

  // If we see 10+ water packets since the last navilink packet, assume that 
  // it is no longer present.
  if (water_packets_since_announce < 10) {
    water_packets_since_announce++;
  } else {
    navilink_present = false;
    state.announce.navilink_present = false;
  }

  if (on_water_packet_cb) on_water_packet_cb(&state.water[device_number]);
}

void Navien::parse_gas() {
  if (recv_buffer.gas.cmd_type != CMD_TYPE_GAS) {
    Serial.println("Unknown gas packet");
    return;
  }
  state.gas.set_temp = Navien::t2c(recv_buffer.gas.set_temp);
  state.gas.outlet_temp = Navien::t2c(recv_buffer.gas.outlet_temp);
  state.gas.inlet_temp = Navien::t2c(recv_buffer.gas.inlet_temp);

  char buffer[10];

  sprintf(buffer, "%d.%d", recv_buffer.gas.controller_version_hi, recv_buffer.gas.controller_version_lo);
  state.gas.controller_version = atof(buffer);
  sprintf(buffer, "%d.%d", recv_buffer.gas.panel_version_hi, recv_buffer.gas.panel_version_lo);
  state.gas.panel_version = atof(buffer);

  uint32_t raw_gas = ((uint32_t)recv_buffer.gas.cumulative_gas_b3 << 24 |
                      (uint32_t)recv_buffer.gas.cumulative_gas_b2 << 16 |
                      (uint32_t)recv_buffer.gas.cumulative_gas_hi << 8  |
                      recv_buffer.gas.cumulative_gas_lo);
  state.gas.accumulated_gas_usage = 0.1f * raw_gas;

  uint32_t raw_water = ((uint32_t)recv_buffer.gas.cumulative_water_usage_b3 << 24 |
                        (uint32_t)recv_buffer.gas.cumulative_water_usage_b2 << 16 |
                        (uint32_t)recv_buffer.gas.cumulative_water_usage_hi << 8  |
                        recv_buffer.gas.cumulative_water_usage_lo);
  state.gas.accumulated_water_usage = 0.1f * raw_water;

  state.gas.current_gas_usage = (uint16_t)recv_buffer.gas.current_gas_hi << 8 | recv_buffer.gas.current_gas_lo;
  state.gas.target_gas_usage = (uint16_t)recv_buffer.gas.target_burner_power_hi << 8 | recv_buffer.gas.target_burner_power_lo;

  uint32_t raw_time = ((uint32_t)recv_buffer.gas.total_operating_time_b3 << 24 |
                       (uint32_t)recv_buffer.gas.total_operating_time_b2 << 16 |
                       (uint32_t)recv_buffer.gas.total_operating_time_hi << 8  |
                       recv_buffer.gas.total_operating_time_lo);
  state.gas.total_operating_time = 60 * raw_time;  // hours -> minutes

  state.gas.elapsed_install_days = (uint16_t)recv_buffer.gas.elapsed_install_days_hi << 8 | recv_buffer.gas.elapsed_install_days_lo;

  // Convert domestic usage count using 32-bit arithmetic
  uint32_t raw_usage = ((uint32_t)recv_buffer.gas.cumulative_domestic_usage_cnt_hi << 8 |
                        recv_buffer.gas.cumulative_domestic_usage_cnt_lo);
  state.gas.accumulated_domestic_usage_cnt = 10 * raw_usage;
  state.gas.recirculation_enabled = recv_buffer.gas.recirculation_enabled ? true : false;

  if (on_gas_packet_cb) on_gas_packet_cb(&(state.gas));
}

void Navien::parse_status_packet() {
  uint8_t pkt_type = recv_buffer.hdr.packet_type;

  if (pkt_type >= PACKET_TYPE_WATER_MIN && pkt_type <= PACKET_TYPE_WATER_MAX) {
    parse_water();
    return;
  }
  switch (pkt_type) {
    case Navien::PACKET_TYPE_GAS:
      parse_gas();
      break;
    default:
      if (on_error_cb)
        on_error_cb(__func__, "Unknown status packet type received.");
      Navien::print_buffer(recv_buffer.raw_data, recv_buffer.hdr.len + HDR_SIZE, on_error_cb);
      break;
  }
}

void Navien::parse_announce(bool from_local_send) {
  if (recv_buffer.announce.cmd_type != CMD_TYPE_ANNOUNCE) {
    Serial.println("Unknown announce packet");
    return;
  }
  //Navien::print_buffer(recv_buffer.raw_data, recv_buffer.hdr.len + HDR_SIZE, on_error_cb);

  size_t plen = HDR_SIZE + recv_buffer.hdr.len + 1;
  if (last_announced_raw_len == plen && plen <= sizeof(last_announced_raw) &&
      last_announce_callback_ms != 0 &&
      (unsigned long)(millis() - last_announce_callback_ms) <= ANNOUNCE_CALLBACK_DEDUPE_MS &&
      memcmp(last_announced_raw, recv_buffer.raw_data, plen) == 0) {
    return;
  }

  if (!from_local_send) {
    // If there are any announce packets seen, then there is a navilink present
    navilink_present = true;
    state.announce.navilink_present = true;
    water_packets_since_announce = 0;
  }

  if (plen <= sizeof(last_announced_raw)) {
    memcpy(last_announced_raw, recv_buffer.raw_data, plen);
    last_announced_raw_len = (uint8_t)plen;
    last_announce_callback_ms = millis();
  }

  if (on_announce_packet_cb) on_announce_packet_cb(&(state));
}

void Navien::parse_command(bool from_local_send) {
  (void)from_local_send;
  if (recv_buffer.cmd.cmd_type != CMD_TYPE_CMD) {
    Serial.println("Unknown command packet");
    return;
  }
  //Navien::print_buffer(recv_buffer.raw_data, recv_buffer.hdr.len + HDR_SIZE, on_error_cb);

  memset(&(state.command), 0x0, sizeof(state.command));

  if (recv_buffer.cmd.system_power == Navien::SYSTEM_POWER_ON) {
    state.command.power_command = true;
    state.command.power_on = true;
  } else if (recv_buffer.cmd.system_power == Navien::SYSTEM_POWER_OFF) {
    state.command.power_command = true;
    state.command.power_on = false;
  }

  if (recv_buffer.cmd.set_temp > 0) {
    state.command.set_temp_command = true;
    state.command.set_temp = (float)recv_buffer.cmd.set_temp / 2.0;
  }

  if (recv_buffer.cmd.hot_button_recirculation & Navien::HOT_BUTTON_DOWN) {
    state.command.hot_button_command = true;
  }

  if (recv_buffer.cmd.hot_button_recirculation & Navien::RECIRCULATION_ON) {
    state.command.recirculation_command = true;
    state.command.recirculation_on = true;
  } else if (recv_buffer.cmd.hot_button_recirculation & Navien::RECIRCULATION_OFF) {
    state.command.recirculation_command = true;
    state.command.recirculation_on = false;
  }

  state.command.cmd_data = recv_buffer.cmd.cmd_data;

  if (on_command_packet_cb) on_command_packet_cb(&(state));
}

void Navien::parse_control_packet(bool from_local_send) {
  switch (recv_buffer.cmd.cmd_type) {
    case Navien::CONTROL_ANNOUNCE:
      parse_announce(from_local_send);
      break;
    case Navien::CONTROL_COMMAND:
      parse_command(from_local_send);
      break;
    default:
      if (on_error_cb)
        on_error_cb(__func__, "Unknown control packet type received.");
      Navien::print_buffer(recv_buffer.raw_data, recv_buffer.hdr.len + HDR_SIZE, on_error_cb);
      break;
  }
}

void Navien::parse_packet() {
  uint8_t crc_c = 0x00;  // Computed CRC
  uint8_t crc_r = 0x00;  // Received CRC

  char errBuffer[255];

  //Navien::print_buffer(recv_buffer.raw_data, HDR_SIZE + recv_buffer.hdr.len + 1);
  crc_r = recv_buffer.raw_data[HDR_SIZE + recv_buffer.hdr.len];

  switch (recv_buffer.hdr.direction) {
    case Navien::PACKET_DIRECTION_STATUS:
      crc_c = Navien::checksum(recv_buffer.raw_data, HDR_SIZE + recv_buffer.hdr.len, CHECKSUM_SEED_4B);
      if (crc_c != crc_r) {
        sprintf(errBuffer, "Status Packet checksum error: 0x%02X (calc) != 0x%02X (recv)", crc_c, crc_r);
        if (on_error_cb)
          on_error_cb(__func__, errBuffer);
        Navien::print_buffer(recv_buffer.raw_data, recv_buffer.hdr.len + HDR_SIZE, on_error_cb);
        break;
      }
      parse_status_packet();
      break;
    case Navien::PACKET_DIRECTION_CONTROL:
      crc_c = Navien::checksum(recv_buffer.raw_data, HDR_SIZE + recv_buffer.hdr.len, CHECKSUM_SEED_62);
      if (crc_c != crc_r) {
        sprintf(errBuffer, "Control Packet checksum error: 0x%02X (calc) != 0x%02X (recv)", crc_c, crc_r);
        if (on_error_cb)
          on_error_cb(__func__, errBuffer);
        break;
      }
      size_t recv_len = HDR_SIZE + recv_buffer.hdr.len + 1;
      // Fixed periodic announce: match raw bytes only (avoid struct/overlay quirks on cmd_type).
      if (last_periodic_announce_time != 0 && recv_len == sizeof(ANNOUNCE_PACKET) &&
          memcmp(recv_buffer.raw_data, ANNOUNCE_PACKET, recv_len) == 0 &&
          (unsigned long)(millis() - last_periodic_announce_time) <= OWN_ANNOUNCE_ECHO_SUPPRESS_MS) {
        break;
      }
      // Suppress processing of our own command/announce packet echoed back from RS485.
      if (last_sent_control_packet_len > 0) {
        unsigned long elapsed = millis() - last_sent_control_packet_time;
        if (elapsed <= LOCAL_ECHO_FILTER_WINDOW_MS &&
            recv_len == last_sent_control_packet_len &&
            memcmp(recv_buffer.raw_data, last_sent_control_packet.raw_data, recv_len) == 0) {
          break;
        }
      }
      parse_control_packet(false);
      break;
  }
}


void Navien::loop() {
  uint8_t byte;
  char errBuffer[255];
  int availableBytes = available();

  // Check if we should send a command
  if (recv_state == INITIAL && can_send(availableBytes)) {
    send_cmd();
    availableBytes = available();
  }

  if (!availableBytes) {
    maybe_send_periodic_announce();
    return;
  }

  // Disable test_mode if we're getting real responses
  if (recv_state != INITIAL && test_mode) {
    test_mode = false;
  }

  unsigned long startMillis = millis();
  int loopIterations = 0;
  const int MAX_LOOP_ITERATIONS = 100;

  while (availableBytes && loopIterations++ < MAX_LOOP_ITERATIONS) {
    switch (recv_state) {
      case INITIAL:
        if (seek_to_marker()) {
          recv_state = MARKER_FOUND;
          break;
        }
        // No marker found — wait for more bytes
        return;

      case MARKER_FOUND:
        availableBytes = available();
        if (availableBytes < HDR_SIZE)
          return;

        if (!read(recv_buffer.raw_data, HDR_SIZE)) {
          sprintf(errBuffer, "Failed to read header: %d when told %d is available", HDR_SIZE, availableBytes);
          if (on_error_cb)
            on_error_cb(__func__, errBuffer);
          recv_state = INITIAL;
          return;
        }

        recv_state = HEADER_PARSED;

        if (recv_buffer.hdr.len == 0xFF || recv_buffer.hdr.len >= sizeof(PACKET_BUFFER)) {
          if (recv_buffer.hdr.len == 0xFF) {
            if (on_error_cb)
              on_error_cb(__func__, "Invalid header length, are the 485 wires reversed?");
          } else if (on_error_cb) {
            sprintf(errBuffer, "Buffer too small for packet length data, dropping packet. len: %d, size: %d", recv_buffer.hdr.len, sizeof(PACKET_BUFFER) - 1);
            on_error_cb(__func__, errBuffer);
          }
          recv_state = INITIAL;
          return;
        }
        break;

      case HEADER_PARSED: {
        availableBytes = available();
        uint8_t len = recv_buffer.hdr.len + 1;

        if (availableBytes < len)
          return;

        if (!read(recv_buffer.raw_data + HDR_SIZE, len)) {
          sprintf(errBuffer, "Failed to read %d bytes when told %d is available", len, availableBytes);
          if (on_error_cb)
            on_error_cb(__func__, errBuffer);
          recv_state = INITIAL;
          return;
        }

        parse_packet();
        availableBytes = available();
        recv_state = INITIAL;
        // Track when we complete a packet for collision avoidance
        last_packet_complete_time = millis();
        break;
      }

      default:
        recv_state = INITIAL;
        break;
    }

    // Check elapsed time or max loop iterations
    if (millis() - startMillis > 100) break;

    yield();  // Let the ESP32 breathe for WDT
    availableBytes = available(); // refresh at end
  }
}

/**
 * Convert flow units to liters/min values
 * flow is reported as 0.1 liter units.
 */
float Navien::flow2lpm(uint8_t f) {
  return (float)f / 10.f;
}

float Navien::t2c(uint8_t c) {
  return (float)c / 2.f;
}


void Navien::print_buffer(const uint8_t *data, size_t length, ErrorCallbackFunction on_error_cb) {
  char hex_buffer[100];
  hex_buffer[(3 * 32) + 1] = 0;
  for (size_t i = 0; i < length; i++) {
    snprintf(&hex_buffer[3 * (i % 32)], sizeof(hex_buffer), "%02X ", data[i]);
    if (i % 32 == 31) {
      if (on_error_cb)
        on_error_cb(__func__, hex_buffer);
    }
  }
  if (length % 32) {
    // null terminate if incomplete line
    hex_buffer[3 * (length % 32) + 2] = 0;
    if (on_error_cb)
      on_error_cb(__func__, hex_buffer);
  }
}

int Navien::enqueue_send_cmd(const PACKET_BUFFER& pkt) {
  if (send_queue_count >= QUEUE_CAPACITY)
    return -1;
  send_array[send_queue_tail] = pkt;
  send_queue_tail = (send_queue_tail + 1) % QUEUE_CAPACITY;
  ++send_queue_count;
  return 1;
}

int Navien::queue_send_cmd(const PACKET_BUFFER& pkt) {
  if (!test_mode && !navilink_present && last_periodic_announce_time == 0 &&
      pkt.cmd.cmd_type == CONTROL_COMMAND) {
    return -1;
  }
  return enqueue_send_cmd(pkt);
}

bool Navien::can_send(int curr_available) {
  static unsigned long last_received = 0;

  // If there's data available, we're receiving something - don't send
  if (curr_available > 0) {
    last_received = millis();
    return false;
  }

  // Don't send if we're in the middle of parsing a packet
  // (wait until we're back to INITIAL state)
  if (recv_state != INITIAL) {
    return false;
  }

  // Wait for sufficient silence after last byte received
  // This ensures we're not in the middle of a packet transmission
  // At 19200 baud, even the longest packets (~55 bytes) take ~29ms to transmit
  // 50ms provides a safe margin to ensure we're between packets
  unsigned long silence_duration = millis() - last_received;
  if (silence_duration < BUS_SILENCE_MS) {
    return false;
  }

  // Also check time since last complete packet was parsed
  // This gives us an additional safety margin to avoid collisions
  if (last_packet_complete_time > 0) {
    unsigned long time_since_packet = millis() - last_packet_complete_time;
    if (time_since_packet < PACKET_GAP_MS) {
      return false;
    }
  }

  return true;
}

void Navien::maybe_send_periodic_announce() {
  if (test_mode || navilink_present || periodic_announce_pending)
    return;

  unsigned long now = millis();
  if (last_periodic_announce_time != 0 &&
      (unsigned long)(now - last_periodic_announce_time) < PERIODIC_ANNOUNCE_INTERVAL_MS)
    return;

  PACKET_BUFFER buf{};
  memcpy(buf.raw_data, ANNOUNCE_PACKET, sizeof(ANNOUNCE_PACKET));
  if (enqueue_send_cmd(buf) < 0)
    return;
  periodic_announce_pending = true;
}

int Navien::send_cmd() {

  if (send_queue_count == 0)
    return -1;
  PACKET_BUFFER send_buffer = send_array[send_queue_head];
  send_queue_head = (send_queue_head + 1) % QUEUE_CAPACITY;
  --send_queue_count;

  // +1 to include the crc value
  int len = HDR_SIZE + send_buffer.hdr.len + 1;

  // Defer control commands until we've successfully transmitted an announce (bus takeover order).
  if (!test_mode && !navilink_present && last_periodic_announce_time == 0 &&
      send_buffer.cmd.cmd_type == CONTROL_COMMAND) {
    enqueue_send_cmd(send_buffer);
    return -1;
  }

  // Rate limiting: enforce minimum 2 second interval between consecutive
  // CONTROL_COMMANDs. This is needed for commands like recirculation and
  // hotButton that send two commands in sequence (command + follow-up).
  // Does not apply after an announce, only after a prior control command.
  if (last_command_sent_time > 0 &&
      send_buffer.cmd.cmd_type == CONTROL_COMMAND &&
      last_sent_control_packet.cmd.cmd_type == CONTROL_COMMAND) {
    unsigned long time_since_last = millis() - last_command_sent_time;
    if (time_since_last < MIN_COMMAND_INTERVAL_MS) {
      // Not enough time has passed, re-queue the command
      enqueue_send_cmd(send_buffer);
      return -1;
    }
  }

  int sent_len = -1;
  // Allow sending only when NaviLink is not present.
  if (!navilink_present) {
    // Double-check the bus is still clear right before sending
    // This is a final safety check to avoid collisions
    if (can_send(available())) {
      sent_len = write(send_buffer.raw_data, len);
      if (sent_len == len) {
        last_command_sent_time = millis();
        last_sent_control_packet = send_buffer;
        last_sent_control_packet_len = len;
        last_sent_control_packet_time = millis();
        recv_buffer = send_buffer;
        if (send_buffer.cmd.cmd_type == CONTROL_ANNOUNCE) {
          periodic_announce_pending = false;
          last_periodic_announce_time = millis();
        }
        parse_control_packet(true);
      }
    } else if (on_error_cb) {
      on_error_cb(__func__, "Bus not clear for transmission, command queued again");
      enqueue_send_cmd(send_buffer);
      return -1;
    }
  } else if (on_error_cb) {
    on_error_cb(__func__, "Failed to send the command (navilink present):");
    Navien::print_buffer(send_buffer.raw_data, len, on_error_cb);
    if (send_buffer.cmd.cmd_type == CONTROL_ANNOUNCE)
      periodic_announce_pending = false;
  }

  return sent_len;
}

int Navien::power(bool power_on) {
  PACKET_BUFFER send_buffer{};
  memcpy(&send_buffer, COMMAND_HEADER, sizeof(COMMAND_HEADER));

  if (power_on) {
    send_buffer.cmd.system_power = 0x0a;
  } else {
    send_buffer.cmd.system_power = 0x0b;
  }
  uint8_t crc = Navien::checksum(send_buffer.raw_data, HDR_SIZE + send_buffer.hdr.len, CHECKSUM_SEED_62);
  send_buffer.raw_data[HDR_SIZE + send_buffer.hdr.len] = crc;

  if (test_mode) {
    state.water[0].system_power = power_on;
    return(HDR_SIZE + send_buffer.hdr.len);
  }

  // Queue the command
  return queue_send_cmd(send_buffer);
}


int Navien::setTemp(float temp_degC) {
  PACKET_BUFFER send_buffer{};
  memcpy(&send_buffer, COMMAND_HEADER, sizeof(COMMAND_HEADER));

  send_buffer.cmd.set_temp = int(temp_degC * 2.0);

  uint8_t crc = Navien::checksum(send_buffer.raw_data, HDR_SIZE + send_buffer.hdr.len, CHECKSUM_SEED_62);
  send_buffer.raw_data[HDR_SIZE + send_buffer.hdr.len] = crc;

  if (test_mode) {
    state.gas.set_temp = temp_degC;
    state.water[0].set_temp = temp_degC;
    return(HDR_SIZE + send_buffer.hdr.len);
  }

  // Queue the command
  return queue_send_cmd(send_buffer);
}

uint8_t Navien::generateCmdData() {
  // Use time-of-day seconds to generate cmd_data
  // Increments every 39 seconds, rolls over every ~2.75 hours
  time_t now = time(nullptr);
  return (uint8_t)(now / 39);
}

int Navien::hotButton() {
  uint8_t cmdData = generateCmdData();
  PACKET_BUFFER send_buffer{};
  memcpy(&send_buffer, COMMAND_HEADER, sizeof(COMMAND_HEADER));

  send_buffer.cmd.hot_button_recirculation = Navien::HOT_BUTTON_DOWN;
  send_buffer.cmd.cmd_data = cmdData;

  uint8_t crc = Navien::checksum(send_buffer.raw_data, HDR_SIZE + send_buffer.hdr.len, CHECKSUM_SEED_62);
  send_buffer.raw_data[HDR_SIZE + send_buffer.hdr.len] = crc;

  // Queue the command
  int sent_len = queue_send_cmd(send_buffer);

  // queue up the button release command
  memcpy(&send_buffer, COMMAND_HEADER, sizeof(COMMAND_HEADER));
  send_buffer.cmd.hot_button_recirculation = 0x00;
  send_buffer.cmd.cmd_data = cmdData;
  crc = Navien::checksum(send_buffer.raw_data, HDR_SIZE + send_buffer.hdr.len, CHECKSUM_SEED_62);
  send_buffer.raw_data[HDR_SIZE + send_buffer.hdr.len] = crc;

  return queue_send_cmd(send_buffer);
}

int Navien::recirculation(bool recirc_on) {
  uint8_t cmdData = generateCmdData();
  PACKET_BUFFER send_buffer{};
  memcpy(&send_buffer, COMMAND_HEADER, sizeof(COMMAND_HEADER));

  send_buffer.cmd.hot_button_recirculation = recirc_on ? Navien::RECIRCULATION_ON : Navien::RECIRCULATION_OFF;
  send_buffer.cmd.cmd_data = cmdData;

  uint8_t crc = Navien::checksum(send_buffer.raw_data, HDR_SIZE + send_buffer.hdr.len, CHECKSUM_SEED_62);
  send_buffer.raw_data[HDR_SIZE + send_buffer.hdr.len] = crc;

  // Queue the first command
  int sent_len = queue_send_cmd(send_buffer);

  // queue up the follow-up command (needed for recirculation to work)
  // This command clears the recirculation command flag
  PACKET_BUFFER follow_up_buffer{};
  memcpy(&follow_up_buffer, COMMAND_HEADER, sizeof(COMMAND_HEADER));
  follow_up_buffer.cmd.hot_button_recirculation = 0x00;
  follow_up_buffer.cmd.cmd_data = cmdData;

  crc = Navien::checksum(follow_up_buffer.raw_data, HDR_SIZE + follow_up_buffer.hdr.len, CHECKSUM_SEED_62);
  follow_up_buffer.raw_data[HDR_SIZE + follow_up_buffer.hdr.len] = crc;

  if (test_mode) {
    state.water[0].recirculation_active = recirc_on;
    state.water[0].recirculation_running = recirc_on;
    state.gas.current_gas_usage = recirc_on ? 200 : 0;
    state.water[0].operating_capacity = recirc_on ? 15 : 0;
    sent_len = HDR_SIZE + follow_up_buffer.hdr.len;
    // In test mode, send immediately without delay
    return queue_send_cmd(follow_up_buffer);
  }

  return queue_send_cmd(follow_up_buffer);
}

uint8_t Navien::checksum(const uint8_t *buffer, uint8_t len, uint16_t seed) {
  uint16_t result;

  if (len < 2) {
    result = 0x00;
  } else {
    result = 0xff;

    for (int i = 0; i < len; i++) {
      result = result << 1;

      if (result > 0xff) {
        result = (result & 0xff) ^ seed;
      }

      // this is important!!
      // the checksum is calculated
      // based on the lower byte, i.e.
      // only the lower byte is XOR-ed
      result = ((uint8_t)result) ^ (uint16_t)buffer[i];
    }
  }
  return result;
}