cc: Detach spiral iterator implementation to separate file.

cc/base/spiral_iterator.h

Index: cc/base/spiral_iterator.h
diff --git a/cc/base/spiral_iterator.h b/cc/base/spiral_iterator.h
new file mode 100644
index 0000000000000000000000000000000000000000..d30176d0037d64833da90abf8021c060b5e80cc1
--- /dev/null
+++ b/cc/base/spiral_iterator.h
@@ -0,0 +1,71 @@
+// Copyright 2016 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.
+
+#ifndef CC_BASE_SPIRAL_ITERATOR_H_
+#define CC_BASE_SPIRAL_ITERATOR_H_
+
+#include "base/logging.h"
+#include "cc/base/cc_export.h"
+#include "cc/base/index_rect.h"
+
+namespace cc {
+
+// The spiral iterator which iterates based on directions around the center
+// rect in the given region. If the center rect is at index (2, 2), spiral
+// iterator gives following sequence on iterating.
+//
+//    x 0   1   2   3   4
+//  y ┌───┬───┬───┬───┬───┐
+//  0 │ 16│ 15│ 14│ 13│ 12│
+//    ├───┼───┼───┼───┼───┤
+//  1 │ 17│  4│  3│  2│ 11│
+//    ├───┼───┼───┼───┼───┤
+//  2 │ 18│  5│  *│  1│ 10│
+//    ├───┼───┼───┼───┼───┤
+//  3 │ 19│  6│  7│  8│  9│
+//    ├───┼───┼───┼───┼───┤
+//  4 │ 20│ 21│ 22│ 23│ 24│
+//    └───┴───┴───┴───┴───┘
+class CC_EXPORT SpiralIterator {
+ public:
+  SpiralIterator();
+  SpiralIterator(const IndexRect& around_index_rect,
+                 const IndexRect& consider_index_rect,
+                 const IndexRect& ignore_index_rect);
+
+  ~SpiralIterator() = default;
+
+  operator bool() const;
+  SpiralIterator& operator++();
+  int index_x() const { return index_x_; }
+  int index_y() const { return index_y_; }
+
+ private:
+  int current_step_count() const {
+    return (direction_ == UP || direction_ == DOWN) ? vertical_step_count_
+                                                    : horizontal_step_count_;
+  }
+
+  bool needs_direction_switch() const;
+  void switch_direction();
+
+  enum Direction { UP, LEFT, DOWN, RIGHT };
+
+  IndexRect around_index_rect_;
+  IndexRect consider_index_rect_;
+  IndexRect ignore_index_rect_;
+  int index_x_;
+  int index_y_;
+
+  Direction direction_;
+  int delta_x_;
+  int delta_y_;
+  int current_step_;
+  int horizontal_step_count_;
+  int vertical_step_count_;
+};
+
+}  // namespace cc
+
+#endif  // CC_BASE_SPIRAL_ITERATOR_H_