local socket = require "socket"
local cjson = require "cjson.safe"

local last_error_msg = {}
local last_watch_msg = {}
local last_devices_msg = {}

function gpsd_setup()
    local s, err = socket.tcp4()
    if err ~= nil then return nil, err end
    s:settimeout(1)

    _, err = s:connect('127.0.0.1', 2947)
    if err ~= nil then return nil, err end

    local msg_json, err = s:receive('*l')
    if err ~= nil then
        s:close()
        return nil, err
    end

    local msg, err = cjson.decode(msg_json)
    if err ~= nil then
        s:close()
        return nil, err
    end
    if msg['class'] ~= 'VERSION' then
        s:close()
        return nil, "got unexpected message class from GPSD: " .. tostring(msg['class'])
    end

    _, err = s:send('?WATCH={"enable":"true"}')
    if err ~= nil then
        s:close()
        return nil, err
    end

    return s, msg
end

function gpsd_poll(s)
    _, err = s:send('?DEVICES;')
    if err ~= nil then
        s:close()
        return nil, err
    end
    _, err = s:send('?POLL;')
    if err ~= nil then
        s:close()
        return nil, err
    end

    while true do
        local msg_json, err = s:receive('*l')
        if err ~= nil then
            s:close()
            return nil, err
        end

        local msg, err = cjson.decode(msg_json)
        if err ~= nil then
            s:close()
            return nil, err
        end

        if msg['class'] == 'ERROR' then
            last_error_msg = msg
        elseif msg['class'] == 'WATCH' then
            last_watch_msg = msg
        elseif msg['class'] == 'DEVICES' then
            last_devices_msg = msg
        elseif msg['class'] == 'POLL' then
            return s, msg
        else
            print("ignoring unknown message class from GPSD: " .. tostring(msg['class']))
        end
    end
end

function metric_version_info(version_info)
    local labels = {}
    labels['rev'] = version_info['rev']
    labels['release'] = version_info['release']
    labels['proto_major'] = version_info['proto_major']
    labels['proto_minor'] = version_info['proto_minor']

    metric("gpsd_version_info", "gauge", labels, 1)
end

function metric_devices(devices_msg)
    if devices_msg['devices'] == nil then return end
    for _, dev in ipairs(devices_msg['devices']) do
        local labels = {}
        labels['device'] = dev['path']
        labels['driver'] = dev['driver']
        labels['subtype'] = dev['subtype']
        labels['subtype1'] = dev['subtype1']

        metric("gpsd_device_info", "gauge", labels, 1)
    end
end

function metric_poll(poll_msg)
    for _, tpv in ipairs(poll_msg['tpv']) do
        local labels = {}
        labels['device'] = tpv['device']

        metric("gpsd_tpv_mode", "gauge", labels, tpv['mode'])
        if tpv['alt'] ~= nil then
            metric("gpsd_tpv_altitude_meters", "gauge", labels, tpv['alt'])
        end
        if tpv['lat'] ~= nil then
            metric("gpsd_tpv_latitude_degrees", "gauge", labels, tpv['lat'])
        end
        if tpv['lon'] ~= nil then
            metric("gpsd_tpv_longitude_degrees", "gauge", labels, tpv['lon'])
        end
    end
end

local s, version_info = gpsd_setup()

local function scrape()
    if s == nil then
        print("lost connection to gpsd, reconncting...")
        s, version_info = gpsd_setup()
        if s == nil then return s, version_info end
    end
    metric_version_info(version_info)

    s, poll_msg = gpsd_poll(s)
    if s == nil then return s, poll end

    metric_devices(last_devices_msg)
    metric_poll(poll_msg)
end

return { scrape = scrape }