centre map on loaded track

frontend/src/Main.elm

@@ -166,7 +166,7 @@ init _ url navKey =
                 Just (Timeline (Just s) (Just d)) -> (s, d)
                 _ -> (10,10)
     in
-        ((Model (toCoord 51.60 -0.01) (FineZoomLevel (13*8)) None 0 0 Empty),
+        ((Model (toCoord 0 0) (FineZoomLevel (1*8)) None 0 0 Empty),
              (fetchTrack start duration))
 
 -- SUBSCRIPTIONS
@@ -198,6 +198,11 @@ type Msg
   | UrlChanged
   | Dribble String
 
+
+coordFromPos p =
+    let {lat, lon } = p
+    in toCoord lat lon
+
 update : Msg -> Model -> (Model, Cmd Msg)
 
 update msg model = (updateModel msg model, Cmd.none)
@@ -243,6 +248,8 @@ updateModel msg model =
                     in
                     { model
                         | track = Present trk
+                        , centre = coordFromPos (Point.centre trk)
+                        , zoom = FineZoomLevel ( 13  * 8)
                         , startTime = floor start
                         , duration = ceiling duration
                     }

frontend/src/Point.elm

@@ -1,4 +1,4 @@
-module Point exposing(Pos, Point, decoder, downsample, duration, subseq, startTime)
+module Point exposing(Pos, Point, decoder, downsample, duration, subseq, startTime, centre)
 
 import Json.Decode as D
 
@@ -68,6 +68,30 @@ startTime points =
         (p::ps) -> Just p.time
         _ -> Nothing
 
+type Bound = Bound Pos Pos | NoBound
+
+extendBound : Pos -> Bound -> Bound
+extendBound pos b =
+    let {lat, lon} = pos
+    in case b of
+           (Bound p1 p2) ->
+               Bound
+               (Pos (min lat p1.lat)  (min lon p1.lon) Nothing)
+               (Pos (max lat p2.lat)  (max lon p2.lon) Nothing)
+           NoBound ->
+               Bound pos pos
+
+bounds points =
+    List.foldr extendBound NoBound (List.map .pos points)
+
+centre points =
+    case bounds points of
+        Bound min max -> Pos
+                         ((max.lat + min.lat) / 2)
+                         ((max.lon + min.lon) / 2)
+                             Nothing
+        NoBound -> Pos 0 0 Nothing
+
 subseq points start dur =
     case points of
         [] -> []