Simplify Autofill Scanner semantics

chrome/browser/autofill/autofill_scanner.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/autofill/autofill_scanner.h"
 
 #include "base/logging.h"
 #include "chrome/browser/autofill/autofill_field.h"
 
 AutofillScanner::AutofillScanner(
     const std::vector<const AutofillField*>& fields)
     : cursor_(fields.begin()),
+      saved_cursor_(fields.begin()),
+      begin_(fields.begin()),
       end_(fields.end()) {
 }
 
 AutofillScanner::~AutofillScanner() {
 }
 
 void AutofillScanner::Advance() {
   DCHECK(!IsEnd());
   ++cursor_;
 }
 
 const AutofillField* AutofillScanner::Cursor() const {
   if (IsEnd()) {
     NOTREACHED();
     return NULL;
   }
 
   return *cursor_;
 }
 
 bool AutofillScanner::IsEnd() const {
   return cursor_ == end_;
 }
 
 void AutofillScanner::Rewind() {
-  DCHECK(!saved_cursors_.empty());
-  cursor_ = saved_cursors_.back();
-  saved_cursors_.pop_back();
+  DCHECK(saved_cursor_ != end_);
+  cursor_ = saved_cursor_;
+  saved_cursor_ = end_;
 }
 
-void AutofillScanner::SaveCursor() {
-  saved_cursors_.push_back(cursor_);
+void AutofillScanner::RewindTo(size_t index) {
+  DCHECK(index < static_cast<size_t>(end_ - begin_));
+  cursor_ = begin_ + index;
+  saved_cursor_ = end_;
 }
+
+size_t AutofillScanner::SaveCursor() {
+  saved_cursor_ = cursor_;
+  return static_cast<size_t>(cursor_ - begin_);
+}