Fixes balancing in tidy tree

src/Hierarchy/Tidy.elm

@@ -275,14 +275,20 @@ type alias YList =
     List { index : Int, id : Int, y : Float }
 
 
-moveSubtree : Int -> Maybe { y : Float, id : Int, index : Int } -> Int -> Float -> TidyLayout a -> TidyLayout a
-moveSubtree currentIndex fromMaybe currentId dist lay =
+moveSubtree : Array Id -> Int -> Maybe { y : Float, id : Int, index : Int } -> Int -> Float -> TidyLayout a -> TidyLayout a
+moveSubtree nodeChildren currentIndex fromMaybe currentId dist lay =
     case fromMaybe of
         Just from ->
             if from.index /= currentIndex - 1 then
                 let
                     normDist =
                         dist / toFloat (currentIndex - from.index)
+
+                    -- The slack is distributed across the interior subtrees between
+                    -- `from` and `current`, so the positive acceleration is planted on
+                    -- the first interior sibling (children[from.index + 1])
+                    interiorStartId =
+                        Array.get (from.index + 1) nodeChildren |> Maybe.withDefault from.id
                 in
                 lay
                     |> updateTidyData
@@ -298,7 +304,7 @@ moveSubtree currentIndex fromMaybe currentId dist lay =
                         (\from_ ->
                             { from_ | shiftAcceleration = from_.shiftAcceleration + normDist }
                         )
-                        from.id
+                        interiorStartId
 
             else
                 lay
@@ -430,7 +436,7 @@ separate peerMargin childIndex nodeId lay_ ylist_ =
                             if dist > 0 then
                                 -- left and right are too close. move right part with distance of dist
                                 ( { rightContour | modifierSum = rightContour.modifierSum + dist }
-                                , moveSubtree childIndex (yList2 |> List.head) (nodeChildren |> Maybe.andThen (Array.get childIndex) |> Maybe.withDefault -1) dist lay
+                                , moveSubtree (nodeChildren |> Maybe.withDefault Array.empty) childIndex (yList2 |> List.head) (nodeChildren |> Maybe.andThen (Array.get childIndex) |> Maybe.withDefault -1) dist lay
                                 )
 
                             else
@@ -617,7 +623,7 @@ layout getters tree =
                                                                         update tail
 
                                                                     else
-                                                                        { index = index, y = maxY, id = id } :: tail
+                                                                        { index = index, y = maxY, id = id } :: lst
                                                     in
                                                     { yList = update yList1, layN = lay3, index = index + 1 }
                                                 )

tests/Hierarchy/TidyTests.elm

@@ -33,11 +33,17 @@ suite =
                 tree
                     |> doLayout
                     |> expectNodesToBeOrdered
+        , Test.fuzz fuzzHierarchy "Rule 7: Laying out a mirrored tree gives the horizontal flip of the original layout" <|
+            \tree ->
+                Expect.equalLists (nodeCenters (mirror tree)) (flippedNodeCenters tree)
 
         -- sanity checks
         , test1
         , test2
         , test3
+
+        -- regression for https://github.com/gampleman/elm-visualization/issues/190
+        , test190
         ]
 
 
@@ -169,6 +175,39 @@ doLayout =
     Hierarchy.tidy [ Hierarchy.nodeSize (\( _, w, h ) -> ( w, h )), Hierarchy.parentChildMargin 1, Hierarchy.peerMargin 1 ]
 
 
+{-| Reverse the order of every node's children, top to bottom.
+-}
+mirror : Tree a -> Tree a
+mirror t =
+    Tree.tree (Tree.label t) (List.reverse (List.map mirror (Tree.children t)))
+
+
+{-| The sorted multiset of node-center positions. The layout reports left-edge
+x (center - width / 2), so we add width / 2 back to recover centers.
+-}
+nodeCenters : Tree ( Int, Float, Float ) -> List ( Float, Float )
+nodeCenters tree =
+    doLayout tree
+        |> Tree.toList
+        |> List.map (\n -> ( round3 (n.x + n.width / 2), round3 n.y ))
+        |> List.sort
+
+
+{-| The original layout's node centers, flipped horizontally about x = 0.
+-}
+flippedNodeCenters : Tree ( Int, Float, Float ) -> List ( Float, Float )
+flippedNodeCenters tree =
+    doLayout tree
+        |> Tree.toList
+        |> List.map (\n -> ( round3 -(n.x + n.width / 2), round3 n.y ))
+        |> List.sort
+
+
+round3 : Float -> Float
+round3 v =
+    toFloat (round (v * 1000)) / 1000
+
+
 formatTree : FinishedLayout -> String
 formatTree =
     let
@@ -210,3 +249,36 @@ test3 =
             Tree.tree ( 0, 1, 1 ) [ Tree.tree ( 0, 1, 1 ) [ Tree.tree ( 0, 1, 10 ) [ Tree.singleton ( 0, 1, 1 ), Tree.singleton ( 0, 1, 1 ) ], Tree.tree ( 0, 1, 1 ) [ Tree.singleton ( 0, 1, 1 ) ], Tree.tree ( 0, 1, 10 ) [ Tree.singleton ( 0, 10, 1 ) ] ] ]
                 |> doLayout
                 |> expectNoOverlapNodes
+
+
+
+{-
+   See <https://github.com/gampleman/elm-visualization/issues/190>.
+-}
+
+
+test190 : Test
+test190 =
+    Test.test "Issue 190: slack between a node's children is distributed evenly" <|
+        \() ->
+            let
+                leaf =
+                    Tree.singleton ( 0, 1, 1 )
+
+                xs =
+                    Tree.tree ( 0, 1, 1 )
+                        [ Tree.tree ( 0, 1, 1 ) [ leaf, leaf, leaf, leaf, leaf, leaf, Tree.tree ( 0, 1, 1 ) [ leaf ] ]
+                        , leaf
+                        , leaf
+                        , Tree.tree ( 0, 1, 1 ) [ leaf ]
+                        , leaf
+                        , Tree.tree ( 0, 1, 1 ) [ Tree.tree ( 0, 1, 1 ) [ leaf, leaf, leaf, leaf, leaf, leaf, leaf ] ]
+                        ]
+                        |> doLayout
+                        |> Tree.children
+                        |> List.map (\c -> (Tree.label c).x)
+
+                gaps =
+                    List.map2 (\p q -> q - p) xs (List.drop 1 xs)
+            in
+            Expect.equalLists (List.map round3 gaps) [ 3.067, 3.067, 3.067, 2.4, 2.4 ]