Add Tree.Snapshot method

tree.go

@@ -756,6 +756,30 @@ func (t *Tree) GetNLeafs() (int, error) {
 	return int(nLeafs), nil
 }
 
+// Snapshot returns a copy of the Tree from the given root
+func (t *Tree) Snapshot(rootKey []byte) (*Tree, error) {
+	// TODO currently this method only changes the 'root pointer', but the
+	// db continues being the same. In a future iteration, once the
+	// db.Database interface allows to do database checkpoints, this method
+	// could be updated to do a full checkpoint of the database for the
+	// snapshot, to return a completly new independent tree containing the
+	// snapshot.
+	t.RLock()
+	defer t.RUnlock()
+
+	// allow to define which root to use
+	if rootKey == nil {
+		rootKey = t.Root()
+	}
+	return &Tree{
+		db:           t.db,
+		maxLevels:    t.maxLevels,
+		root:         rootKey,
+		hashFunction: t.hashFunction,
+		dbg:          t.dbg,
+	}, nil
+}
+
 // Iterate iterates through the full Tree, executing the given function on each
 // node of the Tree.
 func (t *Tree) Iterate(rootKey []byte, f func([]byte, []byte)) error {
@@ -844,8 +868,8 @@ func (t *Tree) Dump(rootKey []byte) ([]byte, error) {
 	return b, err
 }
 
-// ImportDump imports the leafs (that have been exported with the ExportLeafs
+// ImportDump imports the leafs (that have been exported with the Dump method)
-// method) in the Tree.
+// in the Tree.
 func (t *Tree) ImportDump(b []byte) error {
 	r := bytes.NewReader(b)
 	var err error

tree_test.go

@@ -427,6 +427,61 @@ func TestSetGetNLeafs(t *testing.T) {
 	c.Assert(n, qt.Equals, maxInt)
 }
 
+func TestSnapshot(t *testing.T) {
+	c := qt.New(t)
+	database, err := db.NewBadgerDB(c.TempDir())
+	c.Assert(err, qt.IsNil)
+	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	c.Assert(err, qt.IsNil)
+
+	// fill the tree
+	bLen := tree.HashFunction().Len()
+	var keys, values [][]byte
+	for i := 0; i < 1000; i++ {
+		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
+		v := BigIntToBytes(bLen, big.NewInt(int64(i)))
+		keys = append(keys, k)
+		values = append(values, v)
+	}
+	indexes, err := tree.AddBatch(keys, values)
+	c.Assert(err, qt.IsNil)
+	c.Check(len(indexes), qt.Equals, 0)
+
+	rootBI := BytesToBigInt(tree.Root())
+	c.Check(rootBI.String(), qt.Equals,
+		"13742386369878513332697380582061714160370929283209286127733983161245560237407")
+
+	tree2, err := tree.Snapshot(nil)
+	c.Assert(err, qt.IsNil)
+	rootBI = BytesToBigInt(tree2.Root())
+	c.Check(rootBI.String(), qt.Equals,
+		"13742386369878513332697380582061714160370929283209286127733983161245560237407")
+
+	// add k-v to original tree
+	k := BigIntToBytes(bLen, big.NewInt(int64(1000)))
+	v := BigIntToBytes(bLen, big.NewInt(int64(1000)))
+	err = tree.Add(k, v)
+	c.Assert(err, qt.IsNil)
+
+	// expect original tree to have new root
+	rootBI = BytesToBigInt(tree.Root())
+	c.Check(rootBI.String(), qt.Equals,
+		"10747149055773881257049574592162159501044114324358186833013814735296193179713")
+
+	// expect snapshot tree to have the old root
+	rootBI = BytesToBigInt(tree2.Root())
+	c.Check(rootBI.String(), qt.Equals,
+		"13742386369878513332697380582061714160370929283209286127733983161245560237407")
+
+	err = tree2.Add(k, v)
+	c.Assert(err, qt.IsNil)
+	// after adding also the k-v into the snapshot tree, expect original
+	// tree to have new root
+	rootBI = BytesToBigInt(tree.Root())
+	c.Check(rootBI.String(), qt.Equals,
+		"10747149055773881257049574592162159501044114324358186833013814735296193179713")
+}
+
 func BenchmarkAdd(b *testing.B) {
 	bLen := 32 // for both Poseidon & Sha256
 	// prepare inputs