Fix GetMerkleProof for odd numbered nodes (#66)

* Fix GetMerkleProof for odd numbered nodes

* Fix lint

Author: galt-tr

go.mod

@@ -1,13 +1,13 @@
 module github.com/bsv-blockchain/go-subtree
 
-go 1.24.3
+go 1.24.6
 
 require (
 	github.com/bsv-blockchain/go-bt/v2 v2.5.1
 	github.com/bsv-blockchain/go-safe-conversion v1.1.0
 	github.com/bsv-blockchain/go-tx-map v1.2.1
 	github.com/stretchr/testify v1.11.1
-	golang.org/x/exp v0.0.0-20250620022241-b7579e27df2b
+	golang.org/x/exp v0.0.0-20251009144603-d2f985daa21b
 )
 
 require (

go.sum

@@ -28,8 +28,8 @@ github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu
 github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
 golang.org/x/crypto v0.45.0 h1:jMBrvKuj23MTlT0bQEOBcAE0mjg8mK9RXFhRH6nyF3Q=
 golang.org/x/crypto v0.45.0/go.mod h1:XTGrrkGJve7CYK7J8PEww4aY7gM3qMCElcJQ8n8JdX4=
-golang.org/x/exp v0.0.0-20250620022241-b7579e27df2b h1:M2rDM6z3Fhozi9O7NWsxAkg/yqS/lQJ6PmkyIV3YP+o=
-golang.org/x/exp v0.0.0-20250620022241-b7579e27df2b/go.mod h1:3//PLf8L/X+8b4vuAfHzxeRUl04Adcb341+IGKfnqS8=
+golang.org/x/exp v0.0.0-20251009144603-d2f985daa21b h1:18qgiDvlvH7kk8Ioa8Ov+K6xCi0GMvmGfGW0sgd/SYA=
+golang.org/x/exp v0.0.0-20251009144603-d2f985daa21b/go.mod h1:j/pmGrbnkbPtQfxEe5D0VQhZC6qKbfKifgD0oM7sR70=
 google.golang.org/protobuf v1.36.10 h1:AYd7cD/uASjIL6Q9LiTjz8JLcrh/88q5UObnmY3aOOE=
 google.golang.org/protobuf v1.36.10/go.mod h1:HTf+CrKn2C3g5S8VImy6tdcUvCska2kB7j23XfzDpco=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=

subtree.go

@@ -529,6 +529,12 @@ func (st *Subtree) GetMerkleProof(index int) ([]*chainhash.Hash, error) {
 // getLeafSiblingHash returns the hash of the sibling node at the leaf level
 func getLeafSiblingHash(nodes []Node, index int) *chainhash.Hash {
 	if index%2 == 0 {
+		// For even index, sibling is at index+1
+		// But if index+1 is out of bounds (odd number of leaves),
+		// duplicate the last node (Bitcoin convention)
+		if index+1 >= len(nodes) {
+			return &nodes[index].Hash
+		}
 		return &nodes[index+1].Hash
 	}
 

subtree_test.go

@@ -1147,3 +1147,97 @@ func Benchmark_NodeIndex(b *testing.B) {
 		require.GreaterOrEqual(b, index, 0)
 	}
 }
+
+func TestGetMerkleProofOddLeaves(t *testing.T) {
+	t.Run("odd number of leaves in power-of-two tree", func(t *testing.T) {
+		// Create a subtree with capacity for 4 nodes but only add 3 (odd number)
+		tree, err := NewTree(2) // height 2 = 2^2 = 4 leaves
+		require.NoError(t, err)
+
+		// Add 3 nodes (odd number)
+		hash1, _ := chainhash.NewHashFromStr("1111111111111111111111111111111111111111111111111111111111111111")
+		hash2, _ := chainhash.NewHashFromStr("2222222222222222222222222222222222222222222222222222222222222222")
+		hash3, _ := chainhash.NewHashFromStr("3333333333333333333333333333333333333333333333333333333333333333")
+
+		err = tree.AddNode(*hash1, 100, 250)
+		require.NoError(t, err)
+		err = tree.AddNode(*hash2, 200, 300)
+		require.NoError(t, err)
+		err = tree.AddNode(*hash3, 150, 275)
+		require.NoError(t, err)
+
+		// Test GetMerkleProof for the last node (index 2)
+		// This should duplicate the last node as its sibling
+		proof, err := tree.GetMerkleProof(2)
+		require.NoError(t, err)
+		require.NotNil(t, proof)
+
+		// The first hash in the proof should be the duplicate of the last node
+		require.Equal(t, hash3.String(), proof[0].String())
+	})
+
+	t.Run("even number of leaves", func(t *testing.T) {
+		// Create a subtree with 4 nodes (even number)
+		tree, err := NewTreeByLeafCount(4)
+		require.NoError(t, err)
+
+		// Add 4 nodes
+		hash1, _ := chainhash.NewHashFromStr("1111111111111111111111111111111111111111111111111111111111111111")
+		hash2, _ := chainhash.NewHashFromStr("2222222222222222222222222222222222222222222222222222222222222222")
+		hash3, _ := chainhash.NewHashFromStr("3333333333333333333333333333333333333333333333333333333333333333")
+		hash4, _ := chainhash.NewHashFromStr("4444444444444444444444444444444444444444444444444444444444444444")
+
+		err = tree.AddNode(*hash1, 100, 250)
+		require.NoError(t, err)
+		err = tree.AddNode(*hash2, 200, 300)
+		require.NoError(t, err)
+		err = tree.AddNode(*hash3, 150, 275)
+		require.NoError(t, err)
+		err = tree.AddNode(*hash4, 175, 325)
+		require.NoError(t, err)
+
+		// Test GetMerkleProof for the second-to-last node (index 2)
+		proof, err := tree.GetMerkleProof(2)
+		require.NoError(t, err)
+		require.NotNil(t, proof)
+
+		// The first hash in the proof should be hash4 (its sibling)
+		require.Equal(t, hash4.String(), proof[0].String())
+
+		// Test GetMerkleProof for the last node (index 3)
+		proof, err = tree.GetMerkleProof(3)
+		require.NoError(t, err)
+		require.NotNil(t, proof)
+
+		// The first hash in the proof should be hash3 (its sibling)
+		require.Equal(t, hash3.String(), proof[0].String())
+	})
+
+	t.Run("large odd number of leaves", func(t *testing.T) {
+		// Test with 541 nodes (the example that was failing)
+		// Tree needs power of 2 capacity: 2^10 = 1024 > 541
+		tree, err := NewTree(10)
+		require.NoError(t, err)
+
+		// Add 541 nodes (odd number)
+		for i := uint64(0); i < uint64(541); i++ {
+			hash, _ := chainhash.NewHashFromStr("0000000000000000000000000000000000000000000000000000000000000001")
+			err = tree.AddNode(*hash, i, i*100)
+			require.NoError(t, err)
+		}
+
+		// Test GetMerkleProof for the last node (index 540)
+		// This was the one causing the panic
+		proof, err := tree.GetMerkleProof(540)
+		require.NoError(t, err)
+		require.NotNil(t, proof)
+
+		// Should not panic and should return a valid proof
+		require.NotEmptyf(t, proof, "Merkle proof should not be empty")
+
+		// The first hash in the proof should be the duplicate of the last node
+		// since 540 is even and has no sibling at index 541
+		lastNodeHash := tree.Nodes[540].Hash
+		require.Equal(t, lastNodeHash.String(), proof[0].String())
+	})
+}