Add initial 'Leaf' node

src/Paprika.Tests/Merkle/NodeTest.cs

@@ -46,4 +46,25 @@ public void Extension_encoding()
         Assert.That(extension.IsDirty, Is.EqualTo(true));
         Assert.That(extension.Type, Is.EqualTo(NodeType.Extension));
     }
+
+
+    [Test]
+    public void Leaf_encoding()
+    {
+        var rawPath = new byte[] { 0x2, 0x4, 0x6 };
+        var nibblePath = new NibblePath(rawPath, 0, rawPath.Length);
+        var leaf = new LeafNode(nibblePath);
+
+        // TODO: Figure how to store var-length encoded `NibblePath`
+        // Assert.That(leaf.Path.Equals(nibblePath));
+
+        // TODO: Test without unsafe
+        unsafe
+        {
+            Assert.That(sizeof(LeafNode), Is.EqualTo(33));
+        }
+
+        Assert.That(leaf.IsDirty, Is.EqualTo(true));
+        Assert.That(leaf.Type, Is.EqualTo(NodeType.Leaf));
+    }
 }

src/Paprika/Merkle/Node.cs

@@ -74,3 +74,31 @@ public Extension(NibblePath path)
         _header = (byte)NodeType.Extension << 1 | IsDirtyFlag;
     }
 }
+
+[StructLayout(LayoutKind.Explicit, Pack = 1, Size = Size)]
+// Temporary name due to clash with existing `Leaf`
+public readonly struct LeafNode
+{
+    // Leaf: 1 byte for type + dirty, var-length for path, 32 bytes for keccak
+    private const int Size = 33;
+    private const byte IsDirtyFlag = 0b1000;
+    private const byte NodeTypeFlag = 0b0110;
+
+    [FieldOffset(0)]
+    private readonly byte _header;
+
+    [FieldOffset(1)]
+    private readonly Keccak _keccak;
+
+    public Keccak Keccak => _keccak;
+
+    public NibblePath Path => default;
+
+    public bool IsDirty => (_header & IsDirtyFlag) != 0;
+    public NodeType Type => (NodeType)((_header & NodeTypeFlag) >> 1);
+
+    public LeafNode(NibblePath path)
+    {
+        _header = (byte)NodeType.Leaf << 1 | IsDirtyFlag;
+    }
+}