Integer#dodecagonal?: Add

lib/numb.rb

@@ -1,10 +1,10 @@
 # coding: utf-8
 
-libs = %w{abundant achilles automorphic balanced_prime carol composite 
-          decagonal deficient cube dihedral_prime dudeney economical emrip
-          equidigital extravagant factorion frugal happy harshad heptagonal
-          hexagonal hilbert hyperperfect impolite kaprekar keith kynea lucas 
-          lucas_carmichael mms_pair mobius narcissistic nivenmorphic 
+libs = %w{abundant achilles automorphic balanced_prime carol composite cube 
+          decagonal deficient dodecagonal dihedral_prime dudeney economical 
+          emrip equidigital extravagant factorion frugal happy harshad 
+          heptagonal hexagonal hilbert hyperperfect impolite kaprekar keith
+          kynea lucas lucas_carmichael mms_pair mobius narcissistic nivenmorphic 
           n_gonal octagonal ordinal parasitic pentagonal perfect 
           perfect_power polite polydivisible powerful practical 
           primitive_pseudoperfect pronic rhonda self self_descriptive 

lib/numb/dodecagonal.rb

@@ -0,0 +1,5 @@
+class Integer
+  def dodecagonal?
+    n_gonal?(12)
+  end
+end

spec/dodecagonal_spec.rb

@@ -0,0 +1,18 @@
+describe Integer, "#dodecagonal?" do
+
+  # A051624
+  DODECAGONAL = [0,1,12,33,64,105,156,217,288,369,460,561,672,793,
+                 924,1065,1216,1377,1548,1729,1920,2121,2332,2553,
+                 2784,3025,3276,3537,3808,4089,4380,4681,4992,5313,
+                 5644,5985,6336,6697,7068,7449,7840,8241,8652]
+
+  it "returns true for dodecagonal numbers" do
+    DODECAGONAL.each{|n| n.should be_dodecagonal}
+  end
+
+  it "returns false for non-dodecagonal numbers" do
+    ((0..DODECAGONAL.last).to_a - DODECAGONAL).each do |n| 
+      n.should_not be_dodecagonal
+    end
+  end
+end