Show item position in detail view

modules/webapp/src/main/elm/Data/ItemNav.elm

@@ -1,7 +1,43 @@
-module Data.ItemNav exposing (ItemNav)
+module Data.ItemNav exposing (ItemNav, fromList)
+
+import Api.Model.ItemLight exposing (ItemLight)
+import Api.Model.ItemLightList exposing (ItemLightList)
+import Util.List
 
 
 type alias ItemNav =
     { prev : Maybe String
     , next : Maybe String
+    , index : Maybe Int
+    , length : Int
+    }
+
+
+fromList : ItemLightList -> String -> ItemNav
+fromList list id =
+    let
+        all : List ItemLight
+        all =
+            List.concatMap .items list.groups
+
+        next =
+            Util.List.findNext (\i -> i.id == id) all
+                |> Maybe.map .id
+
+        prev =
+            Util.List.findPrev (\i -> i.id == id) all
+                |> Maybe.map .id
+
+        len =
+            List.length all
+
+        index : Maybe Int
+        index =
+            Util.List.findIndexed (.id >> (==) id) all
+                |> Maybe.map Tuple.second
+    in
+    { prev = prev
+    , next = next
+    , index = index
+    , length = len
     }