;; each sentence is introduced by $ and terminated by *
;; first two chars are "talker id"
;; 3-5 are the message type
;; fields are comma-delimited

;; we can parse the message based only on type (i.e. the same type is
;; always parsed the same way no matter who the talker)

;; outline: split message into lines, extract values between
;; $ and * as talker, type, fields
;; then call the parser for the type

(import-macros { : define-tests : expect : expect= } :assert)

(fn split-line [line]
  (let [(talker msgtype rest) (string.match line "$(..)(...)(.+)*")
        fields (icollect [k _ (string.gmatch rest ",([^,]*)")] k)]
    { : talker :message-type msgtype :fields fields }))



(expect=
 (split-line "$GLGSV,2,1,07,78,47,295,34,77,35,208,19,88,36,267,30,87,67,025,27,1*74\n")
 {
  :talker "GL"
  :message-type "GSV"
  :fields ["2" "1" "07" "78" "47" "295" "34" "77" "35" "208" "19" "88"
           "36" "267" "30" "87" "67" "025" "27" "1"
           ]
  }
 )

(expect=
 (split-line "$GNGNS,111134.00,5131.348976,N,00005.551003,W,AAAANN,19,0.7,20.3,47.0,,,V*0C\n")
 {
  :talker "GN"
  :message-type "GNS"
  :fields ["111134.00" ; utc
           "5131.348976" "N"
           "00005.551003" "W"
           "AAAANN"
           "19" ; number of satellites
           "0.7" ; hdop
           "20.3" ; height
           "47.0" ; geoidal separation
           "" "" ; differential data age, reference station
           "V" ; not valid for navigation (?)
           ]
  }
 )

(fn try-number [s]
  (if (= s "")
      nil
      (tonumber s)))

(fn parse-coordinate [value sign]
  (if (= value "")
      nil
      (let [(deg min) (string.match value "(..)(.+)")]
        (*
         (+ (tonumber deg)
            (/ (tonumber min) 60))
         (case sign :N 1 :E 1 :S -1 :W -1)))))

(local
 msg-types
 {
  :GGA
  (fn [fields]
    (let [[utc lat latsign lon lonsign
           fix-quality
           total-space-vehicles
           hdop
           altitude _
           geoidal-separation _
           _ _ ] fields
          ]
      { :lat (parse-coordinate lat latsign)
        :lon (parse-coordinate lon lonsign)
        : fix-quality
        :total-space-vehicles (try-number total-space-vehicles)
        :hdop (try-number hdop)
        :altitude (try-number altitude)
        :geoidal-separation (try-number geoidal-separation)
        }))

  :RMC
  (fn [[utc valid lat latsign lon lonsign
        knots bearing-true date
        magn-var magn-var-sign
        mode nav-status &as fields]]
    (let [magnetic-variation (if (= magn-var "")
                                 nil
                                 (* (tonumber magn-var)
                                    (case magn-var-sign :E 1 :W -1)))]
      { : utc
        : valid
        :lat (parse-coordinate lat latsign)
        :lon (parse-coordinate lon lonsign)
        :speed-knots (try-number knots)
        :bearing-true (try-number bearing-true)
        : date
        : magnetic-variation ;;; XXX probably wrong
        : mode
        }))

  :GNS
  (fn [fields]
    (let [[utc lat latsign lon lonsign mode total-space-vehicles
           hdop altitude geoidal-separation _ _ nav-status] fields]
      { :lat (parse-coordinate lat latsign)
        :lon (parse-coordinate lon lonsign)
        : mode
        :total-space-vehicles (try-number total-space-vehicles)
        :hdop (try-number hdop)
        :altitude (try-number altitude)
        :geoidal-separation (try-number geoidal-separation)
        : nav-status
        }
      ))

  :VTG
  (fn [[bearing-true _ bearing-mag _ knots _ kmh _ mode]]
    {
     :bearing-true (try-number bearing-true)
     :bearing-magnetic (try-number bearing-mag)
     :speed-knots (try-number knots)
     :speed-kmh (try-number kmh)
     : mode
     }
    )
  })


(fn parse-line [line]
  (let [{ : message-type : talker : fields &as split} (split-line line)
        parser (. msg-types message-type)]
    (if parser
        (doto (parser fields)
          (tset :message-type message-type)
          (tset :talker talker))
        split)))

;; XXX this is wrong. 5131.348976 is 51 degrees 31.348976 minutes

(expect=
 (parse-line
  "$GNGNS,111134.00,5131.348976,N,00005.551003,W,AAAANN,19,0.7,20.3,47.0,,,V*0C\n")
 {:altitude 20.3
  :geoidal-separation 47
  :hdop 0.7
  :lat 51.522482933333
  :lon -0.092516716666667
  :message-type "GNS"
  :mode "AAAANN"
  :nav-status "V"
  :talker "GN"
  :total-space-vehicles 19})

(expect=
 (parse-line
  "$GNVTG,263.3,T,266.5,M,1.6,N,2.9,K,A*32\n")
 {
  :talker :GN
  :message-type :VTG
  :bearing-true 263.3
  :bearing-magnetic 266.5
  :speed-knots 1.6
  :speed-kmh 2.9
  :mode :A
  })

{ :parse parse-line }