From 0cf525d62ecbc7cf6a4cf3d3643738eaaa679525 Mon Sep 17 00:00:00 2001
From: Anna Shaleva <shaleva.ann@nspcc.ru>
Date: Thu, 7 Apr 2022 18:11:05 +0300
Subject: [PATCH] core: add ability to traverse backwards for Billet

---
 pkg/core/mpt/billet.go | 30 ++++++++++++++++++++++--------
 pkg/core/mpt/trie.go   |  2 +-
 2 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/pkg/core/mpt/billet.go b/pkg/core/mpt/billet.go
index af7edf2be..40ee35de0 100644
--- a/pkg/core/mpt/billet.go
+++ b/pkg/core/mpt/billet.go
@@ -205,7 +205,7 @@ func (b *Billet) incrementRefAndStore(h util.Uint256, bs []byte) {
 // returned from `process` function. It also replaces all HashNodes to their
 // "unhashed" counterparts until the stop condition is satisfied.
 func (b *Billet) Traverse(process func(pathToNode []byte, node Node, nodeBytes []byte) bool, ignoreStorageErr bool) error {
-	r, err := b.traverse(b.root, []byte{}, []byte{}, process, ignoreStorageErr)
+	r, err := b.traverse(b.root, []byte{}, []byte{}, process, ignoreStorageErr, false)
 	if err != nil && !errors.Is(err, errStop) {
 		return err
 	}
@@ -213,7 +213,7 @@ func (b *Billet) Traverse(process func(pathToNode []byte, node Node, nodeBytes [
 	return nil
 }
 
-func (b *Billet) traverse(curr Node, path, from []byte, process func(pathToNode []byte, node Node, nodeBytes []byte) bool, ignoreStorageErr bool) (Node, error) {
+func (b *Billet) traverse(curr Node, path, from []byte, process func(pathToNode []byte, node Node, nodeBytes []byte) bool, ignoreStorageErr bool, backwards bool) (Node, error) {
 	if _, ok := curr.(EmptyNode); ok {
 		// We're not interested in EmptyNodes, and they do not affect the
 		// traversal process, thus remain them untouched.
@@ -227,7 +227,7 @@ func (b *Billet) traverse(curr Node, path, from []byte, process func(pathToNode
 			}
 			return nil, err
 		}
-		return b.traverse(r, path, from, process, ignoreStorageErr)
+		return b.traverse(r, path, from, process, ignoreStorageErr, backwards)
 	}
 	if len(from) == 0 {
 		bytes := slice.Copy(curr.Bytes())
@@ -242,22 +242,36 @@ func (b *Billet) traverse(curr Node, path, from []byte, process func(pathToNode
 		var (
 			startIndex byte
 			endIndex   byte = childrenCount
+			cmp             = func(i int) bool {
+				return i < int(endIndex)
+			}
+			step = 1
 		)
+		if backwards {
+			startIndex, endIndex = lastChild, startIndex
+			cmp = func(i int) bool {
+				return i >= int(endIndex)
+			}
+			step = -1
+		}
 		if len(from) != 0 {
 			endIndex = lastChild
+			if backwards {
+				endIndex = 0
+			}
 			startIndex, from = splitPath(from)
 		}
-		for i := startIndex; i < endIndex; i++ {
+		for i := int(startIndex); cmp(i); i += step {
 			var newPath []byte
 			if i == lastChild {
 				newPath = path
 			} else {
-				newPath = append(path, i)
+				newPath = append(path, byte(i))
 			}
-			if i != startIndex {
+			if byte(i) != startIndex {
 				from = []byte{}
 			}
-			r, err := b.traverse(n.Children[i], newPath, from, process, ignoreStorageErr)
+			r, err := b.traverse(n.Children[i], newPath, from, process, ignoreStorageErr, backwards)
 			if err != nil {
 				if !errors.Is(err, errStop) {
 					return nil, err
@@ -276,7 +290,7 @@ func (b *Billet) traverse(curr Node, path, from []byte, process func(pathToNode
 		} else {
 			return b.tryCollapseExtension(n), nil
 		}
-		r, err := b.traverse(n.next, append(path, n.key...), from, process, ignoreStorageErr)
+		r, err := b.traverse(n.next, append(path, n.key...), from, process, ignoreStorageErr, backwards)
 		if err != nil && !errors.Is(err, errStop) {
 			return nil, err
 		}
diff --git a/pkg/core/mpt/trie.go b/pkg/core/mpt/trie.go
index 608a39cfb..90352af3d 100644
--- a/pkg/core/mpt/trie.go
+++ b/pkg/core/mpt/trie.go
@@ -625,7 +625,7 @@ func (t *Trie) Find(prefix, from []byte, max int) ([]storage.KeyValue, error) {
 		}
 		return count >= max
 	}
-	_, err = b.traverse(start, path, fromP, process, false)
+	_, err = b.traverse(start, path, fromP, process, false, false)
 	if err != nil && !errors.Is(err, errStop) {
 		return nil, err
 	}