Commit 20b6a68

@@ -0,0 +1,34 @@
+require 'spec_helper'
+
+describe "combinations of strings" do
+  def permutation(string)
+    return [string] if string.size < 2
+    character = string[0]
+    substring = string[1..-1]
+    results = permutation(substring).each_with_object([]) do |permutation, result|
+      (0..permutation.size).each do |n|
+        result << permutation.clone.insert(n, character)
+      end
+    end
+    results
+  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