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diff --git a/spec/rubyspec/core/array/permutation_spec.rb b/spec/rubyspec/core/array/permutation_spec.rb
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+require File.expand_path('../../../spec_helper', __FILE__)
+require File.expand_path('../fixtures/classes', __FILE__)
+
+
+describe "Array#permutation" do
+
+  before :each do
+    @numbers = (1..3).to_a
+    @yielded = []
+  end
+
+  it "returns an Enumerator of all permutations when called without a block or arguments" do
+    enum = @numbers.permutation
+    enum.should be_an_instance_of(Enumerator)
+    enum.to_a.sort.should == [
+      [1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]
+    ].sort
+  end
+
+  it "returns an Enumerator of permutations of given length when called with an argument but no block" do
+    enum = @numbers.permutation(1)
+    enum.should be_an_instance_of(Enumerator)
+    enum.to_a.sort.should == [[1],[2],[3]]
+  end
+
+  it "yields all permutations to the block then returns self when called with block but no arguments" do
+    array = @numbers.permutation {|n| @yielded << n}
+    array.should be_an_instance_of(Array)
+    array.sort.should == @numbers.sort
+    @yielded.sort.should == [
+      [1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]
+    ].sort
+  end
+
+  it "yields all permutations of given length to the block then returns self when called with block and argument" do
+    array = @numbers.permutation(2) {|n| @yielded << n}
+    array.should be_an_instance_of(Array)
+    array.sort.should == @numbers.sort
+    @yielded.sort.should == [[1,2],[1,3],[2,1],[2,3],[3,1],[3,2]].sort
+  end
+
+  it "returns the empty permutation ([[]]) when the given length is 0" do
+    @numbers.permutation(0).to_a.should == [[]]
+    @numbers.permutation(0) { |n| @yielded << n }
+    @yielded.should == [[]]
+  end
+
+  it "returns the empty permutation([]) when called on an empty Array" do
+    [].permutation.to_a.should == [[]]
+    [].permutation { |n| @yielded << n }
+    @yielded.should == [[]]
+  end
+
+  it "returns no permutations when the given length has no permutations" do
+    @numbers.permutation(9).entries.size == 0
+    @numbers.permutation(9) { |n| @yielded << n }
+    @yielded.should == []
+  end
+
+  it "handles duplicate elements correctly" do
+    @numbers << 1
+    @numbers.permutation(2).sort.should == [
+      [1,1],[1,1],[1,2],[1,2],[1,3],[1,3],
+      [2,1],[2,1],[2,3],
+      [3,1],[3,1],[3,2]
+    ].sort
+  end
+
+  it "handles nested Arrays correctly" do
+    # The ugliness is due to the order of permutations returned by
+    # permutation being undefined combined with #sort croaking on Arrays of
+    # Arrays.
+    @numbers << [4,5]
+    got = @numbers.permutation(2).to_a
+    expected = [
+       [1, 2],      [1, 3],      [1, [4, 5]],
+       [2, 1],      [2, 3],      [2, [4, 5]],
+       [3, 1],      [3, 2],      [3, [4, 5]],
+      [[4, 5], 1], [[4, 5], 2], [[4, 5], 3]
+    ]
+    expected.each {|e| got.include?(e).should be_true}
+    got.size.should == expected.size
+  end
+
+  it "truncates Float arguments" do
+    @numbers.permutation(3.7).to_a.sort.should ==
+      @numbers.permutation(3).to_a.sort
+  end
+
+  it "returns an Enumerator which works as expected even when the array was modified" do
+    @numbers = [1, 2]
+    enum = @numbers.permutation
+    @numbers << 3
+    enum.to_a.sort.should == [
+      [1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]
+    ].sort
+  end
+
+  it "generates from a defensive copy, ignoring mutations" do
+    accum = []
+    ary = [1,2,3]
+    ary.permutation(3) do |x|
+      accum << x
+      ary[0] = 5
+    end
+
+    accum.should == [[1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1]]
+  end
+
+  describe "when no block is given" do
+    describe "returned Enumerator" do
+      describe "size" do
+        describe "with an array size greater than 0" do
+          it "returns the descending factorial of array size and given length" do
+            @numbers.permutation(4).size.should == 0
+            @numbers.permutation(3).size.should == 6
+            @numbers.permutation(2).size.should == 6
+            @numbers.permutation(1).size.should == 3
+            @numbers.permutation(0).size.should == 1
+          end
+          it "returns the descending factorial of array size with array size when there's no param" do
+            @numbers.permutation.size.should == 6
+            [1,2,3,4].permutation.size.should == 24
+            [1].permutation.size.should == 1
+          end
+        end
+        describe "with an empty array" do
+          it "returns 1 when the given length is 0" do
+            [].permutation(0).size.should == 1
+          end
+          it "returns 1 when there's param" do
+            [].permutation.size.should == 1
+          end
+        end
+      end
+    end
+  end
+end