fixes a bug when subtracting from empty, adds proper multiplication identity, adds overloads for certain operators (add, subtract, negation, multiply)

Author: rfellers

src/TopDownProteomics/Chemistry/ChemicalFormula.cs

@@ -314,6 +314,8 @@ private ChemicalFormula Merge(ChemicalFormula otherFormula, bool add)
 
                     for (int i = 0; i < newCommonElements.Length; i++)
                         newCommonElements[i] = -otherCommonElements[i];
+
+                    formula._commonElements = newCommonElements;
                 }
 
                 formula._commonElementEntities = otherFormula._commonElementEntities;
@@ -380,7 +382,8 @@ private ChemicalFormula Merge(ChemicalFormula otherFormula, bool add)
         /// <returns></returns>
         public ChemicalFormula Multiply(int multiplier)
         {
-            if (multiplier == 0)
+            // If either side is '0' just return Empty
+            if (this == Empty || multiplier == 0)
                 return Empty;
 
             if (multiplier == 1)
@@ -413,6 +416,32 @@ public ChemicalFormula Multiply(int multiplier)
             return formula;
         }
 
+        #region Operator Overloads
+        /// <summary>Implements the operator +.</summary>
+        /// <param name="c1">The c1.</param>
+        /// <param name="c2">The c2.</param>
+        public static ChemicalFormula operator +(ChemicalFormula c1, ChemicalFormula c2) => c1.Merge(c2, true);
+
+        /// <summary>Implements the operator -.</summary>
+        /// <param name="c1">The c1.</param>
+        /// <param name="c2">The c2.</param>
+        public static ChemicalFormula operator -(ChemicalFormula c1, ChemicalFormula c2) => c1.Merge(c2, false);
+
+        /// <summary>Implements the operator - (negation).</summary>
+        /// <param name="c1">The c1.</param>
+        public static ChemicalFormula operator -(ChemicalFormula c1) => c1.Multiply(-1);
+
+        /// <summary>Implements the operator *.</summary>
+        /// <param name="c1">The c1.</param>
+        /// <param name="multiplier">The multiplier.</param>
+        public static ChemicalFormula operator *(ChemicalFormula c1, int multiplier) => c1.Multiply(multiplier);
+
+        /// <summary>Implements the operator *.</summary>
+        /// <param name="multiplier">The multiplier.</param>
+        /// <param name="c1">The c1.</param>
+        public static ChemicalFormula operator *(int multiplier, ChemicalFormula c1) => c1.Multiply(multiplier);
+        #endregion
+
         /// <summary>Parses the string into a chemical formula.</summary>
         /// <param name="formula">The formula.</param>
         /// <param name="elementProvider">The element provider.</param>

tests/TopDownProteomics.Tests/Chemistry/ChemicalFormulaTest.cs

@@ -419,6 +419,49 @@ public void SubtractTest()
             });
         }
 
+        [Test]
+        public void WorkingWithEmptyTest()
+        {
+            var a = ChemicalFormula.Empty;
+            var b = ChemicalFormula.ParseString("CH2".AsSpan(), _elementProvider);
+
+            var diff = a.Add(b);
+
+            this.SimpleParseTest(diff, new[]
+            {
+                Tuple.Create("C", 1),
+                Tuple.Create("H", 2),
+            });
+
+            diff = a.Subtract(b);
+
+            this.SimpleParseTest(diff, new[]
+            {
+                Tuple.Create("C", -1),
+                Tuple.Create("H", -2),
+            });
+
+            diff = a.Multiply(12);
+
+            Assert.AreEqual(ChemicalFormula.Empty, diff);
+        }
+
+        [Test]
+        public void OperatorOverloadsTest()
+        {
+            var a = ChemicalFormula.ParseString("C2H4".AsSpan(), _elementProvider);
+            var b = ChemicalFormula.ParseString("CH2".AsSpan(), _elementProvider);
+
+            this.SimpleParseTest(a + b, new[] { Tuple.Create("C", 3), Tuple.Create("H", 6), });
+            this.SimpleParseTest(a - b, new[] { Tuple.Create("C", 1), Tuple.Create("H", 2), });
+            this.SimpleParseTest(a * 3, new[] { Tuple.Create("C", 6), Tuple.Create("H", 12), });
+            this.SimpleParseTest(3 * a, new[] { Tuple.Create("C", 6), Tuple.Create("H", 12), });
+            this.SimpleParseTest(-a, new[] { Tuple.Create("C", -2), Tuple.Create("H", -4), });
+
+            Assert.IsFalse(a == b);
+            Assert.IsTrue(a != b);
+        }
+
         [Test]
         public void HashCodeTest()
         {