From 403611e75287ba5b4f4f9e1bdd285ec759120e53 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Michael=20R=C3=A4tzel?= <viir@viir.de>
Date: Fri, 2 Aug 2024 19:48:01 +0000
Subject: [PATCH] Fix stability in core library List sorting functions

+ Add an automated test case for a scenario where we found `List.sortBy` violating the stability constraint.
+ Fix the list sorting function on the productive side to maintain stability.
---
 .../src/ElmInteractiveCoreModules.elm         | 25 +++++++------------
 .../steps/634/expected-value.txt              |  1 +
 .../elm-core-list/steps/634/submission.txt    |  2 ++
 .../steps/635/expected-value.txt              |  1 +
 .../elm-core-list/steps/635/submission.txt    |  2 ++
 5 files changed, 15 insertions(+), 16 deletions(-)
 create mode 100644 implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/634/expected-value.txt
 create mode 100644 implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/634/submission.txt
 create mode 100644 implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/635/expected-value.txt
 create mode 100644 implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/635/submission.txt

diff --git a/implement/pine/ElmTime/compile-elm-program/src/ElmInteractiveCoreModules.elm b/implement/pine/ElmTime/compile-elm-program/src/ElmInteractiveCoreModules.elm
index cad497d8c..29eeeb971 100644
--- a/implement/pine/ElmTime/compile-elm-program/src/ElmInteractiveCoreModules.elm
+++ b/implement/pine/ElmTime/compile-elm-program/src/ElmInteractiveCoreModules.elm
@@ -983,19 +983,12 @@ sortWith compareFunc list =
 
 sortWithSplit : List a -> ( List a, List a )
 sortWithSplit list =
-    case list of
-        [] ->
-            ( [], [] )
-
-        [ x ] ->
-            ( [ x ], [] )
-
-        x :: y :: rest ->
-            let
-                ( left, right ) =
-                    sortWithSplit rest
-            in
-            (Pine_kernel.concat [ [ x ], left], Pine_kernel.concat [ [ y ], right])
+    let
+        middleIndex = (Pine_kernel.length list) // 2
+    in
+    ( Pine_kernel.take [ middleIndex, list ]
+    , Pine_kernel.skip [ middleIndex, list ]
+    )
 
 
 sortWithMerge : List a -> List a -> (a -> a -> Order) -> List a
@@ -1009,11 +1002,11 @@ sortWithMerge left right compareFunc =
 
         ( x :: xs, y :: ys ) ->
             case compareFunc x y of
-                LT ->
-                    Pine_kernel.concat [ [ x ], sortWithMerge xs right compareFunc ]
+                GT ->
+                    Pine_kernel.concat [ [ y ], sortWithMerge left ys compareFunc ]
 
                 _ ->
-                    Pine_kernel.concat [ [ y ], sortWithMerge left ys compareFunc ]
+                    Pine_kernel.concat [ [ x ], sortWithMerge xs right compareFunc ]
 
 """
     , """
diff --git a/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/634/expected-value.txt b/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/634/expected-value.txt
new file mode 100644
index 000000000..74bf9e48c
--- /dev/null
+++ b/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/634/expected-value.txt
@@ -0,0 +1 @@
+[(17,[]),(1,["c"]),(13,["d"]),(11,["e"]),(0,["a","b"])]
\ No newline at end of file
diff --git a/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/634/submission.txt b/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/634/submission.txt
new file mode 100644
index 000000000..98db89cb0
--- /dev/null
+++ b/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/634/submission.txt
@@ -0,0 +1,2 @@
+List.sortBy (\( _, dependencies ) -> List.length dependencies)
+    [ (0, ["a","b"]), (1, ["c"]), (13, ["d"]), (17, []), (11, ["e"]) ]
\ No newline at end of file
diff --git a/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/635/expected-value.txt b/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/635/expected-value.txt
new file mode 100644
index 000000000..1ac6a6b80
--- /dev/null
+++ b/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/635/expected-value.txt
@@ -0,0 +1 @@
+[(23,[]),(17,[]),(1,["c"]),(13,["d"]),(11,["e"]),(0,["a","b"])]
\ No newline at end of file
diff --git a/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/635/submission.txt b/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/635/submission.txt
new file mode 100644
index 000000000..313459ef8
--- /dev/null
+++ b/implement/test-and-train/elm-interactive-scenarios-core/elm-core-list/steps/635/submission.txt
@@ -0,0 +1,2 @@
+List.sortBy (\( _, dependencies ) -> List.length dependencies)
+    [ (23, []), (0, ["a","b"]), (1, ["c"]), (13, ["d"]), (17, []), (11, ["e"]) ]
\ No newline at end of file