make transpose call stack size safe

elm-community · Sep 24, 2017 · 8868d00 · 8868d00
1 parent 153f40c
commit 8868d00
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Showing 2 changed files with 14 additions and 4 deletions.
diff --git a/src/List/Extra.elm b/src/List/Extra.elm
@@ -775,13 +775,18 @@ If some rows are shorter than the following rows, their elements are skipped:
 
 -}
 transpose : List (List a) -> List (List a)
-transpose ll =
+transpose =
+    transposeHelp []
+
+
+transposeHelp : List (List a) -> List (List a) -> List (List a)
+transposeHelp acc ll =
     case ll of
         [] ->
-            []
+            acc
 
         [] :: xss ->
-            transpose xss
+            transposeHelp acc xss
 
         (x :: xs) :: xss ->
             let
@@ -791,7 +796,7 @@ transpose ll =
                 tails =
                     filterMap tail xss
             in
-                (x :: heads) :: transpose (xs :: tails)
+                transposeHelp (acc ++ [ x :: heads ]) (xs :: tails)
 
 
 {-| Return the list of all subsequences of a list.

diff --git a/tests/Tests.elm b/tests/Tests.elm
@@ -123,6 +123,11 @@ all =
                     Expect.equal
                         (transpose [ [ 10, 11 ], [ 20 ], [], [ 30, 31, 32 ] ])
                         [ [ 10, 20, 30 ], [ 11, 31 ], [ 32 ] ]
+            , test "transposes large lists" <|
+                \() ->
+                    Expect.equal
+                        (transpose [ List.repeat 10000 1 ])
+                        (List.repeat 10000 [ 1 ])
             ]
         , describe "subsequences" <|
             [ test "computes subsequences" <|