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require_relative '../../spec_helper'
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 coefficient 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
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