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require 'spec_helper'
describe "combinations of strings" do
def permutation(string)
return [string] if string.size < 2
return [string, string.reverse] if string.size == 2
character = string[-1]
permutation(string.chop).inject([]) do |results, permutation|
0.upto(permutation.size) do |position|
results << permutation.clone.insert(position, character)
end
results
end
end
it 'produces each combination of 1' do
expect(permutation('a')).to match_array(['a'])
end
it 'produces each combination of 2' do
# 'ab' => ["ab", "ba"]
results = permutation('ab')
expected = 'ab'.chars.permutation.map(&:join).to_a
expect(results).to match_array(expected)
end
it 'produces each combination of 3' do
# 'abc' => ["abc", "acb", "bac", "bca", "cab", "cba"]
results = permutation('abc')
expected = 'abc'.chars.permutation.map(&:join).to_a
expect(results).to match_array(expected)
expect(results.length).to eql(6)
end
end
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