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-rw-r--r--lib/stdlib/doc/src/lists.xml6
-rw-r--r--lib/stdlib/test/lists_SUITE.erl42
2 files changed, 42 insertions, 6 deletions
diff --git a/lib/stdlib/doc/src/lists.xml b/lib/stdlib/doc/src/lists.xml
index 89ba5238b5..4a4c170bba 100644
--- a/lib/stdlib/doc/src/lists.xml
+++ b/lib/stdlib/doc/src/lists.xml
@@ -708,12 +708,6 @@ splitwith(Pred, List) ->
> <input>lists:subtract("123212", "212").</input>
"312".</pre>
<p><c>lists:subtract(A, B)</c> is equivalent to <c>A -- B</c>.</p>
- <warning><p>The complexity of <c>lists:subtract(A, B)</c> is proportional
- to <c>length(A)*length(B)</c>, meaning that it will be very slow if
- both <c>A</c> and <c>B</c> are long lists.
- (Using ordered lists and
- <seealso marker="ordsets#subtract/2">ordsets:subtract/2</seealso>
- is a much better choice if both lists are long.)</p></warning>
</desc>
</func>
<func>
diff --git a/lib/stdlib/test/lists_SUITE.erl b/lib/stdlib/test/lists_SUITE.erl
index a0f7fd2744..21b2dd1777 100644
--- a/lib/stdlib/test/lists_SUITE.erl
+++ b/lib/stdlib/test/lists_SUITE.erl
@@ -2692,6 +2692,13 @@ subtract(Config) when is_list(Config) ->
?line {'EXIT',_} = (catch sub([a|b], [])),
?line {'EXIT',_} = (catch sub([a|b], [a])),
+ %% Trapping, both crashing and otherwise.
+ [sub_trapping(N) || N <- lists:seq(0, 18)],
+
+ %% The current implementation chooses which algorithm to use based on
+ %% certain thresholds, and we need proper coverage for all corner cases.
+ [sub_thresholds(N) || N <- lists:seq(0, 32)],
+
ok.
sub_non_matching(A, B) ->
@@ -2701,6 +2708,41 @@ sub(A, B) ->
Res = A -- B,
Res = lists:subtract(A, B).
+sub_trapping(N) ->
+ List = lists:duplicate(N + (1 bsl N), gurka),
+ ImproperList = List ++ crash,
+
+ {'EXIT',_} = (catch sub_trapping_1(ImproperList, [])),
+ {'EXIT',_} = (catch sub_trapping_1(List, ImproperList)),
+
+ List = List -- lists:duplicate(N + (1 bsl N), gaffel),
+ ok = sub_trapping_1(List, []).
+
+sub_trapping_1([], _) -> ok;
+sub_trapping_1(L, R) -> sub_trapping_1(L -- R, [gurka | R]).
+
+sub_thresholds(N) ->
+ %% This needs to be long enough to cause trapping.
+ OtherLen = 1 bsl 18,
+ Other = lists:seq(0, OtherLen - 1),
+
+ Disjoint = lists:seq(-N, -1),
+ Subset = lists:seq(1, N),
+
+ %% LHS is disjoint from RHS, so all elements must be retained.
+ Disjoint = Disjoint -- Other,
+
+ %% LHS is covered by RHS, so all elements must be removed.
+ [] = Subset -- Other,
+
+ %% RHS is disjoint from LHS, so all elements must be retained.
+ Other = Other -- Disjoint,
+
+ %% RHS is covered by LHS, so N elements must be removed.
+ N = OtherLen - length(Other -- Subset),
+
+ ok.
+
%% Test lists:droplast/1
droplast(Config) when is_list(Config) ->
?line [] = lists:droplast([x]),