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.

trie/iterator_test.go

@@ -17,6 +17,8 @@
 package trie
 
 import (
+	"bytes"
+	"fmt"
 	"testing"
 
 	"github.com/ethereum/go-ethereum/common"
@@ -42,7 +44,7 @@ func TestIterator(t *testing.T) {
 	trie.Commit()
 
 	found := make(map[string]string)
-	it := NewIterator(trie.NodeIterator())
+	it := NewIterator(trie.NodeIterator(nil))
 	for it.Next() {
 		found[string(it.Key)] = string(it.Value)
 	}
@@ -72,7 +74,7 @@ func TestIteratorLargeData(t *testing.T) {
 		vals[string(value2.k)] = value2
 	}
 
-	it := NewIterator(trie.NodeIterator())
+	it := NewIterator(trie.NodeIterator(nil))
 	for it.Next() {
 		vals[string(it.Key)].t = true
 	}
@@ -99,7 +101,7 @@ func TestNodeIteratorCoverage(t *testing.T) {
 
 	// Gather all the node hashes found by the iterator
 	hashes := make(map[common.Hash]struct{})
-	for it := trie.NodeIterator(); it.Next(true); {
+	for it := trie.NodeIterator(nil); it.Next(true); {
 		if it.Hash() != (common.Hash{}) {
 			hashes[it.Hash()] = struct{}{}
 		}
@@ -117,18 +119,20 @@ func TestNodeIteratorCoverage(t *testing.T) {
 	}
 }
 
-var testdata1 = []struct{ k, v string }{
-	{"bar", "b"},
+type kvs struct{ k, v string }
+
+var testdata1 = []kvs{
 	{"barb", "ba"},
-	{"bars", "bb"},
 	{"bard", "bc"},
+	{"bars", "bb"},
+	{"bar", "b"},
 	{"fab", "z"},
-	{"foo", "a"},
 	{"food", "ab"},
 	{"foos", "aa"},
+	{"foo", "a"},
 }
 
-var testdata2 = []struct{ k, v string }{
+var testdata2 = []kvs{
 	{"aardvark", "c"},
 	{"bar", "b"},
 	{"barb", "bd"},
@@ -140,6 +144,47 @@ var testdata2 = []struct{ k, v string }{
 	{"jars", "d"},
 }
 
+func TestIteratorSeek(t *testing.T) {
+	trie := newEmpty()
+	for _, val := range testdata1 {
+		trie.Update([]byte(val.k), []byte(val.v))
+	}
+
+	// Seek to the middle.
+	it := NewIterator(trie.NodeIterator([]byte("fab")))
+	if err := checkIteratorOrder(testdata1[4:], it); err != nil {
+		t.Fatal(err)
+	}
+
+	// Seek to a non-existent key.
+	it = NewIterator(trie.NodeIterator([]byte("barc")))
+	if err := checkIteratorOrder(testdata1[1:], it); err != nil {
+		t.Fatal(err)
+	}
+
+	// Seek beyond the end.
+	it = NewIterator(trie.NodeIterator([]byte("z")))
+	if err := checkIteratorOrder(nil, it); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func checkIteratorOrder(want []kvs, it *Iterator) error {
+	for it.Next() {
+		if len(want) == 0 {
+			return fmt.Errorf("didn't expect any more values, got key %q", it.Key)
+		}
+		if !bytes.Equal(it.Key, []byte(want[0].k)) {
+			return fmt.Errorf("wrong key: got %q, want %q", it.Key, want[0].k)
+		}
+		want = want[1:]
+	}
+	if len(want) > 0 {
+		return fmt.Errorf("iterator ended early, want key %q", want[0])
+	}
+	return nil
+}
+
 func TestDifferenceIterator(t *testing.T) {
 	triea := newEmpty()
 	for _, val := range testdata1 {
@@ -154,7 +199,7 @@ func TestDifferenceIterator(t *testing.T) {
 	trieb.Commit()
 
 	found := make(map[string]string)
-	di, _ := NewDifferenceIterator(triea.NodeIterator(), trieb.NodeIterator())
+	di, _ := NewDifferenceIterator(triea.NodeIterator(nil), trieb.NodeIterator(nil))
 	it := NewIterator(di)
 	for it.Next() {
 		found[string(it.Key)] = string(it.Value)
@@ -189,7 +234,7 @@ func TestUnionIterator(t *testing.T) {
 	}
 	trieb.Commit()
 
-	di, _ := NewUnionIterator([]NodeIterator{triea.NodeIterator(), trieb.NodeIterator()})
+	di, _ := NewUnionIterator([]NodeIterator{triea.NodeIterator(nil), trieb.NodeIterator(nil)})
 	it := NewIterator(di)
 
 	all := []struct{ k, v string }{