Integer#stirling2: Add

lib/numb/stirling2.rb

@@ -0,0 +1,9 @@
+# coding: utf-8
+class Integer
+  def stirling2(m)
+    n = self
+    return 1 if (n.zero? and m.zero?) or m == n 
+    return 0 if m > n or m.zero? or n.zero?
+    (n-1).stirling2(m-1) + m * (n-1).stirling2(m)
+  end
+end

spec/numb/stirling2_spec.rb

@@ -0,0 +1,23 @@
+# coding: utf-8
+describe Integer, "#stirling2" do
+  @seq = [
+          [1],
+          [1,   1],
+          [1,   3,    1],
+          [1,   7,    6,    1],
+          [1,  15,   25,   10,     1],
+          [1,  31,   90,   65,     15,     1],
+          [1,  63,  301,  350,    140,    21,    1],
+          [1, 127,  966, 1701,   1050,   266,   28,   1],
+          [1, 255, 3025, 7770,   6951,  2646,  462,  36,  1],
+          [1, 511, 9330, 34105, 42525, 22827, 5880, 750, 45, 1]
+  ]
+
+  @seq.to_enum.with_index(1).each do |row, n|
+    row.to_enum.with_index(1).each do |s, m|
+      it "returns #{s} as the #{n}#{n.ordinal} Stirling number of the 2nd kind (m = #{m})" do
+        n.stirling2(m).should == s
+      end
+    end
+  end
+end