Merge pull request #83 from rogeriochaves/master

Make transpose call stack size safe
elm-community · Dec 18, 2017 · ec6c312 · ec6c312
2 parents 4570d3b + 017e824
commit ec6c312
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diff --git a/src/List/Extra.elm b/src/List/Extra.elm
@@ -813,23 +813,18 @@ If some rows are shorter than the following rows, their elements are skipped:
 
 -}
 transpose : List (List a) -> List (List a)
-transpose ll =
-    case ll of
-        [] ->
-            []
+transpose listOfLists =
+    List.foldr (List.map2 (::)) (List.repeat (rowsLength listOfLists) []) listOfLists
 
-        [] :: xss ->
-            transpose xss
 
-        (x :: xs) :: xss ->
-            let
-                heads =
-                    filterMap head xss
+rowsLength : List (List a) -> Int
+rowsLength listOfLists =
+    case listOfLists of
+        [] ->
+            0
 
-                tails =
-                    filterMap tail xss
-            in
-                (x :: heads) :: transpose (xs :: tails)
+        x :: _ ->
+            List.length x
 
 
 {-| Return the list of all subsequences of a list.

diff --git a/tests/Tests.elm b/tests/Tests.elm
@@ -118,11 +118,16 @@ all =
                     Expect.equal
                         (transpose [ [ 1, 2, 3 ], [ 4, 5, 6 ] ])
                         [ [ 1, 4 ], [ 2, 5 ], [ 3, 6 ] ]
-            , test "short rows are skipped" <|
+            , test "truncate the matrix to the shortest row size" <|
                 \() ->
                     Expect.equal
-                        (transpose [ [ 10, 11 ], [ 20 ], [], [ 30, 31, 32 ] ])
-                        [ [ 10, 20, 30 ], [ 11, 31 ], [ 32 ] ]
+                        (transpose [ [ 10, 11 ], [ 20 ], [ 30, 31, 32 ] ])
+                        [ [ 10, 20, 30 ] ]
+            , test "transposes large lists" <|
+                \() ->
+                    Expect.equal
+                        (transpose [ List.repeat 10000 1 ])
+                        (List.repeat 10000 [ 1 ])
             ]
         , describe "subsequences" <|
             [ test "computes subsequences" <|