Fix Add (#43)

* Fix Add

* Fix Int256

Author: benaadams

src/Nethermind.Int256.Test/UInt256Tests.cs

@@ -34,6 +34,18 @@ public virtual void Add((BigInteger A, BigInteger B) test)
             res.Convert(out BigInteger resUInt256);
 
             resUInt256.Should().Be(resBigInt);
+
+            // Test reusing input as output
+            a.Add(b, out a);
+            a.Convert(out resUInt256);
+            resUInt256.Should().Be(resBigInt);
+
+            a = convert(test.A);
+
+            // Test reusing input as output
+            a.Add(b, out b);
+            b.Convert(out resUInt256);
+            resUInt256.Should().Be(resBigInt);
         }
 
         [TestCaseSource(typeof(BinaryOps), nameof(BinaryOps.TestCases))]
@@ -186,6 +198,19 @@ public virtual void Multiply((BigInteger A, BigInteger B) test)
             res.Convert(out BigInteger resUInt256);
 
             resUInt256.Should().Be(resBigInt);
+
+            // Test reusing input as output
+            uint256a.Multiply(uint256b, out uint256a);
+            uint256a.Convert(out resUInt256);
+
+            resUInt256.Should().Be(resBigInt);
+
+            uint256a = convert(test.A);
+
+            uint256a.Multiply(uint256b, out uint256b);
+            uint256b.Convert(out resUInt256);
+
+            resUInt256.Should().Be(resBigInt);
         }
 
         [TestCaseSource(typeof(TernaryOps), nameof(TernaryOps.TestCases))]

src/Nethermind.Int256/Int256.cs

@@ -168,18 +168,18 @@ public static void SubtractMod(in Int256 x, in Int256 y, in Int256 m, out Int256
         public static void Multiply(in Int256 a, in Int256 b, out Int256 res)
         {
             Int256 av = a, bv = b;
-            if (a.Sign < 0)
+            int aSign = a.Sign;
+            int bSign = b.Sign;
+            if (aSign < 0)
             {
                 a.Neg(out av);
             }
-            if (b.Sign < 0)
+            if (bSign < 0)
             {
                 b.Neg(out bv);
             }
             UInt256.Multiply(av._value, bv._value, out UInt256 ures);
             res = new Int256(ures);
-            int aSign = a.Sign;
-            int bSign = b.Sign;
             if ((aSign < 0 && bSign < 0) || (aSign >= 0 && bSign >= 0))
             {
                 return;

src/Nethermind.Int256/UInt256.cs

@@ -407,11 +407,12 @@ public static bool AddImpl(in UInt256 a, in UInt256 b, out UInt256 res)
             if ((a.u1 | a.u2 | a.u3 | b.u1 | b.u2 | b.u3) == 0)
             {
                 // Fast add for numbers less than 2^64 (18,446,744,073,709,551,615)
-                ulong u0 = a.u0 + b.u0;
+                ulong a0 = a.u0; // copy to local as we need to check after result clear
+                ulong u0 = a0 + b.u0;
                 // Assignment to res after in case is used as input for a or b (by ref aliasing)
                 res = default;
                 Unsafe.AsRef(in res.u0) = u0;
-                if (u0 < a.u0)
+                if (u0 < a0)
                 {
                     Unsafe.AsRef(in res.u1) = 1;
                 }