From 76977611dd68e384fdce8c546efda5e1931e67a6 Mon Sep 17 00:00:00 2001
From: knu <knu@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date: Sat, 5 Nov 2016 09:23:14 +0000
Subject: [PATCH] Add Set#compare_by_identity and Set#compare_by_identity?

* lib/set.rb (Set#compare_by_identity, Set#compare_by_identity?):
  New methods. [Feature #12210]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56589 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
---
 ChangeLog        |  5 +++++
 NEWS             |  4 ++++
 lib/set.rb       | 22 ++++++++++++++++++++--
 test/test_set.rb | 19 +++++++++++++++++++
 4 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index c96a81a134..71133697d2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Nov  5 18:17:54 2016  Akinori MUSHA  <knu@iDaemons.org>
+
+	* lib/set.rb (Set#compare_by_identity, Set#compare_by_identity?):
+	  New methods. [Feature #12210]
+
 Sat Nov  5 18:17:08 2016  SHIBATA Hiroshi  <hsbt@ruby-lang.org>
 
 	* lib/rdoc/*, test/rdoc/*: Update rdoc-5.0.0
diff --git a/NEWS b/NEWS
index 439057a6e1..d9f6c6eedb 100644
--- a/NEWS
+++ b/NEWS
@@ -189,6 +189,10 @@ with all sufficient information, see the ChangeLog file or Redmine
 * pathname
   * New method: Pathname#empty? [Feature#12596]
 
+* set
+  * New methods: Set#compare_by_identity and Set#compare_by_identity?.
+    [Feature #12210]
+
 * WEBrick
 
   * Don't allow , as a separator [Bug #12791]
diff --git a/lib/set.rb b/lib/set.rb
index e77af5de2e..43c388ca90 100644
--- a/lib/set.rb
+++ b/lib/set.rb
@@ -3,7 +3,7 @@
 #
 # set.rb - defines the Set class
 #++
-# Copyright (c) 2002-2013 Akinori MUSHA <knu@iDaemons.org>
+# Copyright (c) 2002-2016 Akinori MUSHA <knu@iDaemons.org>
 #
 # Documentation by Akinori MUSHA and Gavin Sinclair.
 #
@@ -37,7 +37,8 @@
 # Set uses Hash as storage, so you must note the following points:
 #
 # * Equality of elements is determined according to Object#eql? and
-#   Object#hash.
+#   Object#hash.  Use Set#compare_by_identity to make a set compare
+#   its elements by their identity.
 # * Set assumes that the identity of each element does not change
 #   while it is stored.  Modifying an element of a set will render the
 #   set to an unreliable state.
@@ -91,6 +92,23 @@ class Set
     end
   end
 
+  # Makes the set compare its elements by their identity and returns
+  # self.  This method may not be supported by all subclasses of Set.
+  def compare_by_identity
+    if @hash.respond_to?(:compare_by_identity)
+      @hash.compare_by_identity
+      self
+    else
+      raise NotImplementedError, "#{self.class.name}\##{__method__} is not implemented"
+    end
+  end
+
+  # Returns true if the set will compare its elements by their
+  # identity.  Also see Set#compare_by_identity.
+  def compare_by_identity?
+    @hash.respond_to?(:compare_by_identity?) && @hash.compare_by_identity?
+  end
+
   def do_with_enum(enum, &block) # :nodoc:
     if enum.respond_to?(:each_entry)
       enum.each_entry(&block) if block
diff --git a/test/test_set.rb b/test/test_set.rb
index 9439974fea..07c404ab79 100644
--- a/test/test_set.rb
+++ b/test/test_set.rb
@@ -715,6 +715,25 @@ class TC_Set < Test::Unit::TestCase
     set1.add(set2)
     assert_equal(true, set1.inspect.include?('#<Set: {...}>'))
   end
+
+  def test_compare_by_identity
+    a1, a2 = "a", "a"
+    b1, b2 = "b", "b"
+    c = "c"
+    array = [a1, b1, c, a2, b2]
+
+    iset = Set.new.compare_by_identity
+    assert_send([iset, :compare_by_identity?])
+    iset.merge(array)
+    assert_equal(5, iset.size)
+    assert_equal(array.map(&:object_id).sort, iset.map(&:object_id).sort)
+
+    set = Set.new
+    assert_not_send([set, :compare_by_identity?])
+    set.merge(array)
+    assert_equal(3, set.size)
+    assert_equal(array.uniq.sort, set.sort)
+  end
 end
 
 class TC_SortedSet < Test::Unit::TestCase