ardupilot/libraries/AP_Scripting/modules/MAVLink/mavlink_msgs.lua

local mavlink_msgs = {}

function mavlink_msgs.get_msgid(msgname)
  local message_map = require("MAVLink/mavlink_msg_" .. msgname)
  if not message_map then
    error("Unknown MAVLink message " .. msgname)
  end
  return message_map.id
end

function mavlink_msgs.decode_header(message)
  -- build up a map of the result
  local result = {}

  local read_marker = 3

  -- id the MAVLink version
  result.protocol_version, read_marker = string.unpack("<B", message, read_marker)
  if (result.protocol_version == 0xFE) then -- mavlink 1
    result.protocol_version = 1
  elseif (result.protocol_version == 0XFD) then --mavlink 2
    result.protocol_version = 2
  else
    error("Invalid magic byte")
  end

  _, read_marker = string.unpack("<B", message, read_marker) -- payload is always the second byte

  -- strip the incompat/compat flags
  result.incompat_flags, result.compat_flags, read_marker = string.unpack("<BB", message, read_marker)

  -- fetch seq/sysid/compid
  result.seq, result.sysid, result.compid, read_marker = string.unpack("<BBB", message, read_marker)

  -- fetch the message id
  result.msgid, read_marker = string.unpack("<I3", message, read_marker)

  return result, read_marker
end

function mavlink_msgs.decode(message, msg_map)
  local result, offset = mavlink_msgs.decode_header(message)
  local message_map = require("MAVLink/mavlink_msg_" .. msg_map[result.msgid])
  if not message_map then
    -- we don't know how to decode this message, bail on it
    return nil
  end

  -- map all the fields out
  for _,v in ipairs(message_map.fields) do
    if v[3] then
      result[v[1]] = {}
      for j=1,v[3] do
        result[v[1]][j], offset = string.unpack(v[2], message, offset)
      end
    else
      result[v[1]], offset = string.unpack(v[2], message, offset)
    end
  end

  -- ignore the idea of a checksum

  return result;
end

function mavlink_msgs.encode(msgname, message)
  local message_map = require("MAVLink/mavlink_msg_" .. msgname)
  if not message_map then
    -- we don't know how to encode this message, bail on it
    error("Unknown MAVLink message " .. msgname)
  end

  local packString = "<"
  local packedTable = {}
  local packedIndex = 1
  for i,v in ipairs(message_map.fields) do
    if v[3] then
      packString = (packString .. string.rep(string.sub(v[2], 2), v[3]))
      for j = 1, v[3] do
        packedTable[packedIndex] = message[message_map.fields[i][1]][j]
        if packedTable[packedIndex] == nil then
          packedTable[packedIndex] = 0
        end
        packedIndex = packedIndex + 1
      end
    else
      packString = (packString .. string.sub(v[2], 2))
      packedTable[packedIndex] = message[message_map.fields[i][1]]
      packedIndex = packedIndex + 1
    end
  end
  return message_map.id, string.pack(packString, table.unpack(packedTable))
end

return mavlink_msgs