[LayoutNG] Introduce block child iterator.

third_party/WebKit/Source/core/layout/ng/ng_block_child_iterator.cc

+// Copyright 2017 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 "core/layout/ng/ng_block_child_iterator.h"
+
+#include "core/layout/ng/ng_block_break_token.h"
+#include "core/layout/ng/ng_layout_input_node.h"
+
+namespace blink {
+
+NGBlockChildIterator::NGBlockChildIterator(NGLayoutInputNode* first_child,
+                                           NGBlockBreakToken* break_token)
+    : child_(first_child), break_token_(break_token), child_token_idx_(0) {}
+
+NGBlockChildIterator::Entry NGBlockChildIterator::NextChild() {
+  NGBreakToken* child_break_token = nullptr;
+
+  if (break_token_) {
+    // If we're resuming layout after a fragmentainer break, we need to skip
+    // siblings that we're done with. We may have been able to fully lay out
+    // some node(s) preceding a node that we had to break inside (and therefore
+    // were not able to fully lay out). This happens when we have parallel
+    // flows [1], which are caused by floats, overflow, etc.
+    //
+    // [1] https://drafts.csswg.org/css-break/#parallel-flows
+    const auto& child_break_tokens = break_token_->ChildBreakTokens();
+
+    do {
+      // Early exit if we've exhausted our child break tokens.
+      if (child_token_idx_ >= child_break_tokens.size())
+        break;
+
+      // This child break token candidate doesn't match the current node, this
+      // node must be unfinished.
+      NGBreakToken* child_break_token_candidate =
+          child_break_tokens[child_token_idx_];
+      if (child_break_token_candidate->InputNode() != child_)
+        break;
+
+      ++child_token_idx_;
+
+      // We have only found a node if its break token is unfinished.
+      if (!child_break_token_candidate->IsFinished()) {
+        child_break_token = child_break_token_candidate;
+        break;
+      }
+    } while ((child_ = child_->NextSibling()));
+  }
+
+  NGLayoutInputNode* child = child_;
+  if (child_)
+    child_ = child_->NextSibling();
+
+  return Entry(child, child_break_token);
+}
+
+}  // namespace blink