Fix edge cases

Author: lduchosal

src/System.Net.IPNetwork/IPNetwork2Operators.cs

@@ -117,6 +117,14 @@ public sealed partial class IPNetwork2
     /// <returns>Adds the specified number of IP addresses to the network range.</returns>
     public static IEnumerable<IPNetwork2> operator +(IPNetwork2 left, int add)
     {
+        if (add < 0)
+        {
+            return left - (add * -1);
+        }
+        if (add == 0)
+        {
+            return new[] { left };
+        }
         var start = ToBigInteger(left.First);
         var last = ToBigInteger(left.Last);
         var end = last+ add;
@@ -148,6 +156,27 @@ public sealed partial class IPNetwork2
     /// <returns>Try to supernet two consecutive cidr equal subnet into a single one, otherwise return both netowkrs.</returns>
     public static IEnumerable<IPNetwork2> operator -(IPNetwork2 left, int subtract)
     {
-        return left + (subtract * -1);
+        if (subtract < 0)
+        {
+            return left + (subtract * -1);
+        }
+        if (subtract == 0)
+        {
+            return new[] { left };
+        }
+        var start = ToBigInteger(left.First);
+        var last = ToBigInteger(left.Last);
+        var end = last- subtract;
+
+        if (end < start)
+        {
+            return [];
+        }
+
+        var startIp = ToIPAddress(start, left.AddressFamily);
+        var endIp = ToIPAddress(end, left.AddressFamily);
+        
+        InternalParseRange(true, startIp, endIp, out IEnumerable<IPNetwork2> networks);
+        return networks;
     }
 }

src/TestProject/IPNetworkTest/IPNetworkOperatorTests.cs

@@ -336,6 +336,7 @@ public void TestOperatorAdd(string left, string right, int count)
     ///     Tests Operator functionality with Operator +.
     /// </summary>
     [TestMethod]
+    [DataRow("10.0.0.0/32", 0, "10.0.0.0/32")]
     [DataRow("10.0.0.0/32", 1, "10.0.0.0/31")]
     [DataRow("10.0.0.0/32", 2, "10.0.0.0/31, 10.0.0.2/32")]
     [DataRow("10.0.0.0/32", 3, "10.0.0.0/30")]
@@ -377,12 +378,12 @@ public void TestOperatorAddOverflow(string left, int right, string expected)
         Assert.ThrowsExactly<OverflowException>(() => ipn1 + right);
     }
 
-
     /// <summary>
     ///     Tests Operator functionality with Operator -.
     /// </summary>
     [TestMethod]
     [DataRow("10.0.0.0/32", 1, "")]
+    [DataRow("255.255.255.255/32", 1, "")]
     [DataRow("10.0.0.0/32", 3, "")]
     [DataRow("10.0.0.0/32", 7, "")]
     [DataRow("10.0.0.0/32", 15, "")]
@@ -403,4 +404,20 @@ public void TestOperatorsubtract2(string left, int right, string expected)
 
         Assert.AreEqual(expected, sresult);
     }
+    
+    /// <summary>
+    ///     Tests Operator functionality with Operator -.
+    /// </summary>
+    [TestMethod]
+    [DataRow("0.0.0.0/0", -1)]
+    [DataRow("255.255.255.255/32", -1)]
+    public void TestOperatorsubtractOverflow(string left, int right)
+    {
+        var ipn1 = IPNetwork2.Parse(left);
+        Assert.ThrowsExactly<OverflowException>(() =>
+        {
+            var result = ipn1 - right;
+            Assert.AreEqual("32", result.ToString());
+        });
+    }
 }