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diff --git a/layout/base/nsCounterManager.h b/layout/base/nsCounterManager.h
new file mode 100644
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+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+// vim:cindent:ai:sw=4:ts=4:et:
+/* 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/. */
+
+/* implementation of CSS counters (for numbering things) */
+
+#ifndef nsCounterManager_h_
+#define nsCounterManager_h_
+
+#include "mozilla/Attributes.h"
+#include "nsGenConList.h"
+#include "nsClassHashtable.h"
+#include "mozilla/Likely.h"
+#include "CounterStyleManager.h"
+
+class nsCounterList;
+struct nsCounterUseNode;
+struct nsCounterChangeNode;
+
+struct nsCounterNode : public nsGenConNode {
+  enum Type {
+    RESET,      // a "counter number" pair in 'counter-reset'
+    INCREMENT,  // a "counter number" pair in 'counter-increment'
+    SET,        // a "counter number" pair in 'counter-set'
+    USE         // counter() or counters() in 'content'
+  };
+
+  Type mType;
+
+  // Counter value after this node
+  int32_t mValueAfter = 0;
+
+  // mScopeStart points to the node (usually a RESET, but not in the
+  // case of an implied 'counter-reset') that created the scope for
+  // this element (for a RESET, its outer scope, i.e., the one it is
+  // inside rather than the one it creates).
+
+  // May be null for all types, but only when mScopePrev is also null.
+  // Being null for a non-RESET means that it is an implied
+  // 'counter-reset'.  Being null for a RESET means it has no outer
+  // scope.
+  nsCounterNode* mScopeStart = nullptr;
+
+  // mScopePrev points to the previous node that is in the same scope,
+  // or for a RESET, the previous node in the scope outside of the
+  // reset.
+
+  // May be null for all types, but only when mScopeStart is also
+  // null.  Following the mScopePrev links will eventually lead to
+  // mScopeStart.  Being null for a non-RESET means that it is an
+  // implied 'counter-reset'.  Being null for a RESET means it has no
+  // outer scope.
+  nsCounterNode* mScopePrev = nullptr;
+
+  inline nsCounterUseNode* UseNode();
+  inline nsCounterChangeNode* ChangeNode();
+
+  // For RESET, INCREMENT and SET nodes, aPseudoFrame need not be a
+  // pseudo-element, and aContentIndex represents the index within the
+  // 'counter-reset', 'counter-increment' or 'counter-set'  property
+  // instead of within the 'content' property but offset to ensure
+  // that (reset, increment, set, use) sort in that order.
+  // (This slight weirdness allows sharing a lot of code with 'quotes'.)
+  nsCounterNode(int32_t aContentIndex, Type aType)
+      : nsGenConNode(aContentIndex), mType(aType) {}
+
+  // to avoid virtual function calls in the common case
+  inline void Calc(nsCounterList* aList, bool aNotify);
+
+  // Is this a <ol reversed> RESET node?
+  inline bool IsContentBasedReset();
+};
+
+struct nsCounterUseNode : public nsCounterNode {
+  mozilla::CounterStylePtr mCounterStyle;
+  nsString mSeparator;
+
+  // false for counter(), true for counters()
+  bool mAllCounters = false;
+
+  bool mForLegacyBullet = false;
+
+  enum ForLegacyBullet { ForLegacyBullet };
+  explicit nsCounterUseNode(enum ForLegacyBullet)
+      : nsCounterNode(0, USE), mForLegacyBullet(true) {
+    mCounterStyle = nsGkAtoms::list_item;
+  }
+
+  // args go directly to member variables here and of nsGenConNode
+  nsCounterUseNode(mozilla::CounterStylePtr aCounterStyle, nsString aSeparator,
+                   uint32_t aContentIndex, bool aAllCounters)
+      : nsCounterNode(aContentIndex, USE),
+        mCounterStyle(std::move(aCounterStyle)),
+        mSeparator(std::move(aSeparator)),
+        mAllCounters(aAllCounters) {
+    NS_ASSERTION(aContentIndex <= INT32_MAX, "out of range");
+  }
+
+  virtual bool InitTextFrame(nsGenConList* aList, nsIFrame* aPseudoFrame,
+                             nsIFrame* aTextFrame) override;
+
+  bool InitBullet(nsGenConList* aList, nsIFrame* aBulletFrame);
+
+  // assign the correct |mValueAfter| value to a node that has been inserted,
+  // and update the value of the text node, notifying if `aNotify` is true.
+  // Should be called immediately after calling |Insert|.
+  void Calc(nsCounterList* aList, bool aNotify);
+
+  // The text that should be displayed for this counter.
+  void GetText(nsString& aResult);
+};
+
+struct nsCounterChangeNode : public nsCounterNode {
+  int32_t mChangeValue;  // the numeric value of the increment, set or reset
+
+  // |aPseudoFrame| is not necessarily a pseudo-element's frame, but
+  // since it is for every other subclass of nsGenConNode, we follow
+  // the naming convention here.
+  // |aPropIndex| is the index of the value within the list in the
+  // 'counter-increment', 'counter-reset' or 'counter-set' property.
+  nsCounterChangeNode(nsIFrame* aPseudoFrame, nsCounterNode::Type aChangeType,
+                      int32_t aChangeValue, int32_t aPropIndex)
+      : nsCounterNode(  // Fake a content index for resets, increments and sets
+                        // that comes before all the real content, with
+                        // the resets first, in order, and then the increments
+                        // and then the sets.
+            aPropIndex + (aChangeType == RESET ? (INT32_MIN)
+                                               : (aChangeType == INCREMENT
+                                                      ? ((INT32_MIN / 3) * 2)
+                                                      : INT32_MIN / 3)),
+            aChangeType),
+        mChangeValue(aChangeValue) {
+    NS_ASSERTION(aPropIndex >= 0, "out of range");
+    NS_ASSERTION(
+        aChangeType == INCREMENT || aChangeType == SET || aChangeType == RESET,
+        "bad type");
+    mPseudoFrame = aPseudoFrame;
+    CheckFrameAssertions();
+  }
+
+  // assign the correct |mValueAfter| value to a node that has been inserted
+  // Should be called immediately after calling |Insert|.
+  void Calc(nsCounterList* aList);
+};
+
+inline nsCounterUseNode* nsCounterNode::UseNode() {
+  NS_ASSERTION(mType == USE, "wrong type");
+  return static_cast<nsCounterUseNode*>(this);
+}
+
+inline nsCounterChangeNode* nsCounterNode::ChangeNode() {
+  MOZ_ASSERT(mType == INCREMENT || mType == SET || mType == RESET);
+  return static_cast<nsCounterChangeNode*>(this);
+}
+
+inline void nsCounterNode::Calc(nsCounterList* aList, bool aNotify) {
+  if (mType == USE)
+    UseNode()->Calc(aList, aNotify);
+  else
+    ChangeNode()->Calc(aList);
+}
+
+inline bool nsCounterNode::IsContentBasedReset() {
+  return mType == RESET &&
+         ChangeNode()->mChangeValue == std::numeric_limits<int32_t>::min();
+}
+
+class nsCounterList : public nsGenConList {
+ public:
+  nsCounterList() : nsGenConList(), mDirty(false) {}
+
+  void Insert(nsCounterNode* aNode) {
+    nsGenConList::Insert(aNode);
+    // Don't SetScope if we're dirty -- we'll reset all the scopes anyway,
+    // and we can't usefully compute scopes right now.
+    if (MOZ_LIKELY(!IsDirty())) {
+      SetScope(aNode);
+    }
+  }
+
+  nsCounterNode* First() {
+    return static_cast<nsCounterNode*>(mList.getFirst());
+  }
+
+  static nsCounterNode* Next(nsCounterNode* aNode) {
+    return static_cast<nsCounterNode*>(nsGenConList::Next(aNode));
+  }
+  static nsCounterNode* Prev(nsCounterNode* aNode) {
+    return static_cast<nsCounterNode*>(nsGenConList::Prev(aNode));
+  }
+
+  static int32_t ValueBefore(nsCounterNode* aNode) {
+    return aNode->mScopePrev ? aNode->mScopePrev->mValueAfter : 0;
+  }
+
+  // Correctly set |aNode->mScopeStart| and |aNode->mScopePrev|
+  void SetScope(nsCounterNode* aNode);
+
+  // Recalculate |mScopeStart|, |mScopePrev|, and |mValueAfter| for
+  // all nodes and update text in text content nodes.
+  void RecalcAll();
+
+  bool IsDirty() { return mDirty; }
+  void SetDirty() { mDirty = true; }
+
+ private:
+  bool mDirty;
+};
+
+/**
+ * The counter manager maintains an |nsCounterList| for each named
+ * counter to keep track of all scopes with that name.
+ */
+class nsCounterManager {
+ public:
+  // Returns true if dirty
+  bool AddCounterChanges(nsIFrame* aFrame);
+
+  // Gets the appropriate counter list, creating it if necessary.
+  // Guaranteed to return non-null. (Uses an infallible hashtable API.)
+  nsCounterList* CounterListFor(nsAtom* aCounterName);
+
+  // Clean up data in any dirty counter lists.
+  void RecalcAll();
+
+  // Set all counter lists dirty
+  void SetAllDirty();
+
+  // Destroy nodes for the frame in any lists, and return whether any
+  // nodes were destroyed.
+  bool DestroyNodesFor(nsIFrame* aFrame);
+
+  // Clear all data.
+  void Clear() { mNames.Clear(); }
+
+#ifdef DEBUG
+  void Dump();
+#endif
+
+  static int32_t IncrementCounter(int32_t aOldValue, int32_t aIncrement) {
+    // Addition of unsigned values is defined to be arithmetic
+    // modulo 2^bits (C++ 2011, 3.9.1 [basic.fundamental], clause 4);
+    // addition of signed values is undefined (and clang does
+    // something very strange if we use it here).  Likewise integral
+    // conversion from signed to unsigned is also defined as modulo
+    // 2^bits (C++ 2011, 4.7 [conv.integral], clause 2); conversion
+    // from unsigned to signed is however undefined (ibid., clause 3),
+    // but to do what we want we must nonetheless depend on that
+    // small piece of undefined behavior.
+    int32_t newValue = int32_t(uint32_t(aOldValue) + uint32_t(aIncrement));
+    // The CSS Working Group resolved that a counter-increment that
+    // exceeds internal limits should not increment at all.
+    // http://lists.w3.org/Archives/Public/www-style/2013Feb/0392.html
+    // (This means, for example, that if aIncrement is 5, the
+    // counter will get stuck at the largest multiple of 5 less than
+    // the maximum 32-bit integer.)
+    if ((aIncrement > 0) != (newValue > aOldValue)) {
+      newValue = aOldValue;
+    }
+    return newValue;
+  }
+
+ private:
+  nsClassHashtable<nsRefPtrHashKey<nsAtom>, nsCounterList> mNames;
+};
+
+#endif /* nsCounterManager_h_ */