introduce two layer trie

db/trie/branchroottrie.go

@@ -74,6 +74,10 @@ func (tr *branchRootTrie) SetRootHash(rootHash []byte) error {
 	return nil
 }
 
+func (tr *branchRootTrie) IsEmpty() bool {
+	return tr.isEmptyRootHash(tr.rootHash)
+}
+
 func (tr *branchRootTrie) Get(key []byte) ([]byte, error) {
 	trieMtc.WithLabelValues("root", "Get").Inc()
 	kt, err := tr.checkKeyType(key)

db/trie/trie.go

@@ -58,6 +58,8 @@ type Trie interface {
 	RootHash() []byte
 	// SetRootHash sets a new root to trie
 	SetRootHash([]byte) error
+	// IsEmpty returns true is this is an empty trie
+	IsEmpty() bool
 	// DB returns the KVStore storing the node data
 	DB() KVStore
 	// deleteNodeFromDB deletes the data of node from db

state/factory/twolayertrie.go

@@ -0,0 +1,100 @@
+// Copyright (c) 2019 IoTeX Foundation
+// This is an alpha (internal) release and is not suitable for production. This source code is provided 'as is' and no
+// warranties are given as to title or non-infringement, merchantability or fitness for purpose and, to the extent
+// permitted by law, all liability for your use of the code is disclaimed. This source code is governed by Apache
+// License 2.0 that can be found in the LICENSE file.
+
+package factory
+
+import (
+	"context"
+
+	"github.com/iotexproject/iotex-core/db/trie"
+	"github.com/pkg/errors"
+)
+
+// TwoLayerTrie is a trie data structure with two layers
+type TwoLayerTrie struct {
+	layerOne trie.Trie
+}
+
+// Start starts the layer one trie
+func (tlt *TwoLayerTrie) Start(ctx context.Context) error {
+	return tlt.layerOne.Start(ctx)
+}
+
+// Stop stops the layer one trie
+func (tlt *TwoLayerTrie) Stop(ctx context.Context) error {
+	return tlt.layerOne.Stop(ctx)
+}
+
+// RootHash returns the layer one trie root
+func (tlt *TwoLayerTrie) RootHash() []byte {
+	return tlt.layerOne.RootHash()
+}
+
+func (tlt *TwoLayerTrie) layerTwoTrie(key []byte) (trie.Trie, error) {
+	value, err := tlt.layerOne.Get(key)
+	switch errors.Cause(err) {
+	case trie.ErrNotExist:
+		return trie.NewTrie(trie.KVStoreOption(tlt.layerOne.DB()))
+	case nil:
+		return trie.NewTrie(trie.KVStoreOption(tlt.layerOne.DB()), trie.RootHashOption(value))
+	default:
+		return nil, err
+	}
+}
+
+// Get returns the layer two value
+func (tlt *TwoLayerTrie) Get(layerOneKey []byte, layerTwoKey []byte) ([]byte, error) {
+	layerTwo, err := tlt.layerTwoTrie(layerOneKey)
+	if err != nil {
+		return nil, err
+	}
+	if err := layerTwo.Start(context.Background()); err != nil {
+		return nil, err
+	}
+	defer layerTwo.Stop(context.Background())
+
+	return layerTwo.Get(layerTwoKey)
+}
+
+// Upsert upserts an item
+func (tlt *TwoLayerTrie) Upsert(layerOneKey []byte, layerTwoKey []byte, value []byte) error {
+	layerTwo, err := tlt.layerTwoTrie(layerOneKey)
+	if err != nil {
+		return err
+	}
+	if err := layerTwo.Start(context.Background()); err != nil {
+		return err
+	}
+	defer layerTwo.Stop(context.Background())
+
+	if err := layerTwo.Upsert(layerTwoKey, value); err != nil {
+		return err
+	}
+
+	return tlt.layerOne.Upsert(layerOneKey, layerTwo.RootHash())
+}
+
+// Delete deletes an item
+func (tlt *TwoLayerTrie) Delete(layerOneKey []byte, layerTwoKey []byte) error {
+	layerTwo, err := tlt.layerTwoTrie(layerOneKey)
+	if err != nil {
+		return err
+	}
+	if err := layerTwo.Start(context.Background()); err != nil {
+		return err
+	}
+	defer layerTwo.Stop(context.Background())
+
+	if err := layerTwo.Delete(layerTwoKey); err != nil {
+		return err
+	}
+
+	if layerTwo.IsEmpty() {
+		return tlt.layerOne.Delete(layerOneKey)
+	}
+
+	return tlt.layerOne.Upsert(layerOneKey, layerTwo.RootHash())
+}

state/factory/twolayertrie_test.go

@@ -0,0 +1,9 @@
+// Copyright (c) 2019 IoTeX Foundation
+// This is an alpha (internal) release and is not suitable for production. This source code is provided 'as is' and no
+// warranties are given as to title or non-infringement, merchantability or fitness for purpose and, to the extent
+// permitted by law, all liability for your use of the code is disclaimed. This source code is governed by Apache
+// License 2.0 that can be found in the LICENSE file.
+
+package factory
+
+// TODO: add unit test for two layer trie