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diff --git a/layout/generic/RubyUtils.h b/layout/generic/RubyUtils.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_RubyUtils_h_
+#define mozilla_RubyUtils_h_
+
+#include "nsCSSAnonBoxes.h"
+#include "nsGkAtoms.h"
+#include "nsIFrame.h"
+#include "nsTArray.h"
+
+#define RTC_ARRAY_SIZE 1
+
+class nsRubyFrame;
+class nsRubyBaseFrame;
+class nsRubyTextFrame;
+class nsRubyContentFrame;
+class nsRubyBaseContainerFrame;
+class nsRubyTextContainerFrame;
+
+namespace mozilla {
+
+/**
+ * Reserved ISize
+ *
+ * With some exceptions, each ruby internal box has two isizes, which
+ * are the reflowed isize and the final isize. The reflowed isize is
+ * what a box itself needs. It is determined when the box gets reflowed.
+ *
+ * The final isize is what a box should be as the final result. For a
+ * ruby base/text box, the final isize is the size of its ruby column.
+ * For a ruby base/text container, the final isize is the size of its
+ * ruby segment. The final isize is never smaller than the reflowed
+ * isize. It is initially determined when a ruby column/segment gets
+ * fully reflowed, and may be advanced when a box is expanded, e.g.
+ * for justification.
+ *
+ * The difference between the reflowed isize and the final isize is
+ * reserved in the line layout after reflowing a box, hence it is called
+ * "Reserved ISize" here. It is used to expand the ruby boxes from their
+ * reflowed isize to the final isize during alignment of the line.
+ *
+ * There are three exceptions for the final isize:
+ * 1. A ruby text container has a larger final isize only if it is for
+ *    a span or collapsed annotations.
+ * 2. A ruby base container has a larger final isize only if at least
+ *    one of its ruby text containers does.
+ * 3. If a ruby text container has a larger final isize, its children
+ *    must not have.
+ */
+
+class RubyUtils {
+ public:
+  static inline bool IsRubyContentBox(LayoutFrameType aFrameType) {
+    return aFrameType == mozilla::LayoutFrameType::RubyBase ||
+           aFrameType == mozilla::LayoutFrameType::RubyText;
+  }
+
+  static inline bool IsRubyContainerBox(LayoutFrameType aFrameType) {
+    return aFrameType == mozilla::LayoutFrameType::RubyBaseContainer ||
+           aFrameType == mozilla::LayoutFrameType::RubyTextContainer;
+  }
+
+  static inline bool IsRubyBox(LayoutFrameType aFrameType) {
+    return aFrameType == mozilla::LayoutFrameType::Ruby ||
+           IsRubyContentBox(aFrameType) || IsRubyContainerBox(aFrameType);
+  }
+
+  static inline bool IsExpandableRubyBox(nsIFrame* aFrame) {
+    mozilla::LayoutFrameType type = aFrame->Type();
+    return IsRubyContentBox(type) || IsRubyContainerBox(type);
+  }
+
+  static inline bool IsRubyPseudo(PseudoStyleType aPseudo) {
+    return aPseudo == PseudoStyleType::blockRubyContent ||
+           aPseudo == PseudoStyleType::ruby ||
+           aPseudo == PseudoStyleType::rubyBase ||
+           aPseudo == PseudoStyleType::rubyText ||
+           aPseudo == PseudoStyleType::rubyBaseContainer ||
+           aPseudo == PseudoStyleType::rubyTextContainer;
+  }
+
+  static void SetReservedISize(nsIFrame* aFrame, nscoord aISize);
+  static void ClearReservedISize(nsIFrame* aFrame);
+  static nscoord GetReservedISize(nsIFrame* aFrame);
+};
+
+/**
+ * This array stores all ruby text containers of the ruby segment
+ * of the given ruby base container.
+ */
+class MOZ_RAII AutoRubyTextContainerArray final
+    : public AutoTArray<nsRubyTextContainerFrame*, RTC_ARRAY_SIZE> {
+ public:
+  explicit AutoRubyTextContainerArray(nsRubyBaseContainerFrame* aBaseContainer);
+};
+
+/**
+ * This enumerator enumerates each ruby segment.
+ */
+class MOZ_STACK_CLASS RubySegmentEnumerator {
+ public:
+  explicit RubySegmentEnumerator(nsRubyFrame* aRubyFrame);
+
+  void Next();
+  bool AtEnd() const { return !mBaseContainer; }
+
+  nsRubyBaseContainerFrame* GetBaseContainer() const { return mBaseContainer; }
+
+ private:
+  nsRubyBaseContainerFrame* mBaseContainer;
+};
+
+/**
+ * Ruby column is a unit consists of one ruby base and all ruby
+ * annotations paired with it.
+ * See http://dev.w3.org/csswg/css-ruby/#ruby-pairing
+ */
+struct MOZ_STACK_CLASS RubyColumn {
+  nsRubyBaseFrame* mBaseFrame;
+  AutoTArray<nsRubyTextFrame*, RTC_ARRAY_SIZE> mTextFrames;
+  bool mIsIntraLevelWhitespace;
+
+  RubyColumn() : mBaseFrame(nullptr), mIsIntraLevelWhitespace(false) {}
+
+  // Helper class to support iteration across the frames within a single
+  // RubyColumn (the column's ruby base and its annotations).
+  class MOZ_STACK_CLASS Iterator {
+   public:
+    nsIFrame* operator*() const;
+
+    Iterator& operator++() {
+      ++mIndex;
+      SkipUntilExistingFrame();
+      return *this;
+    }
+    Iterator operator++(int) {
+      auto ret = *this;
+      ++*this;
+      return ret;
+    }
+
+    friend bool operator==(const Iterator& aIter1, const Iterator& aIter2) {
+      MOZ_ASSERT(&aIter1.mColumn == &aIter2.mColumn,
+                 "Should only compare iterators of the same ruby column");
+      return aIter1.mIndex == aIter2.mIndex;
+    }
+    friend bool operator!=(const Iterator& aIter1, const Iterator& aIter2) {
+      return !(aIter1 == aIter2);
+    }
+
+   private:
+    Iterator(const RubyColumn& aColumn, int32_t aIndex)
+        : mColumn(aColumn), mIndex(aIndex) {
+      MOZ_ASSERT(
+          aIndex == -1 ||
+          (aIndex >= 0 && aIndex <= int32_t(aColumn.mTextFrames.Length())));
+      SkipUntilExistingFrame();
+    }
+    friend struct RubyColumn;  // for the constructor
+
+    void SkipUntilExistingFrame();
+
+    const RubyColumn& mColumn;
+    // -1 means the ruby base frame,
+    // non-negative means the index of ruby text frame
+    // a value of mTextFrames.Length() means we're done iterating
+    int32_t mIndex = -1;
+  };
+
+  Iterator begin() const { return Iterator(*this, -1); }
+  Iterator end() const { return Iterator(*this, mTextFrames.Length()); }
+  Iterator cbegin() const { return begin(); }
+  Iterator cend() const { return end(); }
+};
+
+/**
+ * This enumerator enumerates ruby columns in a segment.
+ */
+class MOZ_STACK_CLASS RubyColumnEnumerator {
+ public:
+  RubyColumnEnumerator(nsRubyBaseContainerFrame* aRBCFrame,
+                       const AutoRubyTextContainerArray& aRTCFrames);
+
+  void Next();
+  bool AtEnd() const;
+
+  uint32_t GetLevelCount() const { return mFrames.Length(); }
+  nsRubyContentFrame* GetFrameAtLevel(uint32_t aIndex) const;
+  void GetColumn(RubyColumn& aColumn) const;
+
+ private:
+  // Frames in this array are NOT necessary part of the current column.
+  // When in doubt, use GetFrameAtLevel to access it.
+  // See GetFrameAtLevel() and Next() for more info.
+  AutoTArray<nsRubyContentFrame*, RTC_ARRAY_SIZE + 1> mFrames;
+  // Whether we are on a column for intra-level whitespaces
+  bool mAtIntraLevelWhitespace;
+};
+
+/**
+ * Stores block-axis leadings produced from ruby annotations.
+ */
+struct RubyBlockLeadings {
+  nscoord mStart = 0;
+  nscoord mEnd = 0;
+
+  void Reset() { mStart = mEnd = 0; }
+  void Update(nscoord aStart, nscoord aEnd) {
+    mStart = std::max(mStart, aStart);
+    mEnd = std::max(mEnd, aEnd);
+  }
+  void Update(const RubyBlockLeadings& aOther) {
+    Update(aOther.mStart, aOther.mEnd);
+  }
+};
+
+}  // namespace mozilla
+
+#endif /* !defined(mozilla_RubyUtils_h_) */