From f2f7b8dd7fd15d2ba959e022736a56cc45a1cca4 Mon Sep 17 00:00:00 2001
From: gogotanaka <mail@tanakakazuki.com>
Date: Tue, 27 Jan 2015 09:46:51 -0800
Subject: [PATCH 1/1] Implement Set#powerset
To: https://bugs.ruby-lang.org/

---
 NEWS             |  4 ++++
 lib/set.rb       | 16 ++++++++++++++++
 test/test_set.rb |  8 ++++++++
 3 files changed, 28 insertions(+)

diff --git a/NEWS b/NEWS
index 13185d1..c38d696 100644
--- a/NEWS
+++ b/NEWS
@@ -30,6 +30,10 @@ with all sufficient information, see the ChangeLog file.
 
 === Stdlib updates (outstanding ones only)
 
+* lib/set.rb
+  * New methods:
+    * Set#powerset returns set of all subsets of +self+.
+
 === Stdlib compatibility issues (excluding feature bug fixes)
 
 * lib/webrick/utils.rb
diff --git a/lib/set.rb b/lib/set.rb
index d1ea6d8..dec44c8 100644
--- a/lib/set.rb
+++ b/lib/set.rb
@@ -513,6 +513,22 @@ class Set
     end
   end
 
+  # Returns set of all subsets of +self+.
+  #
+  # e.g.:
+  #
+  #   require 'set'
+  #   set = Set[1, 2].powerset
+  #   p set     # => #<Set: {#<Set: {}>,
+  #             #            #<Set: {1}>,
+  #             #            #<Set: {2}>,
+  #             #            #<Set: {1, 2}>}>
+  def powerset
+    (0 .. 2 ** length - 1).map { |n|
+      select.with_index { |_, i| (n >> i) & 1 == 1 }.to_set
+    }.to_set
+  end
+
   InspectKey = :__inspect_key__         # :nodoc:
 
   # Returns a string containing a human-readable representation of the
diff --git a/test/test_set.rb b/test/test_set.rb
index 961578b..a4522aa 100644
--- a/test/test_set.rb
+++ b/test/test_set.rb
@@ -570,6 +570,14 @@ class TC_Set < Test::Unit::TestCase
     }
   end
 
+  def test_powerset
+    assert_equal(Set[1, 2].powerset, Set[Set[], Set[1], Set[2], Set[1, 2]])
+    assert_equal(
+      Set[1, 2, 3].powerset,
+      Set[Set[], Set[1], Set[2], Set[3], Set[1, 2], Set[2, 3], Set[3, 1], Set[1, 2, 3]]
+    )
+  end
+
   def test_taintness
     orig = set = Set[1,2,3]
     assert_equal false, set.tainted?
-- 
gogotanaka