add mathematical operators for Value to operate with Numeric objects

lib/gnucash/value.rb

@@ -35,14 +35,48 @@ module Gnucash
       end
     end
 
-    # Add two Value objects
+    # Add to a Value object
+    # +other+ can be another Value or a Numeric
     def +(other)
+      if other.is_a?(Value)
         Value.new(@val + other.val)
+      elsif other.is_a?(Numeric)
+        (to_f + other).round(2)
+      else
+        raise "Unexpected argument"
+      end
     end
 
-    # Subtract two Value objects
+    # Subtract from a Value object
+    # +other+ can be another Value or a Numeric
     def -(other)
+      if other.is_a?(Value)
         Value.new(@val - other.val)
+      elsif other.is_a?(Numeric)
+        (to_f - other).round(2)
+      else
+        raise "Unexpected argument"
+      end
+    end
+
+    # Multiply a Value object
+    # +other+ should be a Numeric
+    def *(other)
+      if other.is_a?(Numeric)
+        (to_f * other).round(2)
+      else
+        raise "Unexpected argument"
+      end
+    end
+
+    # Divide a Value object
+    # +other+ should be a Numeric
+    def /(other)
+      if other.is_a?(Numeric)
+        (to_f / other).round(2)
+      else
+        raise "Unexpected argument"
+      end
     end
 
     # Represent the Value as a string (two decimal places)

spec/gnucash/value_spec.rb

@@ -30,6 +30,13 @@ module Gnucash
       c.to_s.should == "35.79"
     end
 
+    it "allows adding a Value object to a Numeric" do
+      a = Value.new("1234/100")
+      b = 15
+      c = a + b
+      c.should == 27.34
+    end
+
     it "allows subtracting two value objects" do
       a = Value.new("-12300/100")
       b = Value.new("99/100")
@@ -37,6 +44,27 @@ module Gnucash
       c.to_s.should == "-123.99"
     end
 
+    it "allows subtracting a Numeric from a Value object" do
+      a = Value.new("890/100")
+      b = 7.8
+      c = a - b
+      c.should == 1.1
+    end
+
+    it "allows multiplying a Value by a Numeric" do
+      a = Value.new("100/100")
+      b = 12
+      c = a * b
+      c.should == 12.00
+    end
+
+    it "allows dividing a Value by a Numeric" do
+      a = Value.new("150000/100")
+      b = 150
+      c = a / b
+      c.should == 10.0
+    end
+
     it "formats the number with two decimal places" do
       Value.new("1400/100").to_s.should == "14.00"
     end
@@ -46,5 +74,23 @@ module Gnucash
       (Value.new("89/100") < Value.new("100/100")).should be_true
       (Value.new("1234/100") > Value.new("222/100")).should be_true
     end
+
+    context "errors" do
+      it "raises an error when attempting to add an unknown object type" do
+        expect { Value.new(33) + nil }.to raise_error /Unexpected argument/i
+      end
+
+      it "raises an error when attempting to subtract an unknown object type" do
+        expect { Value.new(33) - nil }.to raise_error /Unexpected argument/i
+      end
+
+      it "raises an error when attempting to multiply by an unknown object type" do
+        expect { Value.new(33) * nil }.to raise_error /Unexpected argument/i
+      end
+
+      it "raises an error when attempting to divide by an unknown object type" do
+        expect { Value.new(33) / nil }.to raise_error /Unexpected argument/i
+      end
+    end
   end
 end