nullify the leaf node instead of removing

Makefile

@@ -4,4 +4,8 @@ test:
 
 # Linter
 lint:
-	 golangci-lint --config .golangci.yml run
+	 golangci-lint --config .golangci.yml run
+
+fix-lint:
+# Fix linter
+	golangci-lint --config .golangci.yml run --fix

db/memory/memory_test.go

@@ -9,9 +9,7 @@ import (
 )
 
 func TestMemoryStorageInterface(t *testing.T) {
-	var db merkletree.Storage //nolint:gosimple
-
-	db = NewMemoryStorage()
+	db := NewMemoryStorage()
 	require.NotNil(t, db)
 }
 

db/test/test.go

@@ -109,6 +109,9 @@ func TestAll(t *testing.T, sb StorageBuilder) {
 	t.Run("TestTypesMarshalers", func(t *testing.T) {
 		TestTypesMarshalers(t, sb.NewStorage(t))
 	})
+	t.Run("TestRemoveLeafNearMiddleNode", func(t *testing.T) {
+		TestRemoveLeafNearMiddleNode(t, sb.NewStorage(t))
+	})
 }
 
 // TestReturnKnownErrIfNotExists checks that the implementation of the
@@ -927,3 +930,24 @@ func TestTypesMarshalers(t *testing.T, sto merkletree.Storage) {
 	assert.Nil(t, err)
 	assert.Equal(t, cpp, cpp2)
 }
+
+func TestRemoveLeafNearMiddleNode(t *testing.T, sto merkletree.Storage) {
+	ctx := context.Background()
+	mt, err := merkletree.NewMerkleTree(ctx, sto, 140)
+	require.Nil(t, err)
+
+	values := []*big.Int{big.NewInt(7), big.NewInt(1), big.NewInt(5)}
+
+	for _, v := range values {
+		err = mt.Add(ctx, v, v)
+		require.NoError(t, err)
+	}
+
+	for _, v := range values {
+		err = mt.Delete(ctx, v)
+		require.NoError(t, err)
+		existProof, _, err := mt.GenerateProof(ctx, v, mt.Root())
+		require.NoError(t, err)
+		require.False(t, existProof.Existence)
+	}
+}

merkletree.go

@@ -558,6 +558,37 @@ func (mt *MerkleTree) rmAndUpload(ctx context.Context, path []bool, kHash *Hash,
 			return err
 		}
 	}
+
+	//When deleting a leaf node that is on the same level as middleNode,
+	//need to nullify the leaf node instead of removing it from the tree.
+	nearestSibling, err := mt.db.Get(ctx, toUpload[:])
+	if err != nil {
+		return err
+	}
+	if nearestSibling.Type == NodeTypeMiddle {
+		var newNode *Node
+		if path[len(siblings)-1] {
+			newNode = NewNodeMiddle(toUpload, &HashZero)
+		} else {
+			newNode = NewNodeMiddle(&HashZero, toUpload)
+		}
+		_, err = mt.addNode(ctx, newNode)
+		if err != nil {
+			return err
+		}
+		newRootKey, err := mt.recalculatePathUntilRoot(path, newNode,
+			siblings[:len(siblings)-1])
+		if err != nil {
+			return err
+		}
+		mt.rootKey = newRootKey
+		err = mt.db.SetRoot(ctx, mt.rootKey)
+		if err != nil {
+			return err
+		}
+		return nil
+	}
+
 	for i := len(siblings) - 2; i >= 0; i-- {
 		if !bytes.Equal(siblings[i][:], HashZero[:]) {
 			var newNode *Node