From d98da38562ec79360b58eed87eced3a506f1ff6d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20H=C3=B6gberg?= <john@erlang.org>
Date: Mon, 15 Oct 2018 18:17:12 +0200
Subject: Optimize operator '--' and yield on large inputs

The removal set now uses a red-black tree instead of an array on
large inputs, decreasing runtime complexity from `n*n` to
`n*log(n)`. It will also exit early when there are no more items
left in the removal set, drastically improving performance and
memory use when the items to be removed are present near the head
of the list.

This got a lot more complicated than before as the overhead of
always using a red-black tree was unacceptable when either of the
inputs were small, but this compromise has okay-to-decent
performance regardless of input size.

Co-authored-by: Dmytro Lytovchenko <dmytro.lytovchenko@erlang-solutions.com>
---
 lib/stdlib/doc/src/lists.xml    |  6 ------
 lib/stdlib/test/lists_SUITE.erl | 42 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 6 deletions(-)

(limited to 'lib')

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]),
-- 
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