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diff --git a/xpcom/ds/nsAtom.h b/xpcom/ds/nsAtom.h
new file mode 100644
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+++ b/xpcom/ds/nsAtom.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 nsAtom_h
+#define nsAtom_h
+
+#include <type_traits>
+
+#include "mozilla/Atomics.h"
+#include "mozilla/Char16.h"
+#include "mozilla/MemoryReporting.h"
+#include "nsISupports.h"
+#include "nsString.h"
+
+namespace mozilla {
+struct AtomsSizes;
+}  // namespace mozilla
+
+class nsStaticAtom;
+class nsDynamicAtom;
+
+// This class encompasses both static and dynamic atoms.
+//
+// - In places where static and dynamic atoms can be used, use RefPtr<nsAtom>.
+//   This is by far the most common case.
+//
+// - In places where only static atoms can appear, use nsStaticAtom* to avoid
+//   unnecessary refcounting. This is a moderately common case.
+//
+// - In places where only dynamic atoms can appear, it doesn't matter much
+//   whether you use RefPtr<nsAtom> or RefPtr<nsDynamicAtom>. This is an
+//   extremely rare case.
+//
+class nsAtom {
+ public:
+  void AddSizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf,
+                              mozilla::AtomsSizes& aSizes) const;
+
+  bool Equals(char16ptr_t aString, uint32_t aLength) const {
+    return mLength == aLength &&
+           memcmp(GetUTF16String(), aString, mLength * sizeof(char16_t)) == 0;
+  }
+
+  bool Equals(const nsAString& aString) const {
+    return Equals(aString.BeginReading(), aString.Length());
+  }
+
+  bool IsStatic() const { return mIsStatic; }
+  bool IsDynamic() const { return !IsStatic(); }
+
+  inline const nsStaticAtom* AsStatic() const;
+  inline const nsDynamicAtom* AsDynamic() const;
+  inline nsDynamicAtom* AsDynamic();
+
+  char16ptr_t GetUTF16String() const;
+
+  uint32_t GetLength() const { return mLength; }
+
+  operator mozilla::Span<const char16_t>() const {
+    // Explicitly specify template argument here to avoid instantiating
+    // Span<char16_t> first and then implicitly converting to Span<const
+    // char16_t>
+    return mozilla::Span<const char16_t>{GetUTF16String(), GetLength()};
+  }
+
+  void ToString(nsAString& aString) const;
+  void ToUTF8String(nsACString& aString) const;
+
+  // A hashcode that is better distributed than the actual atom pointer, for
+  // use in situations that need a well-distributed hashcode. It's called hash()
+  // rather than Hash() so we can use mozilla::BloomFilter<N, nsAtom>, because
+  // BloomFilter requires elements to implement a function called hash().
+  //
+  uint32_t hash() const { return mHash; }
+
+  // This function returns true if ToLowercaseASCII would return the string
+  // unchanged.
+  bool IsAsciiLowercase() const { return mIsAsciiLowercase; }
+
+  // This function returns true if this is the empty atom. This is exactly
+  // equivalent to `this == nsGkAtoms::_empty`, but it's a bit less foot-gunny,
+  // since we also have `nsGkAtoms::empty`.
+  //
+  // Defined in nsGkAtoms.h
+  inline bool IsEmpty() const;
+
+  // We can't use NS_INLINE_DECL_THREADSAFE_REFCOUNTING because the refcounting
+  // of this type is special.
+  inline MozExternalRefCountType AddRef();
+  inline MozExternalRefCountType Release();
+
+  using HasThreadSafeRefCnt = std::true_type;
+
+ protected:
+  // Used by nsStaticAtom.
+  constexpr nsAtom(uint32_t aLength, uint32_t aHash, bool aIsAsciiLowercase)
+      : mLength(aLength),
+        mIsStatic(true),
+        mIsAsciiLowercase(aIsAsciiLowercase),
+        mHash(aHash) {}
+
+  // Used by nsDynamicAtom.
+  nsAtom(const nsAString& aString, uint32_t aHash, bool aIsAsciiLowercase)
+      : mLength(aString.Length()),
+        mIsStatic(false),
+        mIsAsciiLowercase(aIsAsciiLowercase),
+        mHash(aHash) {}
+
+  ~nsAtom() = default;
+
+  const uint32_t mLength : 30;
+  const uint32_t mIsStatic : 1;
+  const uint32_t mIsAsciiLowercase : 1;
+  const uint32_t mHash;
+};
+
+// This class would be |final| if it wasn't for nsCSSAnonBoxPseudoStaticAtom
+// and nsCSSPseudoElementStaticAtom, which are trivial subclasses used to
+// ensure only certain static atoms are passed to certain functions.
+class nsStaticAtom : public nsAtom {
+ public:
+  // These are deleted so it's impossible to RefPtr<nsStaticAtom>. Raw
+  // nsStaticAtom pointers should be used instead.
+  MozExternalRefCountType AddRef() = delete;
+  MozExternalRefCountType Release() = delete;
+
+  // The static atom's precomputed hash value is an argument here, but it
+  // must be the same as would be computed by mozilla::HashString(aStr),
+  // which is what we use when atomizing strings. We compute this hash in
+  // Atom.py and assert in nsAtomTable::RegisterStaticAtoms that the two
+  // hashes match.
+  constexpr nsStaticAtom(uint32_t aLength, uint32_t aHash,
+                         uint32_t aStringOffset, bool aIsAsciiLowercase)
+      : nsAtom(aLength, aHash, aIsAsciiLowercase),
+        mStringOffset(aStringOffset) {}
+
+  const char16_t* String() const {
+    return reinterpret_cast<const char16_t*>(uintptr_t(this) - mStringOffset);
+  }
+
+  already_AddRefed<nsAtom> ToAddRefed() {
+    return already_AddRefed<nsAtom>(static_cast<nsAtom*>(this));
+  }
+
+ private:
+  // This is an offset to the string chars, which must be at a lower address in
+  // memory.
+  uint32_t mStringOffset;
+};
+
+class nsDynamicAtom : public nsAtom {
+ public:
+  // We can't use NS_INLINE_DECL_THREADSAFE_REFCOUNTING because the refcounting
+  // of this type is special.
+  MozExternalRefCountType AddRef() {
+    MOZ_ASSERT(int32_t(mRefCnt) >= 0, "illegal refcnt");
+    nsrefcnt count = ++mRefCnt;
+    if (count == 1) {
+      gUnusedAtomCount--;
+    }
+    return count;
+  }
+
+  MozExternalRefCountType Release() {
+#ifdef DEBUG
+    // We set a lower GC threshold for atoms in debug builds so that we exercise
+    // the GC machinery more often.
+    static const int32_t kAtomGCThreshold = 20;
+#else
+    static const int32_t kAtomGCThreshold = 10000;
+#endif
+
+    MOZ_ASSERT(int32_t(mRefCnt) > 0, "dup release");
+    nsrefcnt count = --mRefCnt;
+    if (count == 0) {
+      if (++gUnusedAtomCount >= kAtomGCThreshold) {
+        GCAtomTable();
+      }
+    }
+
+    return count;
+  }
+
+  const char16_t* String() const {
+    return reinterpret_cast<const char16_t*>(this + 1);
+  }
+
+  static nsDynamicAtom* FromChars(char16_t* chars) {
+    return reinterpret_cast<nsDynamicAtom*>(chars) - 1;
+  }
+
+ private:
+  friend class nsAtomTable;
+  friend class nsAtomSubTable;
+  friend int32_t NS_GetUnusedAtomCount();
+
+  static mozilla::Atomic<int32_t, mozilla::ReleaseAcquire> gUnusedAtomCount;
+  static void GCAtomTable();
+
+  // These shouldn't be used directly, even by friend classes. The
+  // Create()/Destroy() methods use them.
+  nsDynamicAtom(const nsAString& aString, uint32_t aHash,
+                bool aIsAsciiLowercase);
+  ~nsDynamicAtom() = default;
+
+  static nsDynamicAtom* Create(const nsAString& aString, uint32_t aHash);
+  static void Destroy(nsDynamicAtom* aAtom);
+
+  mozilla::ThreadSafeAutoRefCnt mRefCnt;
+
+  // The atom's chars are stored at the end of the struct.
+};
+
+const nsStaticAtom* nsAtom::AsStatic() const {
+  MOZ_ASSERT(IsStatic());
+  return static_cast<const nsStaticAtom*>(this);
+}
+
+const nsDynamicAtom* nsAtom::AsDynamic() const {
+  MOZ_ASSERT(IsDynamic());
+  return static_cast<const nsDynamicAtom*>(this);
+}
+
+nsDynamicAtom* nsAtom::AsDynamic() {
+  MOZ_ASSERT(IsDynamic());
+  return static_cast<nsDynamicAtom*>(this);
+}
+
+MozExternalRefCountType nsAtom::AddRef() {
+  return IsStatic() ? 2 : AsDynamic()->AddRef();
+}
+
+MozExternalRefCountType nsAtom::Release() {
+  return IsStatic() ? 1 : AsDynamic()->Release();
+}
+
+// The four forms of NS_Atomize (for use with |RefPtr<nsAtom>|) return the
+// atom for the string given. At any given time there will always be one atom
+// representing a given string. Atoms are intended to make string comparison
+// cheaper by simplifying it to pointer equality. A pointer to the atom that
+// does not own a reference is not guaranteed to be valid.
+
+// Find an atom that matches the given UTF-8 string. The string is assumed to
+// be zero terminated. Never returns null.
+already_AddRefed<nsAtom> NS_Atomize(const char* aUTF8String);
+
+// Find an atom that matches the given UTF-8 string. Never returns null.
+already_AddRefed<nsAtom> NS_Atomize(const nsACString& aUTF8String);
+
+// Find an atom that matches the given UTF-16 string. The string is assumed to
+// be zero terminated. Never returns null.
+already_AddRefed<nsAtom> NS_Atomize(const char16_t* aUTF16String);
+
+// Find an atom that matches the given UTF-16 string. Never returns null.
+already_AddRefed<nsAtom> NS_Atomize(const nsAString& aUTF16String);
+
+// An optimized version of the method above for the main thread.
+already_AddRefed<nsAtom> NS_AtomizeMainThread(const nsAString& aUTF16String);
+
+// Return a count of the total number of atoms currently alive in the system.
+//
+// Note that the result is imprecise and racy if other threads are currently
+// operating on atoms. It's also slow, since it triggers a GC before counting.
+// Currently this function is only used in tests, which should probably remain
+// the case.
+nsrefcnt NS_GetNumberOfAtoms();
+
+// Return a pointer for a static atom for the string or null if there's no
+// static atom for this string.
+nsStaticAtom* NS_GetStaticAtom(const nsAString& aUTF16String);
+
+class nsAtomString : public nsString {
+ public:
+  explicit nsAtomString(const nsAtom* aAtom) { aAtom->ToString(*this); }
+};
+
+class nsAtomCString : public nsCString {
+ public:
+  explicit nsAtomCString(const nsAtom* aAtom) { aAtom->ToUTF8String(*this); }
+};
+
+class nsDependentAtomString : public nsDependentString {
+ public:
+  explicit nsDependentAtomString(const nsAtom* aAtom)
+      : nsDependentString(aAtom->GetUTF16String(), aAtom->GetLength()) {}
+};
+
+// Checks if the ascii chars in a given atom are already lowercase.
+// If they are, no-op. Otherwise, converts all the ascii uppercase
+// chars to lowercase and atomizes, storing the result in the inout
+// param.
+void ToLowerCaseASCII(RefPtr<nsAtom>& aAtom);
+
+#endif  // nsAtom_h