src/third_party/WebKit/Source/WTF/wtf/RefCountedArray.h

/*
 * Copyright (C) 2011 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
 * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 
 */

#ifndef RefCountedArray_h
#define RefCountedArray_h

#include <wtf/FastMalloc.h>
#include <wtf/StdLibExtras.h>
#include <wtf/Vector.h>

// This implements a reference counted array for POD** values, which is optimized for:
// - An empty array only uses one word.
// - A copy of the array only uses one word (i.e. assignment means aliasing).
// - The vector can't grow beyond 2^32-1 elements.
// - In all other regards this has similar space usage to a Vector.
//
// ** This could be modified to support non-POD values quite easily. It just
//    hasn't been, so far, because there has been no need. Moreover, even now,
//    it's used for things that aren't quite POD according to the official
//    defintion, such as JSC::Instruction.

namespace WTF {

template<typename T>
class RefCountedArray {
public:
    RefCountedArray()
        : m_data(0)
    {
    }
    
    RefCountedArray(const RefCountedArray& other)
        : m_data(other.m_data)
    {
        if (m_data)
            Header::fromPayload(m_data)->refCount++;
    }
    
    explicit RefCountedArray(const Vector<T>& other)
    {
        if (other.isEmpty()) {
            m_data = 0;
            return;
        }
        
        m_data = (static_cast<Header*>(fastMalloc(Header::size() + sizeof(T) * other.size())))->payload();
        Header::fromPayload(m_data)->refCount = 1;
        Header::fromPayload(m_data)->length = other.size();
        ASSERT(Header::fromPayload(m_data)->length == other.size());
        memcpy(m_data, other.begin(), sizeof(T) * other.size());
    }
    
    RefCountedArray& operator=(const RefCountedArray& other)
    {
        T* oldData = m_data;
        m_data = other.m_data;
        if (m_data)
            Header::fromPayload(m_data)->refCount++;
        
        if (!oldData)
            return *this;
        if (--Header::fromPayload(oldData)->refCount)
            return *this;
        fastFree(Header::fromPayload(oldData));
        return *this;
    }
    
    ~RefCountedArray()
    {
        if (!m_data)
            return;
        if (--Header::fromPayload(m_data)->refCount)
            return;
        fastFree(Header::fromPayload(m_data));
    }
    
    size_t size() const
    {
        if (!m_data)
            return 0;
        return Header::fromPayload(m_data)->length;
    }
    
    T* data() { return m_data; }
    T* begin() { return m_data; }
    T* end()
    {
        if (!m_data)
            return 0;
        return m_data + Header::fromPayload(m_data)->length;
    }
    
    const T* data() const { return m_data; }
    const T* begin() const { return m_data; }
    const T* end() const { return const_cast<RefCountedArray*>(this)->end(); }
    
    T& at(size_t i)
    {
        ASSERT(i < size());
        return begin()[i];
    }
    
    const T& at(size_t i) const
    {
        ASSERT(i < size());
        return begin()[i];
    }
    
    T& operator[](size_t i) { return at(i); }
    const T& operator[](size_t i) const { return at(i); }
    
private:
    struct Header {
        unsigned refCount;
        unsigned length;
        
        static size_t size()
        {
            return (sizeof(Header) + 7) & ~7;
        }
        
        T* payload()
        {
            char* result = reinterpret_cast<char*>(this) + size();
            ASSERT(!(bitwise_cast<uintptr_t>(result) & 7));
            return reinterpret_cast_ptr<T*>(result);
        }
        
        static Header* fromPayload(T* payload)
        {
            return reinterpret_cast_ptr<Header*>(reinterpret_cast<char*>(payload) - size());
        }
    };
    
    T* m_data;
};

} // namespace WTF

using WTF::RefCountedArray;

#endif // RefCountedArray_h