Power arithmetic added

lib/money/money/arithmetic.rb

@@ -173,6 +173,36 @@ def *(value)
       end
     end
 
+
+    # Raises the monetary value to the power of the given number and returns a new
+    # +Money+ object with the modified monetary value and the same currency.
+    #
+    # Note that you can exponentiate a Money object by another +Money+ object only
+    # if they share the same currency.
+    #
+    # @param [Numeric, Money] value The exponent to raise the monetary value to,
+    #   or a +Money+ object of the same currency to exponentiate by.
+    #
+    # @return [Money] The resulting money after exponentiation.
+    #
+    # @raise [TypeError] If the input +value+ is not a number, or if it's a +Money+ object with a different currency.
+    #
+    # @example Exponentiating by a number:
+    #   Money.new(2, :USD) ** 3 #=> #<Money @fractional=8>
+    #
+    # @example Exponentiating by a Money object (same currency):
+    #   Money.new(2, :USD) ** Money.new(3, :USD) #=> #<Money @fractional=8>
+    #
+    def **(value)
+      if value.is_a?(Numeric)
+        dup_with(fractional: fractional ** value)
+      elsif value.is_a?(self.class) && value.currency == currency
+        dup_with(fractional: fractional ** value.fractional)
+      else
+        raise TypeError, "Can't exponentiate a #{self.class.name} by a #{value.class.name}"
+      end
+    end
+
     # Divides the monetary value with the given number and returns a new +Money+
     # object with this monetary value and the same currency.
     # Can also divide by another +Money+ object to get a ratio.

spec/money/arithmetic_spec.rb

@@ -306,6 +306,41 @@
     it_behaves_like 'instance with custom bank', :*, 1
   end
 
+  describe "#**" do
+    it "Exponentiates Money by Integer and returns Money" do
+      ts = [
+        {a: Money.new( 2, :USD), b: 4, c: Money.new(16, :USD)},
+        {a: Money.new( 2, :USD), b: -4, c: Money.new((1/16), :USD)},
+        {a: Money.new(-2, :USD), b: 4, c: Money.new(16, :USD)},
+        {a: Money.new(-2, :USD), b: -4, c: Money.new(-(1/16), :USD)},
+        {a: Money.new(2, :USD), b: 0, c: Money.new(1, :USD)},
+        {a: Money.new(-2, :USD), b: 0, c: Money.new(1, :USD)},
+      ]
+      ts.each do |t|
+        expect(t[:a] ** t[:b]).to eq t[:c]
+      end
+    end
+
+    it "does exponantiate Money by Money (same currency)" do
+      expect(Money.new(10, :USD) ** Money.new(0, :USD)).to eq Money.new(1, :USD)
+    end
+
+    it "does not exponentiate Money by Money (different currency)" do
+      expect { Money.new(10, :USD) ** Money.new(4, :EUR) }.to raise_error(TypeError)
+    end
+
+    it "does not exponentiate Money by an object which is not a number" do
+      expect { Money.new(10, :USD) **  'abc' }.to raise_error(TypeError)
+    end
+
+    it "preserves the class in the result when using a subclass of Money" do
+      special_money_class = Class.new(Money)
+      expect(special_money_class.new(10_00, "USD") ** 2).to be_a special_money_class
+    end
+
+    it_behaves_like 'instance with custom bank', :**, 1
+  end
+
   describe "#/" do
     it "divides Money by Integer and returns Money" do
       ts = [