+ permutations

lib/itertools/combinatoric_generators.rb

@@ -7,5 +7,35 @@ def product *iters
       Itertools.iter p.product *pools
     end
 
+    def permutations iter, r=nil
+      Fiber.new do
+        pool    = array Itertools.iter(iter)
+        n       = pool.length
+        r     ||= n
+        indices = (0...n).to_a
+        cycles  = ((n-r+1)..n).to_a.reverse
+        exhausted if n == 0
+
+        Fiber.yield indices.slice(0,r).map { |i| pool[i] }
+        while true
+          finished = true
+          (r-1).downto(0) do |i|
+            cycles[i] -= 1
+            if cycles[i] == 0
+              indices = indices.slice(0,i) + indices.slice(i+1,n) + [indices[i]]
+              cycles[i] = n - i
+            else
+              j = cycles[i]
+              indices[i], indices[-j] = indices[-j], indices[i]
+              Fiber.yield indices.slice(0,r).map { |i| pool[i] }
+              finished = false
+              break
+            end
+          end
+          exhausted if finished
+        end
+      end
+    end
+
   end
 end

spec/combinatoric_generators_spec.rb

@@ -15,4 +15,28 @@
       end
     end
   end
+  context "#permutations" do
+    context "with a string and a length" do
+      it "permutes" do
+        seq = Itertools.permutations "ABCD", 2
+        seq.should begin_with [['A','B'], ['A','C'], ['A','D'], ['B','A'],
+                               ['B','C'], ['B','D'], ['C','A'], ['C','B'],
+                               ['C','D'], ['D','A'], ['D','B'], ['D','C']]
+        seq.should be_exhausted
+      end
+    end
+    context "with a string but no length" do
+      it "defaults to the length of the string" do
+        seq = Itertools.permutations "ABC"
+        seq.should begin_with [['A','B','C'], ['A','C','B'], ['B','A','C'],
+                               ['B','C','A'], ['C','A','B'], ['C','B','A']]
+        seq.should be_exhausted
+      end
+    end
+    context "with an empty array" do
+      it "gives nothing" do
+        Itertools.permutations([]).should be_exhausted
+      end
+    end
+  end
 end