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Task/Animate-a-pendulum/Elm/animate-a-pendulum.elm

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+import Color exposing (..)
+import Collage exposing (..)
+import Element exposing (..)
+import Html exposing (..)
+import Time exposing (..)
+import Html.App exposing (program)
+
+dt = 0.01
+scale = 100
+
+type alias Model =
+  { angle : Float
+  , angVel : Float
+  , length : Float
+  , gravity : Float
+  }
+
+type Msg
+    = Tick Time
+
+init : (Model,Cmd Msg)
+init =
+  ( { angle = 3 * pi / 4
+    , angVel = 0.0
+    , length = 2
+    , gravity = -9.81
+    }
+  , Cmd.none)
+
+update : Msg -> Model -> (Model, Cmd Msg)
+update _ model =
+  let
+    angAcc = -1.0 * (model.gravity / model.length) * sin (model.angle)
+    angVel' = model.angVel + angAcc * dt
+    angle' = model.angle + angVel' * dt
+  in
+    ( { model
+        | angle = angle'
+        , angVel = angVel'
+      }
+    , Cmd.none )
+
+view : Model -> Html Msg
+view model =
+  let
+    endPoint = ( 0, scale * model.length )
+    pendulum =
+      group
+        [ segment ( 0, 0 ) endPoint
+            |> traced { defaultLine | width = 2, color = red }
+        , circle 8
+            |> filled blue
+        , ngon 3 10
+            |> filled green
+            |> rotate (pi/2)
+            |> move endPoint
+        ]
+  in
+    toHtml <|
+      collage 700 500
+        [ pendulum |> rotate model.angle ]
+
+subscriptions : Model -> Sub Msg
+subscriptions _ =
+    Time.every (dt * second) Tick
+
+main =
+  program
+      { init = init
+      , view = view
+      , update = update
+      , subscriptions = subscriptions
+      }