From bf5886b790f8f386ed425f543506a4bebb48448c 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    |  8 --------
 lib/stdlib/test/lists_SUITE.erl | 42 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 8 deletions(-)

(limited to 'lib')

diff --git a/lib/stdlib/doc/src/lists.xml b/lib/stdlib/doc/src/lists.xml
index 7efafedc82..55227aaee5 100644
--- a/lib/stdlib/doc/src/lists.xml
+++ b/lib/stdlib/doc/src/lists.xml
@@ -838,14 +838,6 @@ 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 is very slow if both
-            <c>A</c> and <c>B</c> are long lists. (If both lists are long, it
-            is a much better choice to use ordered lists and
-            <seealso marker="ordsets#subtract/2">
-            <c>ordsets:subtract/2</c></seealso>.</p>
-        </warning>
       </desc>
     </func>
 
diff --git a/lib/stdlib/test/lists_SUITE.erl b/lib/stdlib/test/lists_SUITE.erl
index 7c99244b36..c380b3bba1 100644
--- a/lib/stdlib/test/lists_SUITE.erl
+++ b/lib/stdlib/test/lists_SUITE.erl
@@ -2597,6 +2597,13 @@ subtract(Config) when is_list(Config) ->
     {'EXIT',_} = (catch sub([a|b], [])),
     {'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) ->
@@ -2606,6 +2613,41 @@ 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) ->
     [] = lists:droplast([x]),
-- 
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