Add addition

index.js

@@ -71,6 +71,12 @@ Ratio.prototype =
 
         return this.times(x.reciprocal())
     }
+, plus: function(x, d) {
+        if (d) x = Ratio(x, d)
+        else x = Ratio(x)
+
+        return Ratio(this.n * x.d + x.n * this.d, this.d * x.d)
+    }
 }
 
 module.exports = Ratio

test/index.js

@@ -183,5 +183,32 @@ describe("Ratio", function() {
                 ratio(1,2).neg().toString().should.equal("-1/2")
             })
         })
+
+        describe("Addition", function() {
+
+            it("behaves reasonably with primitive addition", function() {
+                (ratio(1,2) + ratio(1,4)).should.equal(0.75)
+            })
+
+            it("can add ratios", function() {
+                ratio(1,7).plus(ratio(2,7)).toString().should.equal("3/7")
+            })
+
+            it("can add (n,d) pair representing a ratio", function() {
+                ratio(1,7).plus(2,7).toString().should.equal("3/7")
+            })
+
+            it("can add integers", function() {
+                ratio(1).plus(2).toString().should.equal("3")
+            })
+
+            it("can add decimals", function() {
+                ratio(1,4).plus(0.75).toString().should.equal("1")
+            })
+
+            it("can add with different bases", function() {
+                ratio(2,3).plus(1,6).toString().should.equal("5/6")
+            })
+        })
     })
 })