fix: restore the Equals/GetHashCode contract on B2Vec2

src/Box2D.NET/B2BitSets.cs

@@ -42,15 +42,20 @@ public static void b2SetBitGrow(ref B2BitSet bitSet, int bitIndex)
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static void b2ClearBit(ref B2BitSet bitSet, uint bitIndex)
+        public static void b2ClearBit(ref B2BitSet bitSet, int bitIndex)
         {
-            uint blockIndex = bitIndex / 64;
+            if (bitIndex < 0)
+            {
+                return;
+            }
+
+            int blockIndex = bitIndex / 64;
             if (blockIndex >= bitSet.blockCount)
             {
                 return;
             }
 
-            bitSet.bits[blockIndex] &= ~((ulong)1 << (int)(bitIndex % 64));
+            bitSet.bits[blockIndex] &= ~((ulong)1 << (bitIndex % 64));
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -137,9 +142,13 @@ public static void b2GrowBitSet(ref B2BitSet bitSet, int blockCount)
                 ulong[] newBits = b2Alloc<ulong>(bitSet.blockCapacity);
                 //memset( newBits, 0, bitSet->blockCapacity * sizeof( ulong ) );
                 Array.Fill(newBits, 0UL, 0, bitSet.blockCapacity);
-                B2_ASSERT(bitSet.bits != null);
                 //memcpy( newBits, bitSet->bits, oldCapacity * sizeof( ulong ) );
-                Array.Copy(bitSet.bits, newBits, oldCapacity);
+                // b2Alloc returns null for a zero capacity, and C's memcpy from NULL with a zero
+                // length is harmless, so guard rather than fault on the first grow.
+                if (oldCapacity > 0)
+                {
+                    Array.Copy(bitSet.bits, newBits, oldCapacity);
+                }
                 b2Free(bitSet.bits, oldCapacity);
                 bitSet.bits = newBits;
             }

src/Box2D.NET/B2ConstraintGraphs.cs

@@ -207,8 +207,8 @@ internal static void b2RemoveContactFromGraph(B2World world, int bodyIdA, int bo
             if (colorIndex != B2_OVERFLOW_INDEX)
             {
                 // This might clear a bit for a kinematic or static body, but this has no effect
-                b2ClearBit(ref color.bodySet, (uint)bodyIdA);
-                b2ClearBit(ref color.bodySet, (uint)bodyIdB);
+                b2ClearBit(ref color.bodySet, bodyIdA);
+                b2ClearBit(ref color.bodySet, bodyIdB);
             }
 
             int movedIndex = b2Array_RemoveSwap(ref color.contactSims, localIndex);
@@ -324,8 +324,8 @@ internal static void b2RemoveJointFromGraph(B2World world, int bodyIdA, int body
             if (colorIndex != B2_OVERFLOW_INDEX)
             {
                 // May clear static bodies, no effect
-                b2ClearBit(ref color.bodySet, (uint)bodyIdA);
-                b2ClearBit(ref color.bodySet, (uint)bodyIdB);
+                b2ClearBit(ref color.bodySet, bodyIdA);
+                b2ClearBit(ref color.bodySet, bodyIdB);
             }
 
             int movedIndex = b2Array_RemoveSwap(ref color.jointSims, localIndex);

src/Box2D.NET/B2Diagnostics.cs

@@ -97,7 +97,7 @@ public static void b2SetAssertFcn(b2AssertFcn assertFcn)
             b2AssertHandler = assertFcn;
         }
 
-        internal static int b2InternalAssert(string condition, string fileName, int lineNumber)
+        public static int b2InternalAssert(string condition, string fileName, int lineNumber)
         {
             return b2AssertHandler(condition, fileName, lineNumber);
         }

src/Box2D.NET/B2SolverSets.cs

@@ -330,8 +330,8 @@ internal static void b2TrySleepIsland(B2World world, int islandId)
                     if (colorIndex != B2_OVERFLOW_INDEX)
                     {
                         // might clear a bit for a static body, but this has no effect
-                        b2ClearBit(ref color.bodySet, (uint)contact.edges[0].bodyId);
-                        b2ClearBit(ref color.bodySet, (uint)contact.edges[1].bodyId);
+                        b2ClearBit(ref color.bodySet, contact.edges[0].bodyId);
+                        b2ClearBit(ref color.bodySet, contact.edges[1].bodyId);
                     }
 
                     int localIndex = contact.localIndex;
@@ -380,8 +380,8 @@ internal static void b2TrySleepIsland(B2World world, int islandId)
                     if (colorIndex != B2_OVERFLOW_INDEX)
                     {
                         // might clear a bit for a static body, but this has no effect
-                        b2ClearBit(ref color.bodySet, (uint)joint.edges[0].bodyId);
-                        b2ClearBit(ref color.bodySet, (uint)joint.edges[1].bodyId);
+                        b2ClearBit(ref color.bodySet, joint.edges[0].bodyId);
+                        b2ClearBit(ref color.bodySet, joint.edges[1].bodyId);
                     }
 
                     int sleepJointIndex = sleepSet.jointSims.count;

src/Box2D.NET/B2Vec2.cs

@@ -78,9 +78,11 @@ public B2Vec2(float x, float y)
             return !(a == b);
         }
 
+        // Not "this == other". The operator follows C and uses IEEE compare, where NaN != NaN.
+        // Equals must stay reflexive and agree with GetHashCode, so it compares bitwise.
         public bool Equals(B2Vec2 other)
         {
-            return this == other;
+            return X.Equals(other.X) && Y.Equals(other.Y);
         }
 
         public override bool Equals(object obj)

src/Box2D.NET/B2Worlds.cs

@@ -1152,7 +1152,7 @@ internal static bool DrawQueryCallback(int proxyId, ulong userData, ref B2DrawCo
         // todo this has varying order for moving shapes, causing flicker when overlapping shapes are moving
         // solution: display order by shape id modulus 3, keep 3 buckets in GLSolid* and flush in 3 passes.
         /// Call this to draw shapes and other debug draw data
-        public static void b2World_Draw(B2WorldId worldId, B2DebugDraw draw)
+        public static void b2World_Draw(B2WorldId worldId, in B2DebugDraw draw)
         {
             B2World world = b2GetWorldFromId(worldId);
             B2_ASSERT(world.locked == false);