trie: add start key to NodeIterator constructors

The 'step' method is split into two parts, 'peek' and 'push'. peek
returns the next state but doesn't make it current.

The end of iteration was previously tracked by setting 'trie' to nil.
End of iteration is now tracked using the 'iteratorEnd' error, which is
slightly cleaner and requires less code.

1 files changed, 76 insertions, 51 deletions

diff --git a/trie/iterator.go b/trie/iterator.go
index fef5b2593..26ae1d5ad 100644
--- a/trie/iterator.go
+++ b/trie/iterator.go
@@ -19,10 +19,13 @@ package trie
 import (
 	"bytes"
 	"container/heap"
+	"errors"
 
 	"github.com/ethereum/go-ethereum/common"
 )
 
+var iteratorEnd = errors.New("end of iteration")
+
 // Iterator is a key-value trie iterator that traverses a Trie.
 type Iterator struct {
 	nodeIt NodeIterator
@@ -79,25 +82,24 @@ type nodeIteratorState struct {
 	hash    common.Hash // Hash of the node being iterated (nil if not standalone)
 	node    node        // Trie node being iterated
 	parent  common.Hash // Hash of the first full ancestor node (nil if current is the root)
-	child   int         // Child to be processed next
+	index   int         // Child to be processed next
 	pathlen int         // Length of the path to this node
 }
 
 type nodeIterator struct {
 	trie  *Trie                // Trie being iterated
 	stack []*nodeIteratorState // Hierarchy of trie nodes persisting the iteration state
-
-	err error // Failure set in case of an internal error in the iterator
-
-	path []byte // Path to the current node
+	err   error                // Failure set in case of an internal error in the iterator
+	path  []byte               // Path to the current node
 }
 
-// newNodeIterator creates an post-order trie iterator.
-func newNodeIterator(trie *Trie) NodeIterator {
+func newNodeIterator(trie *Trie, start []byte) NodeIterator {
 	if trie.Hash() == emptyState {
 		return new(nodeIterator)
 	}
-	return &nodeIterator{trie: trie}
+	it := &nodeIterator{trie: trie}
+	it.seek(start)
+	return it
 }
 
 // Hash returns the hash of the current node
@@ -147,6 +149,9 @@ func (it *nodeIterator) Path() []byte {
 
 // Error returns the error set in case of an internal error in the iterator
 func (it *nodeIterator) Error() error {
+	if it.err == iteratorEnd {
+		return nil
+	}
 	return it.err
 }
 
@@ -155,47 +160,54 @@ func (it *nodeIterator) Error() error {
 // sets the Error field to the encountered failure. If `descend` is false,
 // skips iterating over any subnodes of the current node.
 func (it *nodeIterator) Next(descend bool) bool {
-	// If the iterator failed previously, don't do anything
 	if it.err != nil {
 		return false
 	}
 	// Otherwise step forward with the iterator and report any errors
-	if err := it.step(descend); err != nil {
+	state, parentIndex, path, err := it.peek(descend)
+	if err != nil {
 		it.err = err
 		return false
 	}
-	return it.trie != nil
+	it.push(state, parentIndex, path)
+	return true
 }
 
-// step moves the iterator to the next node of the trie.
-func (it *nodeIterator) step(descend bool) error {
-	if it.trie == nil {
-		// Abort if we reached the end of the iteration
-		return nil
+func (it *nodeIterator) seek(prefix []byte) {
+	// The path we're looking for is the hex encoded key without terminator.
+	key := keybytesToHex(prefix)
+	key = key[:len(key)-1]
+	// Move forward until we're just before the closest match to key.
+	for {
+		state, parentIndex, path, err := it.peek(bytes.HasPrefix(key, it.path))
+		if err != nil || bytes.Compare(path, key) >= 0 {
+			it.err = err
+			return
+		}
+		it.push(state, parentIndex, path)
 	}
+}
+
+// peek creates the next state of the iterator.
+func (it *nodeIterator) peek(descend bool) (*nodeIteratorState, *int, []byte, error) {
 	if len(it.stack) == 0 {
 		// Initialize the iterator if we've just started.
 		root := it.trie.Hash()
-		state := &nodeIteratorState{node: it.trie.root, child: -1}
+		state := &nodeIteratorState{node: it.trie.root, index: -1}
 		if root != emptyRoot {
 			state.hash = root
 		}
-		it.stack = append(it.stack, state)
-		return nil
+		return state, nil, nil, nil
 	}
-
 	if !descend {
 		// If we're skipping children, pop the current node first
-		it.path = it.path[:it.stack[len(it.stack)-1].pathlen]
-		it.stack = it.stack[:len(it.stack)-1]
+		it.pop()
 	}
 
 	// Continue iteration to the next child
-outer:
 	for {
 		if len(it.stack) == 0 {
-			it.trie = nil
-			return nil
+			return nil, nil, nil, iteratorEnd
 		}
 		parent := it.stack[len(it.stack)-1]
 		ancestor := parent.hash
@@ -203,63 +215,76 @@ outer:
 			ancestor = parent.parent
 		}
 		if node, ok := parent.node.(*fullNode); ok {
-			// Full node, iterate over children
-			for parent.child++; parent.child < len(node.Children); parent.child++ {
-				child := node.Children[parent.child]
+			// Full node, move to the first non-nil child.
+			for i := parent.index + 1; i < len(node.Children); i++ {
+				child := node.Children[i]
 				if child != nil {
 					hash, _ := child.cache()
-					it.stack = append(it.stack, &nodeIteratorState{
+					state := &nodeIteratorState{
 						hash:    common.BytesToHash(hash),
 						node:    child,
 						parent:  ancestor,
-						child:   -1,
+						index:   -1,
 						pathlen: len(it.path),
-					})
-					it.path = append(it.path, byte(parent.child))
-					break outer
+					}
+					path := append(it.path, byte(i))
+					parent.index = i - 1
+					return state, &parent.index, path, nil
 				}
 			}
 		} else if node, ok := parent.node.(*shortNode); ok {
 			// Short node, return the pointer singleton child
-			if parent.child < 0 {
-				parent.child++
+			if parent.index < 0 {
 				hash, _ := node.Val.cache()
-				it.stack = append(it.stack, &nodeIteratorState{
+				state := &nodeIteratorState{
 					hash:    common.BytesToHash(hash),
 					node:    node.Val,
 					parent:  ancestor,
-					child:   -1,
+					index:   -1,
 					pathlen: len(it.path),
-				})
+				}
+				var path []byte
 				if hasTerm(node.Key) {
-					it.path = append(it.path, node.Key[:len(node.Key)-1]...)
+					path = append(it.path, node.Key[:len(node.Key)-1]...)
 				} else {
-					it.path = append(it.path, node.Key...)
+					path = append(it.path, node.Key...)
 				}
-				break
+				return state, &parent.index, path, nil
 			}
 		} else if hash, ok := parent.node.(hashNode); ok {
 			// Hash node, resolve the hash child from the database
-			if parent.child < 0 {
-				parent.child++
+			if parent.index < 0 {
 				node, err := it.trie.resolveHash(hash, nil, nil)
 				if err != nil {
-					return err
+					return it.stack[len(it.stack)-1], &parent.index, it.path, err
 				}
-				it.stack = append(it.stack, &nodeIteratorState{
+				state := &nodeIteratorState{
 					hash:    common.BytesToHash(hash),
 					node:    node,
 					parent:  ancestor,
-					child:   -1,
+					index:   -1,
 					pathlen: len(it.path),
-				})
-				break
+				}
+				return state, &parent.index, it.path, nil
 			}
 		}
-		it.path = it.path[:parent.pathlen]
-		it.stack = it.stack[:len(it.stack)-1]
+		// No more child nodes, move back up.
+		it.pop()
 	}
-	return nil
+}
+
+func (it *nodeIterator) push(state *nodeIteratorState, parentIndex *int, path []byte) {
+	it.path = path
+	it.stack = append(it.stack, state)
+	if parentIndex != nil {
+		*parentIndex += 1
+	}
+}
+
+func (it *nodeIterator) pop() {
+	parent := it.stack[len(it.stack)-1]
+	it.path = it.path[:parent.pathlen]
+	it.stack = it.stack[:len(it.stack)-1]
 }
 
 func compareNodes(a, b NodeIterator) int {