js/src/ds/OrderedHashTable.h
author Nika Layzell <nika@thelayzells.com>
Wed, 16 Sep 2020 20:47:55 +0000
changeset 549331 ab7d302fd3186b10ada9264528c80f6840e44571
parent 515237 f449a588de9aa51bbce98437f5e125eaf901c224
permissions -rw-r--r--
Bug 1659696 - Check PendingInitialization before targeting in window.open, r=kmag This requires adding the flag as a synced field on the BrowsingContext, and checking it in a few more places. Attempts to open a new window in this racy manner will now raise an exception. This should avoid the issue from bug 1658854 by blocking the buggy attempts to load before the nested event loop has been exited. Differential Revision: https://phabricator.services.mozilla.com/D87927

/* -*- 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 ds_OrderedHashTable_h
#define ds_OrderedHashTable_h

/*
 * Define two collection templates, js::OrderedHashMap and js::OrderedHashSet.
 * They are like js::HashMap and js::HashSet except that:
 *
 *   - Iterating over an Ordered hash table visits the entries in the order in
 *     which they were inserted. This means that unlike a HashMap, the behavior
 *     of an OrderedHashMap is deterministic (as long as the HashPolicy methods
 *     are effect-free and consistent); the hashing is a pure performance
 *     optimization.
 *
 *   - Range objects over Ordered tables remain valid even when entries are
 *     added or removed or the table is resized. (However in the case of
 *     removing entries, note the warning on class Range below.)
 *
 *   - The API is a little different, so it's not a drop-in replacement.
 *     In particular, the hash policy is a little different.
 *     Also, the Ordered templates lack the Ptr and AddPtr types.
 *
 * Hash policies
 *
 * See the comment about "Hash policy" in HashTable.h for general features that
 * hash policy classes must provide. Hash policies for OrderedHashMaps and Sets
 * differ in that the hash() method takes an extra argument:
 *     static js::HashNumber hash(Lookup, const HashCodeScrambler&);
 * They must additionally provide a distinguished "empty" key value and the
 * following static member functions:
 *     bool isEmpty(const Key&);
 *     void makeEmpty(Key*);
 */

#include "mozilla/HashFunctions.h"

#include <utility>

#include "js/HashTable.h"

namespace js {

namespace detail {

/*
 * detail::OrderedHashTable is the underlying data structure used to implement
 * both OrderedHashMap and OrderedHashSet. Programs should use one of those two
 * templates rather than OrderedHashTable.
 */
template <class T, class Ops, class AllocPolicy>
class OrderedHashTable {
 public:
  using Key = typename Ops::KeyType;
  using Lookup = typename Ops::Lookup;

  struct Data {
    T element;
    Data* chain;

    Data(const T& e, Data* c) : element(e), chain(c) {}
    Data(T&& e, Data* c) : element(std::move(e)), chain(c) {}
  };

  class Range;
  friend class Range;

 private:
  Data** hashTable;       // hash table (has hashBuckets() elements)
  Data* data;             // data vector, an array of Data objects
                          // data[0:dataLength] are constructed
  uint32_t dataLength;    // number of constructed elements in data
  uint32_t dataCapacity;  // size of data, in elements
  uint32_t liveCount;     // dataLength less empty (removed) entries
  uint32_t hashShift;     // multiplicative hash shift
  Range* ranges;  // list of all live Ranges on this table in malloc memory
  Range*
      nurseryRanges;  // list of all live Ranges on this table in the GC nursery
  AllocPolicy alloc;
  mozilla::HashCodeScrambler hcs;  // don't reveal pointer hash codes

  // TODO: This should be templated on a functor type and receive lambda
  // arguments but this causes problems for the hazard analysis builds. See
  // bug 1398213.
  template <void (*f)(Range* range, uint32_t arg)>
  void forEachRange(uint32_t arg = 0) {
    Range* next;
    for (Range* r = ranges; r; r = next) {
      next = r->next;
      f(r, arg);
    }
    for (Range* r = nurseryRanges; r; r = next) {
      next = r->next;
      f(r, arg);
    }
  }

 public:
  OrderedHashTable(AllocPolicy ap, mozilla::HashCodeScrambler hcs)
      : hashTable(nullptr),
        data(nullptr),
        dataLength(0),
        dataCapacity(0),
        liveCount(0),
        hashShift(0),
        ranges(nullptr),
        nurseryRanges(nullptr),
        alloc(std::move(ap)),
        hcs(hcs) {}

  MOZ_MUST_USE bool init() {
    MOZ_ASSERT(!hashTable, "init must be called at most once");

    uint32_t buckets = initialBuckets();
    Data** tableAlloc = alloc.template pod_malloc<Data*>(buckets);
    if (!tableAlloc) {
      return false;
    }
    for (uint32_t i = 0; i < buckets; i++) {
      tableAlloc[i] = nullptr;
    }

    uint32_t capacity = uint32_t(buckets * fillFactor());
    Data* dataAlloc = alloc.template pod_malloc<Data>(capacity);
    if (!dataAlloc) {
      alloc.free_(tableAlloc, buckets);
      return false;
    }

    // clear() requires that members are assigned only after all allocation
    // has succeeded, and that this->ranges is left untouched.
    hashTable = tableAlloc;
    data = dataAlloc;
    dataLength = 0;
    dataCapacity = capacity;
    liveCount = 0;
    hashShift = js::kHashNumberBits - initialBucketsLog2();
    MOZ_ASSERT(hashBuckets() == buckets);
    return true;
  }

  ~OrderedHashTable() {
    forEachRange<Range::onTableDestroyed>();
    if (hashTable) {
      // |hashBuckets()| isn't valid when |hashTable| hasn't been created.
      alloc.free_(hashTable, hashBuckets());
    }
    freeData(data, dataLength, dataCapacity);
  }

  /* Return the number of elements in the table. */
  uint32_t count() const { return liveCount; }

  /* True if any element matches l. */
  bool has(const Lookup& l) const { return lookup(l) != nullptr; }

  /* Return a pointer to the element, if any, that matches l, or nullptr. */
  T* get(const Lookup& l) {
    Data* e = lookup(l, prepareHash(l));
    return e ? &e->element : nullptr;
  }

  /* Return a pointer to the element, if any, that matches l, or nullptr. */
  const T* get(const Lookup& l) const {
    return const_cast<OrderedHashTable*>(this)->get(l);
  }

  /*
   * If the table already contains an entry that matches |element|,
   * replace that entry with |element|. Otherwise add a new entry.
   *
   * On success, return true, whether there was already a matching element or
   * not. On allocation failure, return false. If this returns false, it
   * means the element was not added to the table.
   */
  template <typename ElementInput>
  MOZ_MUST_USE bool put(ElementInput&& element) {
    HashNumber h = prepareHash(Ops::getKey(element));
    if (Data* e = lookup(Ops::getKey(element), h)) {
      e->element = std::forward<ElementInput>(element);
      return true;
    }

    if (dataLength == dataCapacity) {
      // If the hashTable is more than 1/4 deleted data, simply rehash in
      // place to free up some space. Otherwise, grow the table.
      uint32_t newHashShift =
          liveCount >= dataCapacity * 0.75 ? hashShift - 1 : hashShift;
      if (!rehash(newHashShift)) {
        return false;
      }
    }

    h >>= hashShift;
    liveCount++;
    Data* e = &data[dataLength++];
    new (e) Data(std::forward<ElementInput>(element), hashTable[h]);
    hashTable[h] = e;
    return true;
  }

  /*
   * If the table contains an element matching l, remove it and set *foundp
   * to true. Otherwise set *foundp to false.
   *
   * Return true on success, false if we tried to shrink the table and hit an
   * allocation failure. Even if this returns false, *foundp is set correctly
   * and the matching element was removed. Shrinking is an optimization and
   * it's OK for it to fail.
   */
  bool remove(const Lookup& l, bool* foundp) {
    // Note: This could be optimized so that removing the last entry,
    // data[dataLength - 1], decrements dataLength. LIFO use cases would
    // benefit.

    // If a matching entry exists, empty it.
    Data* e = lookup(l, prepareHash(l));
    if (e == nullptr) {
      *foundp = false;
      return true;
    }

    *foundp = true;
    liveCount--;
    Ops::makeEmpty(&e->element);

    // Update active Ranges.
    uint32_t pos = e - data;
    forEachRange<&Range::onRemove>(pos);

    // If many entries have been removed, try to shrink the table.
    if (hashBuckets() > initialBuckets() &&
        liveCount < dataLength * minDataFill()) {
      if (!rehash(hashShift + 1)) {
        return false;
      }
    }
    return true;
  }

  /*
   * Remove all entries.
   *
   * Returns false on OOM, leaving the OrderedHashTable and any live Ranges
   * in the old state.
   *
   * The effect on live Ranges is the same as removing all entries; in
   * particular, those Ranges are still live and will see any entries added
   * after a successful clear().
   */
  MOZ_MUST_USE bool clear() {
    if (dataLength != 0) {
      Data** oldHashTable = hashTable;
      Data* oldData = data;
      uint32_t oldHashBuckets = hashBuckets();
      uint32_t oldDataLength = dataLength;
      uint32_t oldDataCapacity = dataCapacity;

      hashTable = nullptr;
      if (!init()) {
        // init() only mutates members on success; see comment above.
        hashTable = oldHashTable;
        return false;
      }

      alloc.free_(oldHashTable, oldHashBuckets);
      freeData(oldData, oldDataLength, oldDataCapacity);
      forEachRange<&Range::onClear>();
    }

    MOZ_ASSERT(hashTable);
    MOZ_ASSERT(data);
    MOZ_ASSERT(dataLength == 0);
    MOZ_ASSERT(liveCount == 0);
    return true;
  }

  /*
   * Ranges are used to iterate over OrderedHashTables.
   *
   * Suppose 'Map' is some instance of OrderedHashMap, and 'map' is a Map.
   * Then you can walk all the key-value pairs like this:
   *
   *     for (Map::Range r = map.all(); !r.empty(); r.popFront()) {
   *         Map::Entry& pair = r.front();
   *         ... do something with pair ...
   *     }
   *
   * Ranges remain valid for the lifetime of the OrderedHashTable, even if
   * entries are added or removed or the table is resized. Don't do anything
   * to a Range, except destroy it, after the OrderedHashTable has been
   * destroyed. (We support destroying the two objects in either order to
   * humor the GC, bless its nondeterministic heart.)
   *
   * Warning: The behavior when the current front() entry is removed from the
   * table is subtly different from js::HashTable<>::Enum::removeFront()!
   * HashTable::Enum doesn't skip any entries when you removeFront() and then
   * popFront(). OrderedHashTable::Range does! (This is useful for using a
   * Range to implement JS Map.prototype.iterator.)
   *
   * The workaround is to call popFront() as soon as possible,
   * before there's any possibility of modifying the table:
   *
   *     for (Map::Range r = map.all(); !r.empty(); ) {
   *         Key key = r.front().key;         // this won't modify map
   *         Value val = r.front().value;     // this won't modify map
   *         r.popFront();
   *         // ...do things that might modify map...
   *     }
   */
  class Range {
    friend class OrderedHashTable;

    // Cannot be a reference since we need to be able to do
    // |offsetof(Range, ht)|.
    OrderedHashTable* ht;

    /* The index of front() within ht->data. */
    uint32_t i;

    /*
     * The number of nonempty entries in ht->data to the left of front().
     * This is used when the table is resized or compacted.
     */
    uint32_t count;

    /*
     * Links in the doubly-linked list of active Ranges on ht.
     *
     * prevp points to the previous Range's .next field;
     *   or to ht->ranges if this is the first Range in the list.
     * next points to the next Range;
     *   or nullptr if this is the last Range in the list.
     *
     * Invariant: *prevp == this.
     */
    Range** prevp;
    Range* next;

    /*
     * Create a Range over all the entries in ht.
     * (This is private on purpose. End users must use ht->all().)
     */
    Range(OrderedHashTable* ht, Range** listp)
        : ht(ht), i(0), count(0), prevp(listp), next(*listp) {
      *prevp = this;
      if (next) {
        next->prevp = &next;
      }
      seek();
    }

   public:
    Range(const Range& other)
        : ht(other.ht),
          i(other.i),
          count(other.count),
          prevp(&ht->ranges),
          next(ht->ranges) {
      *prevp = this;
      if (next) {
        next->prevp = &next;
      }
    }

    ~Range() {
      *prevp = next;
      if (next) {
        next->prevp = prevp;
      }
    }

   private:
    // Prohibit copy assignment.
    Range& operator=(const Range& other) = delete;

    void seek() {
      while (i < ht->dataLength &&
             Ops::isEmpty(Ops::getKey(ht->data[i].element))) {
        i++;
      }
    }

    /*
     * The hash table calls this when an entry is removed.
     * j is the index of the removed entry.
     */
    void onRemove(uint32_t j) {
      MOZ_ASSERT(valid());
      if (j < i) {
        count--;
      }
      if (j == i) {
        seek();
      }
    }

    /*
     * The hash table calls this when the table is resized or compacted.
     * Since |count| is the number of nonempty entries to the left of
     * front(), discarding the empty entries will not affect count, and it
     * will make i and count equal.
     */
    void onCompact() {
      MOZ_ASSERT(valid());
      i = count;
    }

    /* The hash table calls this when cleared. */
    void onClear() {
      MOZ_ASSERT(valid());
      i = count = 0;
    }

    bool valid() const { return next != this; }

    void onTableDestroyed() {
      MOZ_ASSERT(valid());
      prevp = &next;
      next = this;
    }

   public:
    bool empty() const {
      MOZ_ASSERT(valid());
      return i >= ht->dataLength;
    }

    /*
     * Return the first element in the range. This must not be called if
     * this->empty().
     *
     * Warning: Removing an entry from the table also removes it from any
     * live Ranges, and a Range can become empty that way, rendering
     * front() invalid. If in doubt, check empty() before calling front().
     */
    T& front() {
      MOZ_ASSERT(valid());
      MOZ_ASSERT(!empty());
      return ht->data[i].element;
    }

    /*
     * Remove the first element from this range.
     * This must not be called if this->empty().
     *
     * Warning: Removing an entry from the table also removes it from any
     * live Ranges, and a Range can become empty that way, rendering
     * popFront() invalid. If in doubt, check empty() before calling
     * popFront().
     */
    void popFront() {
      MOZ_ASSERT(valid());
      MOZ_ASSERT(!empty());
      MOZ_ASSERT(!Ops::isEmpty(Ops::getKey(ht->data[i].element)));
      count++;
      i++;
      seek();
    }

    /*
     * Change the key of the front entry.
     *
     * This calls Ops::hash on both the current key and the new key.
     * Ops::hash on the current key must return the same hash code as
     * when the entry was added to the table.
     */
    void rekeyFront(const Key& k) {
      MOZ_ASSERT(valid());
      Data& entry = ht->data[i];
      HashNumber oldHash =
          ht->prepareHash(Ops::getKey(entry.element)) >> ht->hashShift;
      HashNumber newHash = ht->prepareHash(k) >> ht->hashShift;
      Ops::setKey(entry.element, k);
      if (newHash != oldHash) {
        // Remove this entry from its old hash chain. (If this crashes
        // reading nullptr, it would mean we did not find this entry on
        // the hash chain where we expected it. That probably means the
        // key's hash code changed since it was inserted, breaking the
        // hash code invariant.)
        Data** ep = &ht->hashTable[oldHash];
        while (*ep != &entry) {
          ep = &(*ep)->chain;
        }
        *ep = entry.chain;

        // Add it to the new hash chain. We could just insert it at the
        // beginning of the chain. Instead, we do a bit of work to
        // preserve the invariant that hash chains always go in reverse
        // insertion order (descending memory order). No code currently
        // depends on this invariant, so it's fine to kill it if
        // needed.
        ep = &ht->hashTable[newHash];
        while (*ep && *ep > &entry) {
          ep = &(*ep)->chain;
        }
        entry.chain = *ep;
        *ep = &entry;
      }
    }

    static size_t offsetOfHashTable() { return offsetof(Range, ht); }
    static size_t offsetOfI() { return offsetof(Range, i); }
    static size_t offsetOfCount() { return offsetof(Range, count); }
    static size_t offsetOfPrevP() { return offsetof(Range, prevp); }
    static size_t offsetOfNext() { return offsetof(Range, next); }

    static void onTableDestroyed(Range* range, uint32_t arg) {
      range->onTableDestroyed();
    }
    static void onRemove(Range* range, uint32_t arg) { range->onRemove(arg); }
    static void onClear(Range* range, uint32_t arg) { range->onClear(); }
    static void onCompact(Range* range, uint32_t arg) { range->onCompact(); }
  };

  Range all() { return Range(this, &ranges); }

  /*
   * Allocate a new Range, possibly in nursery memory. The buffer must be
   * large enough to hold a Range object.
   *
   * All nursery-allocated ranges can be freed in one go by calling
   * destroyNurseryRanges().
   */
  Range* createRange(void* buffer, bool inNursery) {
    auto range = static_cast<Range*>(buffer);
    new (range) Range(this, inNursery ? &nurseryRanges : &ranges);
    return range;
  }

  void destroyNurseryRanges() { nurseryRanges = nullptr; }

  /*
   * Change the value of the given key.
   *
   * This calls Ops::hash on both the current key and the new key.
   * Ops::hash on the current key must return the same hash code as
   * when the entry was added to the table.
   */
  void rekeyOneEntry(const Key& current, const Key& newKey, const T& element) {
    if (current == newKey) {
      return;
    }

    Data* entry = lookup(current, prepareHash(current));
    if (!entry) {
      return;
    }

    HashNumber oldHash = prepareHash(current) >> hashShift;
    HashNumber newHash = prepareHash(newKey) >> hashShift;

    entry->element = element;

    // Remove this entry from its old hash chain. (If this crashes
    // reading nullptr, it would mean we did not find this entry on
    // the hash chain where we expected it. That probably means the
    // key's hash code changed since it was inserted, breaking the
    // hash code invariant.)
    Data** ep = &hashTable[oldHash];
    while (*ep != entry) {
      ep = &(*ep)->chain;
    }
    *ep = entry->chain;

    // Add it to the new hash chain. We could just insert it at the
    // beginning of the chain. Instead, we do a bit of work to
    // preserve the invariant that hash chains always go in reverse
    // insertion order (descending memory order). No code currently
    // depends on this invariant, so it's fine to kill it if
    // needed.
    ep = &hashTable[newHash];
    while (*ep && *ep > entry) {
      ep = &(*ep)->chain;
    }
    entry->chain = *ep;
    *ep = entry;
  }

  static size_t offsetOfDataLength() {
    return offsetof(OrderedHashTable, dataLength);
  }
  static size_t offsetOfData() { return offsetof(OrderedHashTable, data); }
  static constexpr size_t offsetOfDataElement() {
    static_assert(offsetof(Data, element) == 0,
                  "RangeFront and RangePopFront depend on offsetof(Data, "
                  "element) being 0");
    return offsetof(Data, element);
  }
  static constexpr size_t sizeofData() { return sizeof(Data); }

 private:
  /* Logarithm base 2 of the number of buckets in the hash table initially. */
  static uint32_t initialBucketsLog2() { return 1; }
  static uint32_t initialBuckets() { return 1 << initialBucketsLog2(); }

  /*
   * The maximum load factor (mean number of entries per bucket).
   * It is an invariant that
   *     dataCapacity == floor(hashBuckets() * fillFactor()).
   *
   * The fill factor should be between 2 and 4, and it should be chosen so that
   * the fill factor times sizeof(Data) is close to but <= a power of 2.
   * This fixed fill factor was chosen to make the size of the data
   * array, in bytes, close to a power of two when sizeof(T) is 16.
   */
  static double fillFactor() { return 8.0 / 3.0; }

  /*
   * The minimum permitted value of (liveCount / dataLength).
   * If that ratio drops below this value, we shrink the table.
   */
  static double minDataFill() { return 0.25; }

 public:
  HashNumber prepareHash(const Lookup& l) const {
    return mozilla::ScrambleHashCode(Ops::hash(l, hcs));
  }

 private:
  /* The size of hashTable, in elements. Always a power of two. */
  uint32_t hashBuckets() const {
    return 1 << (js::kHashNumberBits - hashShift);
  }

  static void destroyData(Data* data, uint32_t length) {
    for (Data* p = data + length; p != data;) {
      (--p)->~Data();
    }
  }

  void freeData(Data* data, uint32_t length, uint32_t capacity) {
    destroyData(data, length);
    alloc.free_(data, capacity);
  }

  Data* lookup(const Lookup& l, HashNumber h) {
    for (Data* e = hashTable[h >> hashShift]; e; e = e->chain) {
      if (Ops::match(Ops::getKey(e->element), l)) {
        return e;
      }
    }
    return nullptr;
  }

  const Data* lookup(const Lookup& l) const {
    return const_cast<OrderedHashTable*>(this)->lookup(l, prepareHash(l));
  }

  /* This is called after rehashing the table. */
  void compacted() {
    // If we had any empty entries, compacting may have moved live entries
    // to the left within |data|. Notify all live Ranges of the change.
    forEachRange<&Range::onCompact>();
  }

  /* Compact the entries in |data| and rehash them. */
  void rehashInPlace() {
    for (uint32_t i = 0, N = hashBuckets(); i < N; i++) {
      hashTable[i] = nullptr;
    }
    Data* wp = data;
    Data* end = data + dataLength;
    for (Data* rp = data; rp != end; rp++) {
      if (!Ops::isEmpty(Ops::getKey(rp->element))) {
        HashNumber h = prepareHash(Ops::getKey(rp->element)) >> hashShift;
        if (rp != wp) {
          wp->element = std::move(rp->element);
        }
        wp->chain = hashTable[h];
        hashTable[h] = wp;
        wp++;
      }
    }
    MOZ_ASSERT(wp == data + liveCount);

    while (wp != end) {
      (--end)->~Data();
    }
    dataLength = liveCount;
    compacted();
  }

  /*
   * Grow, shrink, or compact both |hashTable| and |data|.
   *
   * On success, this returns true, dataLength == liveCount, and there are no
   * empty elements in data[0:dataLength]. On allocation failure, this
   * leaves everything as it was and returns false.
   */
  MOZ_MUST_USE bool rehash(uint32_t newHashShift) {
    // If the size of the table is not changing, rehash in place to avoid
    // allocating memory.
    if (newHashShift == hashShift) {
      rehashInPlace();
      return true;
    }

    size_t newHashBuckets = size_t(1) << (js::kHashNumberBits - newHashShift);
    Data** newHashTable = alloc.template pod_malloc<Data*>(newHashBuckets);
    if (!newHashTable) {
      return false;
    }
    for (uint32_t i = 0; i < newHashBuckets; i++) {
      newHashTable[i] = nullptr;
    }

    uint32_t newCapacity = uint32_t(newHashBuckets * fillFactor());
    Data* newData = alloc.template pod_malloc<Data>(newCapacity);
    if (!newData) {
      alloc.free_(newHashTable, newHashBuckets);
      return false;
    }

    Data* wp = newData;
    Data* end = data + dataLength;
    for (Data* p = data; p != end; p++) {
      if (!Ops::isEmpty(Ops::getKey(p->element))) {
        HashNumber h = prepareHash(Ops::getKey(p->element)) >> newHashShift;
        new (wp) Data(std::move(p->element), newHashTable[h]);
        newHashTable[h] = wp;
        wp++;
      }
    }
    MOZ_ASSERT(wp == newData + liveCount);

    alloc.free_(hashTable, hashBuckets());
    freeData(data, dataLength, dataCapacity);

    hashTable = newHashTable;
    data = newData;
    dataLength = liveCount;
    dataCapacity = newCapacity;
    hashShift = newHashShift;
    MOZ_ASSERT(hashBuckets() == newHashBuckets);

    compacted();
    return true;
  }

  // Not copyable.
  OrderedHashTable& operator=(const OrderedHashTable&) = delete;
  OrderedHashTable(const OrderedHashTable&) = delete;
};

}  // namespace detail

template <class Key, class Value, class OrderedHashPolicy, class AllocPolicy>
class OrderedHashMap {
 public:
  class Entry {
    template <class, class, class>
    friend class detail::OrderedHashTable;
    void operator=(const Entry& rhs) {
      const_cast<Key&>(key) = rhs.key;
      value = rhs.value;
    }

    void operator=(Entry&& rhs) {
      MOZ_ASSERT(this != &rhs, "self-move assignment is prohibited");
      const_cast<Key&>(key) = std::move(rhs.key);
      value = std::move(rhs.value);
    }

   public:
    Entry() : key(), value() {}
    template <typename V>
    Entry(const Key& k, V&& v) : key(k), value(std::forward<V>(v)) {}
    Entry(Entry&& rhs) : key(std::move(rhs.key)), value(std::move(rhs.value)) {}

    const Key key;
    Value value;

    static size_t offsetOfKey() { return offsetof(Entry, key); }
    static size_t offsetOfValue() { return offsetof(Entry, value); }
  };

 private:
  struct MapOps : OrderedHashPolicy {
    using KeyType = Key;
    static void makeEmpty(Entry* e) {
      OrderedHashPolicy::makeEmpty(const_cast<Key*>(&e->key));

      // Clear the value. Destroying it is another possibility, but that
      // would complicate class Entry considerably.
      e->value = Value();
    }
    static const Key& getKey(const Entry& e) { return e.key; }
    static void setKey(Entry& e, const Key& k) { const_cast<Key&>(e.key) = k; }
  };

  typedef detail::OrderedHashTable<Entry, MapOps, AllocPolicy> Impl;
  Impl impl;

 public:
  using Range = typename Impl::Range;

  OrderedHashMap(AllocPolicy ap, mozilla::HashCodeScrambler hcs)
      : impl(std::move(ap), hcs) {}
  MOZ_MUST_USE bool init() { return impl.init(); }
  uint32_t count() const { return impl.count(); }
  bool has(const Key& key) const { return impl.has(key); }
  Range all() { return impl.all(); }
  const Entry* get(const Key& key) const { return impl.get(key); }
  Entry* get(const Key& key) { return impl.get(key); }
  bool remove(const Key& key, bool* foundp) { return impl.remove(key, foundp); }
  MOZ_MUST_USE bool clear() { return impl.clear(); }

  template <typename V>
  MOZ_MUST_USE bool put(const Key& key, V&& value) {
    return impl.put(Entry(key, std::forward<V>(value)));
  }

  HashNumber hash(const Key& key) const { return impl.prepareHash(key); }

  void rekeyOneEntry(const Key& current, const Key& newKey) {
    const Entry* e = get(current);
    if (!e) {
      return;
    }
    return impl.rekeyOneEntry(current, newKey, Entry(newKey, e->value));
  }

  Range* createRange(void* buffer, bool inNursery) {
    return impl.createRange(buffer, inNursery);
  }

  void destroyNurseryRanges() { impl.destroyNurseryRanges(); }

  static size_t offsetOfEntryKey() { return Entry::offsetOfKey(); }
  static size_t offsetOfImplDataLength() { return Impl::offsetOfDataLength(); }
  static size_t offsetOfImplData() { return Impl::offsetOfData(); }
  static constexpr size_t offsetOfImplDataElement() {
    return Impl::offsetOfDataElement();
  }
  static constexpr size_t sizeofImplData() { return Impl::sizeofData(); }
};

template <class T, class OrderedHashPolicy, class AllocPolicy>
class OrderedHashSet {
 private:
  struct SetOps : OrderedHashPolicy {
    using KeyType = const T;
    static const T& getKey(const T& v) { return v; }
    static void setKey(const T& e, const T& v) { const_cast<T&>(e) = v; }
  };

  typedef detail::OrderedHashTable<T, SetOps, AllocPolicy> Impl;
  Impl impl;

 public:
  using Range = typename Impl::Range;

  explicit OrderedHashSet(AllocPolicy ap, mozilla::HashCodeScrambler hcs)
      : impl(std::move(ap), hcs) {}
  MOZ_MUST_USE bool init() { return impl.init(); }
  uint32_t count() const { return impl.count(); }
  bool has(const T& value) const { return impl.has(value); }
  Range all() { return impl.all(); }
  MOZ_MUST_USE bool put(const T& value) { return impl.put(value); }
  bool remove(const T& value, bool* foundp) {
    return impl.remove(value, foundp);
  }
  MOZ_MUST_USE bool clear() { return impl.clear(); }

  HashNumber hash(const T& value) const { return impl.prepareHash(value); }

  void rekeyOneEntry(const T& current, const T& newKey) {
    return impl.rekeyOneEntry(current, newKey, newKey);
  }

  Range* createRange(void* buffer, bool inNursery) {
    return impl.createRange(buffer, inNursery);
  }

  void destroyNurseryRanges() { impl.destroyNurseryRanges(); }

  static size_t offsetOfEntryKey() { return 0; }
  static size_t offsetOfImplDataLength() { return Impl::offsetOfDataLength(); }
  static size_t offsetOfImplData() { return Impl::offsetOfData(); }
  static constexpr size_t offsetOfImplDataElement() {
    return Impl::offsetOfDataElement();
  }
  static constexpr size_t sizeofImplData() { return Impl::sizeofData(); }
};

}  // namespace js

#endif /* ds_OrderedHashTable_h */