[LayoutNG] Initial support for multicol, introducing NGBlockBreakToken.

third_party/WebKit/Source/core/layout/ng/ng_column_mapper.h

Index: third_party/WebKit/Source/core/layout/ng/ng_column_mapper.h
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_column_mapper.h b/third_party/WebKit/Source/core/layout/ng/ng_column_mapper.h
new file mode 100644
index 0000000000000000000000000000000000000000..b4ab09f918c4ee7ce7c22075180d1b81a8cfd928
--- /dev/null
+++ b/third_party/WebKit/Source/core/layout/ng/ng_column_mapper.h
@@ -0,0 +1,82 @@
+// 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.
+
+#ifndef NGColumnMapper_h
+#define NGColumnMapper_h
+
+#include "core/CoreExport.h"
+#include "core/layout/ng/ng_block_break_token.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+// Keeps track of the current column, and is used to map offsets from the
+// coordinate established by the fragmented flow [1] to visual coordinates.
+//
+// [1] https://drafts.csswg.org/css-break/#fragmented-flow
+//
+// TODO(mstensho): Add a NGFragmentainerMapper super class. There are other
+// fragmentation types than multicol (regions, pagination for printing).
+class CORE_EXPORT NGColumnMapper
+    : public GarbageCollectedFinalized<NGColumnMapper> {
+ public:
+  NGColumnMapper(LayoutUnit inline_progression, LayoutUnit block_size)
+      : inline_progression_(inline_progression), block_size_(block_size) {}
+
+  // Column block length.
+  LayoutUnit BlockSize() const { return block_size_; }
+
+  // Offset for next break, in the fragmented flow coordinate space.
+  LayoutUnit NextBreakOffset() const {
+    return last_break_block_offset_ + block_size_;
+  }
+
+  // Convert an offset in the fragmented flow coordinate space to the visual
+  // coordinate space.
+  void ToVisualOffset(NGLogicalOffset& offset) {
+    offset.inline_offset += inline_offset_;
+    offset.block_offset -= last_break_block_offset_;
+  }
+
+  // Specify where in the content to start layout of the next column.
+  void SetBreakToken(NGBlockBreakToken* token) {
+    DCHECK(!break_token_);
+    break_token_ = token;
+  }
+  bool HasBreakToken() const { return break_token_; }
+
+  // Advance to the next column. Returns the break token to resume at. If
+  // nullptr is returned, there's no more content to process.
+  NGBlockBreakToken* Advance() {

[eae, 2017/01/12 23:36:23]

DCHECK(break_token_) or are there cases when we might call advance with a
break_token_ of null?

[mstensho (USE GERRIT), 2017/01/13 14:30:18]

The idea was that it should be legal for break_token_ to be null (and we do cope
with such a situation both here and at the call site), but right now I cannot
think of a situation where it may happen.

+    NGBlockBreakToken* token = break_token_;
+    break_token_ = nullptr;
+    if (token) {
+      inline_offset_ += inline_progression_;
+      last_break_block_offset_ += block_size_;
+    }
+    return token;
+  }
+
+  DEFINE_INLINE_TRACE() { visitor->trace(break_token_); }
+
+ private:
+  // Where to resume in the next column.
+  Member<NGBlockBreakToken> break_token_;
+
+  // The sum of used column-width and column_gap.
+  LayoutUnit inline_progression_;
+
+  // Inline offset to the current column.
+  LayoutUnit inline_offset_;
+
+  // Block size of a column.
+  LayoutUnit block_size_;
+
+  // Amount of content held by previous columns.
+  LayoutUnit last_break_block_offset_;
+};
+
+}  // namespace blink
+
+#endif  // NGColumnMapper_h