JSArray.cpp 70.2 KB
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/*
 *  Copyright (C) 1999-2000 Harri Porten (porten@kde.org)
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 *  Copyright (C) 2003, 2007, 2008, 2009 Apple Inc. All rights reserved.
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 *  Copyright (C) 2003 Peter Kelly (pmk@post.com)
 *  Copyright (C) 2006 Alexey Proskuryakov (ap@nypop.com)
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Lesser General Public
 *  License as published by the Free Software Foundation; either
 *  version 2 of the License, or (at your option) any later version.
 *
 *  This library is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *  Lesser General Public License for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public
 *  License along with this library; if not, write to the Free Software
 *  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 *
 */

#include "config.h"
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#include "JSArray.h"
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#include "ArrayPrototype.h"
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#include "CachedCall.h"
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#include "Error.h"
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#include "Executable.h"
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#include "GetterSetter.h"
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#include "PropertyNameArray.h"
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#include <wtf/AVLTree.h>
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#include <wtf/Assertions.h>
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#include <wtf/OwnPtr.h>
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#include <Operations.h>
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using namespace std;
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using namespace WTF;
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namespace JSC {
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ASSERT_CLASS_FITS_IN_CELL(JSArray);

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// Overview of JSArray
//
// Properties of JSArray objects may be stored in one of three locations:
//   * The regular JSObject property map.
//   * A storage vector.
//   * A sparse map of array entries.
//
// Properties with non-numeric identifiers, with identifiers that are not representable
// as an unsigned integer, or where the value is greater than  MAX_ARRAY_INDEX
// (specifically, this is only one property - the value 0xFFFFFFFFU as an unsigned 32-bit
// integer) are not considered array indices and will be stored in the JSObject property map.
//
// All properties with a numeric identifer, representable as an unsigned integer i,
// where (i <= MAX_ARRAY_INDEX), are an array index and will be stored in either the
// storage vector or the sparse map.  An array index i will be handled in the following
// fashion:
//
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//   * Where (i < MIN_SPARSE_ARRAY_INDEX) the value will be stored in the storage vector,
//     unless the array is in SparseMode in which case all properties go into the map.
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//   * Where (MIN_SPARSE_ARRAY_INDEX <= i <= MAX_STORAGE_VECTOR_INDEX) the value will either
//     be stored in the storage vector or in the sparse array, depending on the density of
//     data that would be stored in the vector (a vector being used where at least
//     (1 / minDensityMultiplier) of the entries would be populated).
//   * Where (MAX_STORAGE_VECTOR_INDEX < i <= MAX_ARRAY_INDEX) the value will always be stored
//     in the sparse array.

// The definition of MAX_STORAGE_VECTOR_LENGTH is dependant on the definition storageSize
// function below - the MAX_STORAGE_VECTOR_LENGTH limit is defined such that the storage
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// size calculation cannot overflow.  (sizeof(ArrayStorage) - sizeof(WriteBarrier<Unknown>)) +
// (vectorLength * sizeof(WriteBarrier<Unknown>)) must be <= 0xFFFFFFFFU (which is maximum value of size_t).
#define MAX_STORAGE_VECTOR_LENGTH static_cast<unsigned>((0xFFFFFFFFU - (sizeof(ArrayStorage) - sizeof(WriteBarrier<Unknown>))) / sizeof(WriteBarrier<Unknown>))
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// These values have to be macros to be used in max() and min() without introducing
// a PIC branch in Mach-O binaries, see <rdar://problem/5971391>.
#define MIN_SPARSE_ARRAY_INDEX 10000U
#define MAX_STORAGE_VECTOR_INDEX (MAX_STORAGE_VECTOR_LENGTH - 1)
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// 0xFFFFFFFF is a bit weird -- is not an array index even though it's an integer.
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#define MAX_ARRAY_INDEX 0xFFFFFFFEU
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// The value BASE_VECTOR_LEN is the maximum number of vector elements we'll allocate
// for an array that was created with a sepcified length (e.g. a = new Array(123))
#define BASE_VECTOR_LEN 4U
    
// The upper bound to the size we'll grow a zero length array when the first element
// is added.
#define FIRST_VECTOR_GROW 4U

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// Our policy for when to use a vector and when to use a sparse map.
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// For all array indices under MIN_SPARSE_ARRAY_INDEX, we always use a vector.
// When indices greater than MIN_SPARSE_ARRAY_INDEX are involved, we use a vector
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// as long as it is 1/8 full. If more sparse than that, we use a map.
static const unsigned minDensityMultiplier = 8;

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const ClassInfo JSArray::s_info = {"Array", &JSNonFinalObject::s_info, 0, 0, CREATE_METHOD_TABLE(JSArray)};
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// We keep track of the size of the last array after it was grown.  We use this
// as a simple heuristic for as the value to grow the next array from size 0.
// This value is capped by the constant FIRST_VECTOR_GROW defined above.
static unsigned lastArraySize = 0;

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static inline size_t storageSize(unsigned vectorLength)
{
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    ASSERT(vectorLength <= MAX_STORAGE_VECTOR_LENGTH);

    // MAX_STORAGE_VECTOR_LENGTH is defined such that provided (vectorLength <= MAX_STORAGE_VECTOR_LENGTH)
    // - as asserted above - the following calculation cannot overflow.
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    size_t size = (sizeof(ArrayStorage) - sizeof(WriteBarrier<Unknown>)) + (vectorLength * sizeof(WriteBarrier<Unknown>));
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    // Assertion to detect integer overflow in previous calculation (should not be possible, provided that
    // MAX_STORAGE_VECTOR_LENGTH is correctly defined).
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    ASSERT(((size - (sizeof(ArrayStorage) - sizeof(WriteBarrier<Unknown>))) / sizeof(WriteBarrier<Unknown>) == vectorLength) && (size >= (sizeof(ArrayStorage) - sizeof(WriteBarrier<Unknown>))));
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    return size;
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}

static inline bool isDenseEnoughForVector(unsigned length, unsigned numValues)
{
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    return length <= MIN_SPARSE_ARRAY_INDEX || length / minDensityMultiplier <= numValues;
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}

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#if !CHECK_ARRAY_CONSISTENCY

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inline void JSArray::checkConsistency(ConsistencyCheckType)
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{
}

#endif

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JSArray::JSArray(JSGlobalData& globalData, Structure* structure)
    : JSNonFinalObject(globalData, structure)
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    , m_storage(0)
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{
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}

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void JSArray::finishCreation(JSGlobalData& globalData, unsigned initialLength)
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{
    Base::finishCreation(globalData);
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    ASSERT(inherits(&s_info));

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    unsigned initialVectorLength = BASE_VECTOR_LEN;
    unsigned initialStorageSize = storageSize(initialVectorLength);
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    m_storage = static_cast<ArrayStorage*>(fastMalloc(initialStorageSize));
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    m_storage->m_allocBase = m_storage;
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    m_storage->m_length = initialLength;
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    m_indexBias = 0;
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    m_vectorLength = initialVectorLength;
    m_storage->m_sparseValueMap = 0;
    m_storage->subclassData = 0;
    m_storage->m_numValuesInVector = 0;
#if CHECK_ARRAY_CONSISTENCY
    m_storage->m_inCompactInitialization = false;
#endif
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    WriteBarrier<Unknown>* vector = m_storage->m_vector;
    for (size_t i = 0; i < initialVectorLength; ++i)
        vector[i].clear();
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    checkConsistency();
    
    Heap::heap(this)->reportExtraMemoryCost(initialStorageSize);
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}

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JSArray* JSArray::tryFinishCreationUninitialized(JSGlobalData& globalData, unsigned initialLength)
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{
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    Base::finishCreation(globalData);
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    ASSERT(inherits(&s_info));

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    // Check for lengths larger than we can handle with a vector.
    if (initialLength > MAX_STORAGE_VECTOR_LENGTH)
        return 0;

    unsigned initialVectorLength = max(initialLength, BASE_VECTOR_LEN);
    unsigned initialStorageSize = storageSize(initialVectorLength);

    m_storage = static_cast<ArrayStorage*>(fastMalloc(initialStorageSize));
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    m_storage->m_allocBase = m_storage;
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    m_storage->m_length = 0;
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    m_indexBias = 0;
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    m_vectorLength = initialVectorLength;
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    m_storage->m_sparseValueMap = 0;
    m_storage->subclassData = 0;
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    m_storage->m_numValuesInVector = initialLength;
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#if CHECK_ARRAY_CONSISTENCY
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    m_storage->m_inCompactInitialization = true;
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#endif
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    WriteBarrier<Unknown>* vector = m_storage->m_vector;
    for (size_t i = initialLength; i < initialVectorLength; ++i)
        vector[i].clear();

    Heap::heap(this)->reportExtraMemoryCost(initialStorageSize);
    return this;
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}

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JSArray::~JSArray()
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{
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    ASSERT(jsCast<JSArray*>(this));
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    // If we are unable to allocate memory for m_storage then this may be null.
    if (!m_storage)
        return;

    checkConsistency(DestructorConsistencyCheck);
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    delete m_storage->m_sparseValueMap;
    fastFree(m_storage->m_allocBase);
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}

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void JSArray::destroy(JSCell* cell)
{
    jsCast<JSArray*>(cell)->JSArray::~JSArray();
}

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inline std::pair<SparseArrayValueMap::iterator, bool> SparseArrayValueMap::add(JSArray* array, unsigned i)
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{
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    SparseArrayEntry entry;
    std::pair<iterator, bool> result = m_map.add(i, entry);
    size_t capacity = m_map.capacity();
    if (capacity != m_reportedCapacity) {
        Heap::heap(array)->reportExtraMemoryCost((capacity - m_reportedCapacity) * (sizeof(unsigned) + sizeof(WriteBarrier<Unknown>)));
        m_reportedCapacity = capacity;
    }
    return result;
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}

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inline void SparseArrayValueMap::put(ExecState* exec, JSArray* array, unsigned i, JSValue value)
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{
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    SparseArrayEntry& entry = add(array, i).first->second;

    if (!(entry.attributes & (Getter | Setter))) {
        if (entry.attributes & ReadOnly) {
            // FIXME: should throw if being called from strict mode.
            // throwTypeError(exec, StrictModeReadonlyPropertyWriteError);
            return;
        }

        entry.set(exec->globalData(), array, value);
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        return;
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    }
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    JSValue accessor = entry.Base::get();
    ASSERT(accessor.isGetterSetter());
    JSObject* setter = asGetterSetter(accessor)->setter();
    
    if (!setter) {
        throwTypeError(exec, "setting a property that has only a getter");
        return;
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    }
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    CallData callData;
    CallType callType = setter->methodTable()->getCallData(setter, callData);
    MarkedArgumentBuffer args;
    args.append(value);
    call(exec, setter, callType, callData, array, args);
}

inline void SparseArrayEntry::get(PropertySlot& slot) const
{
    JSValue value = Base::get();
    ASSERT(value);

    if (LIKELY(!value.isGetterSetter())) {
        slot.setValue(value);
        return;
    }

    JSObject* getter = asGetterSetter(value)->getter();
    if (!getter) {
        slot.setUndefined();
        return;
    }

    slot.setGetterSlot(getter);
}

inline void SparseArrayEntry::get(PropertyDescriptor& descriptor) const
{
    descriptor.setDescriptor(Base::get(), attributes);
}

inline JSValue SparseArrayEntry::get(ExecState* exec, JSArray* array) const
{
    JSValue result = Base::get();
    ASSERT(result);

    if (LIKELY(!result.isGetterSetter()))
        return result;

    JSObject* getter = asGetterSetter(result)->getter();
    if (!getter)
        return jsUndefined();

    CallData callData;
    CallType callType = getter->methodTable()->getCallData(getter, callData);
    return call(exec, getter, callType, callData, array, exec->emptyList());
}

inline JSValue SparseArrayEntry::getNonSparseMode() const
{
    ASSERT(!attributes);
    return Base::get();
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}

inline void SparseArrayValueMap::visitChildren(SlotVisitor& visitor)
{
    iterator end = m_map.end();
    for (iterator it = m_map.begin(); it != end; ++it)
        visitor.append(&it->second);
}

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void JSArray::enterSparseMode(JSGlobalData& globalData)
{
    ArrayStorage* storage = m_storage;
    SparseArrayValueMap* map = storage->m_sparseValueMap;

    if (!map)
        map = storage->m_sparseValueMap = new SparseArrayValueMap;

    if (map->sparseMode())
        return;

    map->setSparseMode();

    unsigned usedVectorLength = min(storage->m_length, m_vectorLength);
    for (unsigned i = 0; i < usedVectorLength; ++i) {
        JSValue value = storage->m_vector[i].get();
        // This will always be a new entry in the map, so no need to check we can write,
        // and attributes are default so no need to set them.
        if (value)
            map->add(this, i).first->second.set(globalData, this, value);
    }

    ArrayStorage* newStorage = static_cast<ArrayStorage*>(fastMalloc(storageSize(0)));
    memcpy(newStorage, m_storage, storageSize(0));
    newStorage->m_allocBase = newStorage;
    fastFree(m_storage);
    m_storage = newStorage;
    m_indexBias = 0;
    m_vectorLength = 0;
}

void JSArray::putDescriptor(ExecState* exec, SparseArrayEntry* entryInMap, PropertyDescriptor& descriptor, PropertyDescriptor& oldDescriptor)
{
    if (descriptor.isDataDescriptor()) {
        if (descriptor.value())
            entryInMap->set(exec->globalData(), this, descriptor.value());
        entryInMap->attributes = descriptor.attributesOverridingCurrent(oldDescriptor) & ~(Getter | Setter);
        return;
    }

    if (descriptor.isAccessorDescriptor()) {
        JSObject* getter = 0;
        if (descriptor.getter() && descriptor.getter().isObject())
            getter = asObject(descriptor.getter());
        if (!getter && oldDescriptor.isAccessorDescriptor()) {
            if (oldDescriptor.getter() && oldDescriptor.getter().isObject())
                getter = asObject(oldDescriptor.getter());
        }
        JSObject* setter = 0;
        if (descriptor.setter() && descriptor.setter().isObject())
            setter = asObject(descriptor.setter());
        if (!setter && oldDescriptor.isAccessorDescriptor()) {
            if (oldDescriptor.setter() && oldDescriptor.setter().isObject())
                setter = asObject(oldDescriptor.setter());
        }

        GetterSetter* accessor = GetterSetter::create(exec);
        if (getter)
            accessor->setGetter(exec->globalData(), getter);
        if (setter)
            accessor->setSetter(exec->globalData(), setter);

        entryInMap->set(exec->globalData(), this, accessor);
        entryInMap->attributes = descriptor.attributesOverridingCurrent(oldDescriptor) & ~DontDelete;
        return;
    }

    ASSERT(descriptor.isGenericDescriptor());
    entryInMap->attributes = descriptor.attributesOverridingCurrent(oldDescriptor);
}

static bool reject(ExecState* exec, bool throwException, const char* message)
{
    if (throwException)
        throwTypeError(exec, message);
    return false;
}

// Defined in ES5.1 8.12.9
bool JSArray::defineOwnNumericProperty(ExecState* exec, unsigned index, PropertyDescriptor& descriptor, bool throwException)
{
    ASSERT(index != 0xFFFFFFFF);

    if (!inSparseMode()) {
        // Fast case: we're putting a regular property to a regular array
        // FIXME: this will pessimistically assume that if attributes are missing then they'll default to false
        // – however if the property currently exists missing attributes will override from their current 'true'
        // state (i.e. defineOwnProperty could be used to set a value without needing to entering 'SparseMode').
        if (!descriptor.attributes()) {
            ASSERT(!descriptor.isAccessorDescriptor());
            putByIndex(this, exec, index, descriptor.value());
            return true;
        }

        enterSparseMode(exec->globalData());
    }

    SparseArrayValueMap* map = m_storage->m_sparseValueMap;
    ASSERT(map);

    // 1. Let current be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
    std::pair<SparseArrayValueMap::iterator, bool> result = map->add(this, index);
    SparseArrayEntry* entryInMap = &result.first->second;

    // 2. Let extensible be the value of the [[Extensible]] internal property of O.
    // 3. If current is undefined and extensible is false, then Reject.
    // 4. If current is undefined and extensible is true, then
    if (result.second) {
        if (!isExtensible()) {
            map->remove(result.first);
            return reject(exec, throwException, "Attempting to define property on object that is not extensible.");
        }

        // 4.a. If IsGenericDescriptor(Desc) or IsDataDescriptor(Desc) is true, then create an own data property
        // named P of object O whose [[Value]], [[Writable]], [[Enumerable]] and [[Configurable]] attribute values
        // are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly
        // created property is set to its default value.
        // 4.b. Else, Desc must be an accessor Property Descriptor so, create an own accessor property named P of
        // object O whose [[Get]], [[Set]], [[Enumerable]] and [[Configurable]] attribute values are described by
        // Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property
        // is set to its default value.
        // 4.c. Return true.

        PropertyDescriptor defaults;
        entryInMap->setWithoutWriteBarrier(jsUndefined());
        entryInMap->attributes = DontDelete | DontEnum | ReadOnly;
        entryInMap->get(defaults);

        putDescriptor(exec, entryInMap, descriptor, defaults);
        if (index >= m_storage->m_length)
            m_storage->m_length = index + 1;
        return true;
    }

    // 5. Return true, if every field in Desc is absent.
    // 6. Return true, if every field in Desc also occurs in current and the value of every field in Desc is the same value as the corresponding field in current when compared using the SameValue algorithm (9.12).
    PropertyDescriptor current;
    entryInMap->get(current);
    if (descriptor.isEmpty() || descriptor.equalTo(exec, current))
        return true;

    // 7. If the [[Configurable]] field of current is false then
    if (!current.configurable()) {
        // 7.a. Reject, if the [[Configurable]] field of Desc is true.
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        if (descriptor.configurablePresent() && !descriptor.configurable())
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            return reject(exec, throwException, "Attempting to change configurable attribute of unconfigurable property.");
        // 7.b. Reject, if the [[Enumerable]] field of Desc is present and the [[Enumerable]] fields of current and Desc are the Boolean negation of each other.
        if (descriptor.enumerablePresent() && current.enumerable() != descriptor.enumerable())
            return reject(exec, throwException, "Attempting to change enumerable attribute of unconfigurable property.");
    }

    // 8. If IsGenericDescriptor(Desc) is true, then no further validation is required.
    if (!descriptor.isGenericDescriptor()) {
        // 9. Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) have different results, then
        if (current.isDataDescriptor() != descriptor.isDataDescriptor()) {
            // 9.a. Reject, if the [[Configurable]] field of current is false.
            if (!current.configurable())
                return reject(exec, throwException, "Attempting to change access mechanism for an unconfigurable property.");
            // 9.b. If IsDataDescriptor(current) is true, then convert the property named P of object O from a
            // data property to an accessor property. Preserve the existing values of the converted property‘s
            // [[Configurable]] and [[Enumerable]] attributes and set the rest of the property‘s attributes to
            // their default values.
            // 9.c. Else, convert the property named P of object O from an accessor property to a data property.
            // Preserve the existing values of the converted property‘s [[Configurable]] and [[Enumerable]]
            // attributes and set the rest of the property‘s attributes to their default values.
        } else if (current.isDataDescriptor() && descriptor.isDataDescriptor()) {
            // 10. Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) are both true, then
            // 10.a. If the [[Configurable]] field of current is false, then
            if (!current.configurable() && !current.writable()) {
                // 10.a.i. Reject, if the [[Writable]] field of current is false and the [[Writable]] field of Desc is true.
                if (descriptor.writable())
                    return reject(exec, throwException, "Attempting to change writable attribute of unconfigurable property.");
                // 10.a.ii. If the [[Writable]] field of current is false, then
                // 10.a.ii.1. Reject, if the [[Value]] field of Desc is present and SameValue(Desc.[[Value]], current.[[Value]]) is false.
                if (descriptor.value() && !sameValue(exec, descriptor.value(), current.value()))
                    return reject(exec, throwException, "Attempting to change value of a readonly property.");
            }
            // 10.b. else, the [[Configurable]] field of current is true, so any change is acceptable.
        } else {
            // 11. Else, IsAccessorDescriptor(current) and IsAccessorDescriptor(Desc) are both true so, if the [[Configurable]] field of current is false, then
            if (!current.configurable()) {
                // 11.i. Reject, if the [[Set]] field of Desc is present and SameValue(Desc.[[Set]], current.[[Set]]) is false.
                if (descriptor.setterPresent() && descriptor.setter() != current.setter())
                    return reject(exec, throwException, "Attempting to change the setter of an unconfigurable property.");
                // 11.ii. Reject, if the [[Get]] field of Desc is present and SameValue(Desc.[[Get]], current.[[Get]]) is false.
                if (descriptor.getterPresent() && descriptor.getter() != current.getter())
                    return reject(exec, throwException, "Attempting to change the getter of an unconfigurable property.");
            }
        }
    }

    // 12. For each attribute field of Desc that is present, set the correspondingly named attribute of the property named P of object O to the value of the field.
    putDescriptor(exec, entryInMap, descriptor, current);
    // 13. Return true.
    return true;
}

void JSArray::setLengthWritable(ExecState* exec, bool writable)
{
    ASSERT(isLengthWritable() || !writable);
    if (!isLengthWritable() || writable)
        return;

    enterSparseMode(exec->globalData());

    SparseArrayValueMap* map = m_storage->m_sparseValueMap;
    ASSERT(map);
    map->setLengthIsReadOnly();
}

// Defined in ES5.1 15.4.5.1
bool JSArray::defineOwnProperty(JSObject* object, ExecState* exec, const Identifier& propertyName, PropertyDescriptor& descriptor, bool throwException)
{
    JSArray* array = static_cast<JSArray*>(object);

    // 3. If P is "length", then
    if (propertyName == exec->propertyNames().length) {
        // All paths through length definition call the default [[DefineOwnProperty]], hence:
        // from ES5.1 8.12.9 7.a.
        if (descriptor.configurable())
            return reject(exec, throwException, "Attempting to change configurable attribute of unconfigurable property.");
        // from ES5.1 8.12.9 7.b.
        if (descriptor.enumerable())
            return reject(exec, throwException, "Attempting to change enumerable attribute of unconfigurable property.");

        // a. If the [[Value]] field of Desc is absent, then
        // a.i. Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", Desc, and Throw as arguments.
        if (descriptor.isAccessorDescriptor())
            return reject(exec, throwException, "Attempting to change access mechanism for an unconfigurable property.");
        // from ES5.1 8.12.9 10.a.
        if (!array->isLengthWritable() && descriptor.writable())
            return reject(exec, throwException, "Attempting to change writable attribute of unconfigurable property.");
        // This descriptor is either just making length read-only, or changing nothing!
        if (!descriptor.value()) {
            array->setLengthWritable(exec, descriptor.writable());
            return true;
        }
        
        // b. Let newLenDesc be a copy of Desc.
        // c. Let newLen be ToUint32(Desc.[[Value]]).
        unsigned newLen = descriptor.value().toUInt32(exec);
        // d. If newLen is not equal to ToNumber( Desc.[[Value]]), throw a RangeError exception.
        if (newLen != descriptor.value().toNumber(exec)) {
            throwError(exec, createRangeError(exec, "Invalid array length"));
            return false;
        }

        // Based on SameValue check in 8.12.9, this is always okay.
        if (newLen == array->length()) {
            array->setLengthWritable(exec, descriptor.writable());
            return true;
        }

        // e. Set newLenDesc.[[Value] to newLen.
        // f. If newLen >= oldLen, then
        // f.i. Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", newLenDesc, and Throw as arguments.
        // g. Reject if oldLenDesc.[[Writable]] is false.
        if (!array->isLengthWritable())
            return reject(exec, throwException, "Attempting to change value of a readonly property.");
        
        // h. If newLenDesc.[[Writable]] is absent or has the value true, let newWritable be true.
        // i. Else,
        // i.i. Need to defer setting the [[Writable]] attribute to false in case any elements cannot be deleted.
        // i.ii. Let newWritable be false.
        // i.iii. Set newLenDesc.[[Writable] to true.
        // j. Let succeeded be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", newLenDesc, and Throw as arguments.
        // k. If succeeded is false, return false.
        // l. While newLen < oldLen repeat,
        // l.i. Set oldLen to oldLen – 1.
        // l.ii. Let deleteSucceeded be the result of calling the [[Delete]] internal method of A passing ToString(oldLen) and false as arguments.
        // l.iii. If deleteSucceeded is false, then
        if (!array->setLength(newLen, throwException)) {
            // 1. Set newLenDesc.[[Value] to oldLen+1.
            // 2. If newWritable is false, set newLenDesc.[[Writable] to false.
            // 3. Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", newLenDesc, and false as arguments.
            // 4. Reject.
            array->setLengthWritable(exec, descriptor.writable());
            return false;
        }

        // m. If newWritable is false, then
        // i. Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", Property Descriptor{[[Writable]]: false}, and false as arguments. This call will always return true.
        array->setLengthWritable(exec, descriptor.writable());
        // n. Return true.
        return true;
    }

    // 4. Else if P is an array index (15.4), then
    bool isArrayIndex;
    // a. Let index be ToUint32(P).
    unsigned index = propertyName.toArrayIndex(isArrayIndex);
    if (isArrayIndex) {
        // b. Reject if index >= oldLen and oldLenDesc.[[Writable]] is false.
        if (index >= array->length() && !array->isLengthWritable())
            return reject(exec, throwException, "Attempting to define numeric property on array with non-writable length property.");
        // c. Let succeeded be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing P, Desc, and false as arguments.
        // d. Reject if succeeded is false.
        // e. If index >= oldLen
        // e.i. Set oldLenDesc.[[Value]] to index + 1.
        // e.ii. Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", oldLenDesc, and false as arguments. This call will always return true.
        // f. Return true.
        return array->defineOwnNumericProperty(exec, index, descriptor, throwException);
    }

    return JSObject::defineOwnProperty(object, exec, propertyName, descriptor, throwException);
}

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bool JSArray::getOwnPropertySlotByIndex(JSCell* cell, ExecState* exec, unsigned i, PropertySlot& slot)
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{
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    JSArray* thisObject = jsCast<JSArray*>(cell);
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    ArrayStorage* storage = thisObject->m_storage;
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    if (i >= storage->m_length) {
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        if (i > MAX_ARRAY_INDEX)
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            return thisObject->methodTable()->getOwnPropertySlot(thisObject, exec, Identifier::from(exec, i), slot);
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        return false;
    }

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    if (i < thisObject->m_vectorLength) {
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        JSValue value = storage->m_vector[i].get();
        if (value) {
            slot.setValue(value);
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            return true;
        }
    } else if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
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        SparseArrayValueMap::iterator it = map->find(i);
        if (it != map->notFound()) {
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            it->second.get(slot);
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            return true;
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        }
    }

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    return JSObject::getOwnPropertySlot(thisObject, exec, Identifier::from(exec, i), slot);
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}

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bool JSArray::getOwnPropertySlot(JSCell* cell, ExecState* exec, const Identifier& propertyName, PropertySlot& slot)
{
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    JSArray* thisObject = jsCast<JSArray*>(cell);
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    if (propertyName == exec->propertyNames().length) {
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        slot.setValue(jsNumber(thisObject->length()));
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        return true;
    }

    bool isArrayIndex;
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    unsigned i = propertyName.toArrayIndex(isArrayIndex);
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    if (isArrayIndex)
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        return JSArray::getOwnPropertySlotByIndex(thisObject, exec, i, slot);
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    return JSObject::getOwnPropertySlot(thisObject, exec, propertyName, slot);
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}

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bool JSArray::getOwnPropertyDescriptor(JSObject* object, ExecState* exec, const Identifier& propertyName, PropertyDescriptor& descriptor)
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{
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    JSArray* thisObject = jsCast<JSArray*>(object);
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    if (propertyName == exec->propertyNames().length) {
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        descriptor.setDescriptor(jsNumber(thisObject->length()), DontDelete | DontEnum);
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        return true;
    }
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    ArrayStorage* storage = thisObject->m_storage;
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    bool isArrayIndex;
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    unsigned i = propertyName.toArrayIndex(isArrayIndex);
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    if (isArrayIndex) {
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        if (i >= storage->m_length)
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            return false;
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        if (i < thisObject->m_vectorLength) {
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            WriteBarrier<Unknown>& value = storage->m_vector[i];
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            if (value) {
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                descriptor.setDescriptor(value.get(), 0);
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                return true;
            }
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        } else if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
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            SparseArrayValueMap::iterator it = map->find(i);
            if (it != map->notFound()) {
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                it->second.get(descriptor);
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                return true;
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            }
        }
    }
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    return JSObject::getOwnPropertyDescriptor(thisObject, exec, propertyName, descriptor);
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}

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// ECMA 15.4.5.1
void JSArray::put(JSCell* cell, ExecState* exec, const Identifier& propertyName, JSValue value, PutPropertySlot& slot)
{
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    JSArray* thisObject = jsCast<JSArray*>(cell);
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    bool isArrayIndex;
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    unsigned i = propertyName.toArrayIndex(isArrayIndex);
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    if (isArrayIndex) {
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        putByIndex(thisObject, exec, i, value);
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        return;
    }

    if (propertyName == exec->propertyNames().length) {
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        unsigned newLength = value.toUInt32(exec);
        if (value.toNumber(exec) != static_cast<double>(newLength)) {
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            throwError(exec, createRangeError(exec, "Invalid array length"));
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            return;
        }
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        thisObject->setLength(newLength, slot.isStrictMode());
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        return;
    }

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    JSObject::put(thisObject, exec, propertyName, value, slot);
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}

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void JSArray::putByIndex(JSCell* cell, ExecState* exec, unsigned i, JSValue value)
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{
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    JSArray* thisObject = jsCast<JSArray*>(cell);
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    thisObject->checkConsistency();

    ArrayStorage* storage = thisObject->m_storage;
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    // Fast case - store to the vector.
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    if (i < thisObject->m_vectorLength) {
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        WriteBarrier<Unknown>& valueSlot = storage->m_vector[i];
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        unsigned length = storage->m_length;

        // Update m_length & m_numValuesInVector as necessary.
        if (i >= length) {
            length = i + 1;
            storage->m_length = length;
            ++storage->m_numValuesInVector;
        } else if (!valueSlot)
            ++storage->m_numValuesInVector;

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        valueSlot.set(exec->globalData(), thisObject, value);
        thisObject->checkConsistency();
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        return;
    }

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    // Handle 2^32-1 - this is not an array index (see ES5.1 15.4), and is treated as a regular property.
    if (UNLIKELY(i > MAX_ARRAY_INDEX)) {
        PutPropertySlot slot;
        thisObject->methodTable()->put(thisObject, exec, Identifier::from(exec, i), value, slot);
        return;
    }

    // For all other cases, call putByIndexBeyondVectorLength.
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    thisObject->putByIndexBeyondVectorLength(exec, i, value);
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    thisObject->checkConsistency();
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}

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NEVER_INLINE void JSArray::putByIndexBeyondVectorLength(ExecState* exec, unsigned i, JSValue value)
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{
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    JSGlobalData& globalData = exec->globalData();

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    // i should be a valid array index that is outside of the current vector.
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    ASSERT(i >= m_vectorLength);
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    ASSERT(i <= MAX_ARRAY_INDEX);
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    ArrayStorage* storage = m_storage;
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    SparseArrayValueMap* map = storage->m_sparseValueMap;
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    // First, handle cases where we don't currently have a sparse map.
    if (LIKELY(!map)) {
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        // Update m_length if necessary.
        if (i >= storage->m_length)
            storage->m_length = i + 1;

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        // Check that it is sensible to still be using a vector, and then try to grow the vector.
        if (LIKELY((isDenseEnoughForVector(i, storage->m_numValuesInVector)) && increaseVectorLength(i + 1))) {
            // success! - reread m_storage since it has likely been reallocated, and store to the vector.
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            storage = m_storage;
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            storage->m_vector[i].set(globalData, this, value);
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            ++storage->m_numValuesInVector;
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            return;
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        }
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        // We don't want to, or can't use a vector to hold this property - allocate a sparse map & add the value.
        map = new SparseArrayValueMap;
        storage->m_sparseValueMap = map;
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        map->put(exec, this, i, value);
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        return;
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    }

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    // Update m_length if necessary.
    unsigned length = storage->m_length;
    if (i >= length) {
        // Prohibit growing the array if length is not writable.
        if (map->lengthIsReadOnly()) {
            // FIXME: should throw in strict mode.
            return;
        }
        length = i + 1;
        storage->m_length = length;
    }

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    // We are currently using a map - check whether we still want to be doing so.
    // We will continue  to use a sparse map if SparseMode is set, a vector would be too sparse, or if allocation fails.
    unsigned numValuesInArray = storage->m_numValuesInVector + map->size();
    if (map->sparseMode() || !isDenseEnoughForVector(length, numValuesInArray) || !increaseVectorLength(length)) {
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        map->put(exec, this, i, value);
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        return;
    }
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    // Reread m_storage afterincreaseVectorLength, update m_numValuesInVector.
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    storage = m_storage;
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    storage->m_numValuesInVector = numValuesInArray;
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    // Copy all values from the map into the vector, and delete the map.
    WriteBarrier<Unknown>* vector = storage->m_vector;
    SparseArrayValueMap::const_iterator end = map->end();
    for (SparseArrayValueMap::const_iterator it = map->begin(); it != end; ++it)
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        vector[it->first].set(globalData, this, it->second.getNonSparseMode());
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    delete map;
    storage->m_sparseValueMap = 0;

    // Store the new property into the vector.
    WriteBarrier<Unknown>& valueSlot = vector[i];
    if (!valueSlot)
        ++storage->m_numValuesInVector;
    valueSlot.set(globalData, this, value);
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}

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bool JSArray::deleteProperty(JSCell* cell, ExecState* exec, const Identifier& propertyName)
{
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    JSArray* thisObject = jsCast<JSArray*>(cell);
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    bool isArrayIndex;
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    unsigned i = propertyName.toArrayIndex(isArrayIndex);
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    if (isArrayIndex)
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        return thisObject->methodTable()->deletePropertyByIndex(thisObject, exec, i);
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    if (propertyName == exec->propertyNames().length)
        return false;

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    return JSObject::deleteProperty(thisObject, exec, propertyName);
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}

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bool JSArray::deletePropertyByIndex(JSCell* cell, ExecState* exec, unsigned i)
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{
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    JSArray* thisObject = jsCast<JSArray*>(cell);
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    thisObject->checkConsistency();

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    if (i > MAX_ARRAY_INDEX)
        return thisObject->methodTable()->deleteProperty(thisObject, exec, Identifier::from(exec, i));

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    ArrayStorage* storage = thisObject->m_storage;
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    if (i < thisObject->m_vectorLength) {
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        WriteBarrier<Unknown>& valueSlot = storage->m_vector[i];
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        if (valueSlot) {
            valueSlot.clear();
            --storage->m_numValuesInVector;
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        }
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    } else if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
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        SparseArrayValueMap::iterator it = map->find(i);
        if (it != map->notFound()) {
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            if (it->second.attributes & DontDelete)
                return false;
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            map->remove(it);
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        }
    }

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    thisObject->checkConsistency();
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    return true;
}
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static int compareKeysForQSort(const void* a, const void* b)
{
    uint64_t da = *static_cast<const uint64_t*>(a);
    uint64_t db = *static_cast<const uint64_t*>(b);
    return (da > db) - (da < db);
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}

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void JSArray::getOwnPropertyNames(JSObject* object, ExecState* exec, PropertyNameArray& propertyNames, EnumerationMode mode)
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{
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    JSArray* thisObject = jsCast<JSArray*>(object);
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    // FIXME: Filling PropertyNameArray with an identifier for every integer
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    // is incredibly inefficient for large arrays. We need a different approach,
    // which almost certainly means a different structure for PropertyNameArray.
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    ArrayStorage* storage = thisObject->m_storage;
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    unsigned usedVectorLength = min(storage->m_length, thisObject->m_vectorLength);
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    for (unsigned i = 0; i < usedVectorLength; ++i) {
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        if (storage->m_vector[i])
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            propertyNames.add(Identifier::from(exec, i));
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    }

    if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
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        Vector<uint64_t> keys;
        keys.reserveCapacity(map->size());
        
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        SparseArrayValueMap::const_iterator end = map->end();
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        for (SparseArrayValueMap::const_iterator it = map->begin(); it != end; ++it) {
            if (mode == IncludeDontEnumProperties || !(it->second.attributes & DontEnum))
                keys.append(it->first);
        }

        qsort(keys.begin(), keys.size(), sizeof(uint64_t), compareKeysForQSort);
        for (unsigned i = 0; i < keys.size(); ++i)
            propertyNames.add(Identifier::from(exec, static_cast<unsigned>(keys[i])));
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    }
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    if (mode == IncludeDontEnumProperties)
        propertyNames.add(exec->propertyNames().length);

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    JSObject::getOwnPropertyNames(thisObject, exec, propertyNames, mode);
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}

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ALWAYS_INLINE unsigned JSArray::getNewVectorLength(unsigned desiredLength)
{
    ASSERT(desiredLength <= MAX_STORAGE_VECTOR_LENGTH);

    unsigned increasedLength;
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    unsigned maxInitLength = min(m_storage->m_length, 100000U);
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    if (desiredLength < maxInitLength)
        increasedLength = maxInitLength;
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    else if (!m_vectorLength)
        increasedLength = max(desiredLength, lastArraySize);
    else {
        // Mathematically equivalent to:
        //   increasedLength = (newLength * 3 + 1) / 2;
        // or:
        //   increasedLength = (unsigned)ceil(newLength * 1.5));
        // This form is not prone to internal overflow.
        increasedLength = desiredLength + (desiredLength >> 1) + (desiredLength & 1);
    }

    ASSERT(increasedLength >= desiredLength);

    lastArraySize = min(increasedLength, FIRST_VECTOR_GROW);

    return min(increasedLength, MAX_STORAGE_VECTOR_LENGTH);
}

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bool JSArray::increaseVectorLength(unsigned newLength)
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{
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    // This function leaves the array in an internally inconsistent state, because it does not move any values from sparse value map
    // to the vector. Callers have to account for that, because they can do it more efficiently.
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    if (newLength > MAX_STORAGE_VECTOR_LENGTH)
        return false;
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    ArrayStorage* storage = m_storage;
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    unsigned vectorLength = m_vectorLength;
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    ASSERT(newLength > vectorLength);
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    unsigned newVectorLength = getNewVectorLength(newLength);
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    void* baseStorage = storage->m_allocBase;
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    // Fast case - there is no precapacity. In these cases a realloc makes sense.
    if (LIKELY(!m_indexBias)) {
        if (!tryFastRealloc(baseStorage, storageSize(newVectorLength)).getValue(baseStorage))
            return false;
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        storage = m_storage = reinterpret_cast_ptr<ArrayStorage*>(baseStorage);
        m_storage->m_allocBase = baseStorage;
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        WriteBarrier<Unknown>* vector = storage->m_vector;
        for (unsigned i = vectorLength; i < newVectorLength; ++i)
            vector[i].clear();

        m_vectorLength = newVectorLength;
        
        Heap::heap(this)->reportExtraMemoryCost(storageSize(newVectorLength) - storageSize(vectorLength));
        return true;
    }

    // Remove some, but not all of the precapacity. Atomic decay, & capped to not overflow array length.
    unsigned newIndexBias = min(m_indexBias >> 1, MAX_STORAGE_VECTOR_LENGTH - newVectorLength);
    // Calculate new stoarge capcity, allowing room for the pre-capacity.
    unsigned newStorageCapacity = newVectorLength + newIndexBias;
    void* newAllocBase;
    if (!tryFastMalloc(storageSize(newStorageCapacity)).getValue(newAllocBase))
        return false;
    // The sum of m_vectorLength and m_indexBias will never exceed MAX_STORAGE_VECTOR_LENGTH.
    ASSERT(m_vectorLength <= MAX_STORAGE_VECTOR_LENGTH && (MAX_STORAGE_VECTOR_LENGTH - m_vectorLength) >= m_indexBias);
    unsigned currentCapacity = m_vectorLength + m_indexBias;
    // Currently there is no way to report to the heap that the extra capacity is shrinking!
    if (newStorageCapacity > currentCapacity)
        Heap::heap(this)->reportExtraMemoryCost((newStorageCapacity - currentCapacity) * sizeof(WriteBarrier<Unknown>));
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    m_vectorLength = newVectorLength;
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    m_indexBias = newIndexBias;
    m_storage = reinterpret_cast_ptr<ArrayStorage*>(reinterpret_cast<WriteBarrier<Unknown>*>(newAllocBase) + m_indexBias);

    // Copy the ArrayStorage header & current contents of the vector, clear the new post-capacity.
    memmove(m_storage, storage, storageSize(vectorLength));
    for (unsigned i = vectorLength; i < m_vectorLength; ++i)
        m_storage->m_vector[i].clear();

    // Free the old allocation, update m_allocBase.
    fastFree(m_storage->m_allocBase);
    m_storage->m_allocBase = newAllocBase;
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    return true;
}
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// This method makes room in the vector, but leaves the new space uncleared.
bool JSArray::unshiftCountSlowCase(unsigned count)
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{
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    // If not, we should have handled this on the fast path.
    ASSERT(count > m_indexBias);
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    ArrayStorage* storage = m_storage;
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    // Step 1:
    // Gather 4 key metrics:
    //  * usedVectorLength - how many entries are currently in the vector (conservative estimate - fewer may be in use in sparse vectors).
    //  * requiredVectorLength - how many entries are will there be in the vector, after allocating space for 'count' more.
    //  * currentCapacity - what is the current size of the vector, including any pre-capacity.
    //  * desiredCapacity - how large should we like to grow the vector to - based on 2x requiredVectorLength.

    unsigned length = storage->m_length;
    unsigned usedVectorLength = min(m_vectorLength, length);
    ASSERT(usedVectorLength <= MAX_STORAGE_VECTOR_LENGTH);
    // Check that required vector length is possible, in an overflow-safe fashion.
    if (count > MAX_STORAGE_VECTOR_LENGTH - usedVectorLength)
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        return false;
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    unsigned requiredVectorLength = usedVectorLength + count;
    ASSERT(requiredVectorLength <= MAX_STORAGE_VECTOR_LENGTH);
    // The sum of m_vectorLength and m_indexBias will never exceed MAX_STORAGE_VECTOR_LENGTH.
    ASSERT(m_vectorLength <= MAX_STORAGE_VECTOR_LENGTH && (MAX_STORAGE_VECTOR_LENGTH - m_vectorLength) >= m_indexBias);
    unsigned currentCapacity = m_vectorLength + m_indexBias;
    // The calculation of desiredCapacity won't overflow, due to the range of MAX_STORAGE_VECTOR_LENGTH.
    unsigned desiredCapacity = min(MAX_STORAGE_VECTOR_LENGTH, max(BASE_VECTOR_LEN, requiredVectorLength) << 1);

    // Step 2:
    // We're either going to choose to allocate a new ArrayStorage, or we're going to reuse the existing on.

    void* newAllocBase;
    unsigned newStorageCapacity;
    // If the current storage array is sufficiently large (but not too large!) then just keep using it.
    if (currentCapacity > desiredCapacity && isDenseEnoughForVector(currentCapacity, requiredVectorLength)) {
        newAllocBase = storage->m_allocBase;
        newStorageCapacity = currentCapacity;
    } else {
        if (!tryFastMalloc(storageSize(desiredCapacity)).getValue(newAllocBase))
            return false;
        newStorageCapacity = desiredCapacity;
        // Currently there is no way to report to the heap that the extra capacity is shrinking!
        if (desiredCapacity > currentCapacity)
            Heap::heap(this)->reportExtraMemoryCost((desiredCapacity - currentCapacity) * sizeof(WriteBarrier<Unknown>));
    }

    // Step 3:
    // Work out where we're going to move things to.

    // Determine how much of the vector to use as pre-capacity, and how much as post-capacity.
    // If the vector had no free post-capacity (length >= m_vectorLength), don't give it any.
    // If it did, we calculate the amount that will remain based on an atomic decay - leave the
    // vector with half the post-capacity it had previously.
    unsigned postCapacity = 0;
    if (length < m_vectorLength) {
        // Atomic decay, + the post-capacity cannot be greater than what is available.
        postCapacity = min((m_vectorLength - length) >> 1, newStorageCapacity - requiredVectorLength);
        // If we're moving contents within the same allocation, the post-capacity is being reduced.
        ASSERT(newAllocBase != storage->m_allocBase || postCapacity < m_vectorLength - length);
    }

    m_vectorLength = requiredVectorLength + postCapacity;
    m_indexBias = newStorageCapacity - m_vectorLength;
    m_storage = reinterpret_cast_ptr<ArrayStorage*>(reinterpret_cast<WriteBarrier<Unknown>*>(newAllocBase) + m_indexBias);

    // Step 4:
    // Copy array data / header into their new locations, clear post-capacity & free any old allocation.

    // If this is being moved within the existing buffer of memory, we are always shifting data
    // to the right (since count > m_indexBias). As such this memmove cannot trample the header.
    memmove(m_storage->m_vector + count, storage->m_vector, sizeof(WriteBarrier<Unknown>) * usedVectorLength);
    memmove(m_storage, storage, storageSize(0));

    // Are we copying into a new allocation?
    if (newAllocBase != m_storage->m_allocBase) {
        // Free the old allocation, update m_allocBase.
        fastFree(m_storage->m_allocBase);
        m_storage->m_allocBase = newAllocBase;

        // We need to clear any entries in the vector beyond length. We only need to
        // do this if this was a new allocation, because if we're using an existing
        // allocation the post-capacity will already be cleared, and in an existing
        // allocation we can only beshrinking the amount of post capacity.
        for (unsigned i = requiredVectorLength; i < m_vectorLength; ++i)
            m_storage->m_vector[i].clear();
    }
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    return true;
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}

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bool JSArray::setLength(unsigned newLength, bool throwException)
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{
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    UNUSED_PARAM(throwException);
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    checkConsistency();

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    ArrayStorage* storage = m_storage;
    
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    unsigned length = storage->m_length;
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    if (newLength < length) {
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        unsigned usedVectorLength = min(length, m_vectorLength);
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        for (unsigned i = newLength; i < usedVectorLength; ++i) {
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            WriteBarrier<Unknown>& valueSlot = storage->m_vector[i];
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            bool hadValue = valueSlot;
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            valueSlot.clear();
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            storage->m_numValuesInVector -= hadValue;
        }

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        // FIXME: properties should be removed in reverse numeric order, if any cannot be
        // removed this method should reject (possibly throw / return false), and length
        // should be set the after the last property that could not be removed.
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        if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
            SparseArrayValueMap copy = *map;
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            SparseArrayValueMap::const_iterator end = copy.end();
            for (SparseArrayValueMap::const_iterator it = copy.begin(); it != end; ++it) {
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                if (it->first >= newLength)
                    map->remove(it->first);
            }
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            if (map->isEmpty() && !map->sparseMode()) {
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                delete map;
                storage->m_sparseValueMap = 0;
            }
        }
    }
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    storage->m_length = newLength;
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    checkConsistency();
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    return true;
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}

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JSValue JSArray::pop(ExecState* exec)
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{
    checkConsistency();

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    ArrayStorage* storage = m_storage;
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    unsigned length = storage->m_length;
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    if (!length) {
        if (!isLengthWritable())
            throwTypeError(exec, StrictModeReadonlyPropertyWriteError);
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        return jsUndefined();
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    }
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    --length;

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    JSValue result;
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    if (length < m_vectorLength) {
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        WriteBarrier<Unknown>& valueSlot = storage->m_vector[length];
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        if (valueSlot) {
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            --storage->m_numValuesInVector;
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            result = valueSlot.get();
            valueSlot.clear();
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        } else
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            result = jsUndefined();
    } else {
        result = jsUndefined();
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        if (SparseArrayValueMap* map = storage->m_sparseValueMap) {
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            SparseArrayValueMap::iterator it = map->find(length);
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            if (it != map->notFound()) {
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                unsigned attributes = it->second.attributes;

                result = it->second.get(exec, this);
                if (exec->hadException())
                    return jsUndefined();
                
                if (attributes & DontDelete) {
                    throwError(exec, createTypeError(exec, "Unable to delete property."));
                    checkConsistency();
                    return result;
                }
                
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                map->remove(it);
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                if (map->isEmpty() && !map->sparseMode()) {
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                    delete map;
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                    storage->m_sparseValueMap = 0;
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                }
            }
        }
    }

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    storage->m_length = length;
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    checkConsistency();

    return result;
}

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// Push & putIndex are almost identical, with two small differences.
//  - we always are writing beyond the current array bounds, so it is always necessary to update m_length & m_numValuesInVector.
//  - pushing to an array of length 2^32-1 stores the property, but throws a range error.
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void JSArray::push(ExecState* exec, JSValue value)
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{
    checkConsistency();
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    ArrayStorage* storage = m_storage;
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    // Fast case - push within vector, always update m_length & m_numValuesInVector.
    unsigned length = storage->m_length;
    if (length < m_vectorLength) {
        storage->m_vector[length].set(exec->globalData(), this, value);
        storage->m_length = length + 1;
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        ++storage->m_numValuesInVector;
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        checkConsistency();
        return;
    }