1 files changed, 74 insertions, 0 deletions
diff --git a/spec/ruby/core/array/combination_spec.rb b/spec/ruby/core/array/combination_spec.rb
new file mode 100644
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+++ b/spec/ruby/core/array/combination_spec.rb
@@ -0,0 +1,74 @@
+require File.expand_path('../../../spec_helper', __FILE__)
+
+describe "Array#combination" do
+  before :each do
+    @array = [1, 2, 3, 4]
+  end
+
+  it "returns an enumerator when no block is provided" do
+    @array.combination(2).should be_an_instance_of(Enumerator)
+  end
+
+  it "returns self when a block is given" do
+    @array.combination(2){}.should equal(@array)
+  end
+
+  it "yields nothing for out of bounds length and return self" do
+    @array.combination(5).to_a.should == []
+    @array.combination(-1).to_a.should == []
+  end
+
+  it "yields the expected combinations" do
+    @array.combination(3).to_a.sort.should == [[1,2,3],[1,2,4],[1,3,4],[2,3,4]]
+  end
+
+  it "yields nothing if the argument is out of bounds" do
+    @array.combination(-1).to_a.should == []
+    @array.combination(5).to_a.should == []
+  end
+
+  it "yields a copy of self if the argument is the size of the receiver" do
+    r = @array.combination(4).to_a
+    r.should == [@array]
+    r[0].should_not equal(@array)
+  end
+
+  it "yields [] when length is 0" do
+    @array.combination(0).to_a.should == [[]] # one combination of length 0
+    [].combination(0).to_a.should == [[]] # one combination of length 0
+  end
+
+  it "yields a partition consisting of only singletons" do
+    @array.combination(1).to_a.sort.should == [[1],[2],[3],[4]]
+  end
+
+  it "generates from a defensive copy, ignoring mutations" do
+    accum = []
+    @array.combination(2) do |x|
+      accum << x
+      @array[0] = 1
+    end
+    accum.should == [[1, 2], [1, 3], [1, 4], [2, 3], [2, 4], [3, 4]]
+  end
+
+  describe "when no block is given" do
+    describe "returned Enumerator" do
+      describe "size" do
+        it "returns 0 when the number of combinations is < 0" do
+          @array.combination(-1).size.should == 0
+          [].combination(-2).size.should == 0
+        end
+        it "returns the binomial coeficient between the array size the number of combinations" do
+          @array.combination(5).size.should == 0
+          @array.combination(4).size.should == 1
+          @array.combination(3).size.should == 4
+          @array.combination(2).size.should == 6
+          @array.combination(1).size.should == 4
+          @array.combination(0).size.should == 1
+          [].combination(0).size.should == 1
+          [].combination(1).size.should == 0
+        end
+      end
+    end
+  end
+end