Fix bug with Expression.simplify

Expression.simplify was failing when there was more than one term with
zero variables.

src/expressions.js

@@ -39,6 +39,7 @@ Expression.prototype.simplify = function() {
     }
 
     copy._combineLikeTerms();
+    copy._moveTermsWithDegreeZeroToConstants();
     copy._removeTermsWithCoefficientZero();
     copy.constants = (copy.constant().valueOf() === 0 ? [] : [copy.constant()]);
     copy._sort();
@@ -326,13 +327,27 @@ Expression.prototype._combineLikeTerms = function() {
                 this.terms.splice(j, 1);
             }
         }
+    }
+
+    return this;
+};
+
+Expression.prototype._moveTermsWithDegreeZeroToConstants = function() {
+    var keepTerms = [];
+    var constant = new Fraction(0, 1);
+
+    for (var i = 0; i < this.terms.length; i++) {
+        var thisTerm = this.terms[i];
 
         if (thisTerm.variables.length === 0) {
-            this.constants.push(thisTerm.coefficient());
-            this.terms.splice(i, 1);
+            constant = constant.add(thisTerm.coefficient());
+        } else {
+            keepTerms.push(thisTerm);
         }
     }
 
+    this.constants.push(constant);
+    this.terms = keepTerms;
     return this;
 };
 

test/expression-spec.js

@@ -751,8 +751,20 @@ describe("Expression simplification", function() {
         answer = answer.simplify(); // 12x + 33
 
         expect(answer.toString()).toEqual("12x + 33");
-    })
-});describe("Expression summation", function() {
+    });
+
+    it("should properly simplify if there are only constants", function() {
+        var exp = new Expression("x").add(3); // x + 3
+        exp = exp.pow(2, false); // xx + 3x + 3x + 3 * 3
+        exp = exp.eval({x: 2}, false); // 2 * 2 + 3 * 2 + 3 * 2 + 3 * 3
+
+        var ans = exp.simplify();
+
+        expect(ans.toString()).toEqual("25");
+    });
+});
+
+describe("Expression summation", function() {
 	it("should return a sum expressions whose variables have been substituted", function() {
 		var xplus3 = new Expression("x").add(3);
 		var ans = xplus3.summation(new Expression("x"), 3, 6);