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+port module Main exposing (..)
+
+import Browser
+import Html exposing (Html, button, div, text, br, input, img)
+import Html.Attributes exposing (style, class, value, placeholder, src)
+import Html.Events exposing (onInput, onClick)
+import Html.Lazy exposing (lazy)
+import List exposing (foldr)
+
+import Protocol exposing (decodeState, encodeState)
+import Json.Decode as D
+
+
+main = Browser.document
+    { init = init
+    , update = update
+    , view = view
+    , subscriptions = subscriptions
+    }
+
+ -- subscribe to messages from the websocket
+subscriptions : Model -> Sub Msg
+subscriptions model = recvPort GotMessage
+
+ -- corresponding ports for the subscriptions
+port recvPort : (String -> msg) -> Sub msg
+port sendPort : String -> Cmd msg
+
+
+{- STATE AND STATE TRANSITIONS -}
+
+ -- the main client state
+type alias Model = { slide : Int, max : Int }
+
+ -- start at the first slide, for now just assume that we have ten slides
+init : () -> (Model, Cmd Msg)
+init f = ( { slide = 0, max = 10 }, Cmd.none)
+
+ -- possible state transitions:
+type Msg = GotMessage String
+         | NextSlide
+         | PrevSlide
+
+ -- update stuff
+update : Msg -> Model -> (Model, Cmd Msg)
+update msg model = case msg of
+    NextSlide       -> updateSlide ((+) 1) model
+    PrevSlide       -> updateSlide ((+) -1) model
+    GotMessage text ->
+        case decodeState text of
+          Nothing -> (model, Cmd.none)
+          Just newstate -> ({model | slide = newstate.state }, Cmd.none)
+
+ -- while changing slides we must take care not to end up in an invalid
+ -- state (where slide > max), so this logic gets its own function
+tweakSlide : (Int -> Int) -> Model -> Model
+tweakSlide f model = { model | slide = modBy model.max (f model.slide) }
+
+updateSlide : (Int -> Int) -> Model -> (Model, Cmd Msg)
+updateSlide f model = 
+    let new = tweakSlide f model
+    in (new, sendPort (encodeState { state = new.slide }))
+
+{- VIEW COMPONENTS -}
+
+ -- our body consists of a slide container and a couple controls
+body : Model -> Html Msg
+body model = div []
+        [ div [class "slides"] (slideView model.slide model.max)
+        , div [class "controls"] 
+            [ button [ onClick PrevSlide ] [ text "←" ]
+            , text (String.fromInt (model.slide+1))
+            , button [ onClick NextSlide ] [ text "→" ]
+            ]
+        ]
+
+ -- the slide view is just a list of img tags, one for each slide; note
+ -- that these are all loaded on startup, with visibility done through
+ -- z indices so we won't have lags while switching between them.
+slideView : Int -> Int -> List (Html Msg)
+slideView i max = List.range 0 (max - 1)
+    |> List.map (\x -> ("example/" ++ (padNum 2 (x+1)) ++ ".png", x == i))
+    |> List.map (\(path,t) -> img [src path, onTop t] [])
+
+onTop t = style "z-index" (String.fromInt (if t then 10 else 0))
+
+ -- format an int with prefixed 0's — apparently elm doesn't have a
+ -- standard function for that, but it's needed for slide filenames
+padNum : Int -> Int -> String
+padNum l num = 
+  let str = String.fromInt num
+  in if (String.length str) > l then str else
+    (String.repeat (l - String.length str) "0") ++ str
+
+
+view : Model -> Browser.Document Msg
+view model = { title = "Websockets example"
+             , body = [body model]
+             }
+
+