Source/WTF/wtf/ConcurrentBuffer.h

/*
 * Copyright (C) 2017 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
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 */

#pragma once

#include <wtf/Atomics.h>
#include <wtf/FastMalloc.h>
#include <wtf/HashFunctions.h>
#include <wtf/Lock.h>
#include <wtf/Noncopyable.h>
#include <wtf/Vector.h>

namespace WTF {

// ConcurrentBuffer is suitable for when you plan to store immutable data and sometimes append to it.
// It supports storing data that is not copy-constructable but bit-copyable.
template<typename T>
class ConcurrentBuffer {
    WTF_MAKE_NONCOPYABLE(ConcurrentBuffer);
    WTF_MAKE_FAST_ALLOCATED;
public:
    
    ConcurrentBuffer()
    {
    }
    
    ~ConcurrentBuffer()
    {
        if (Array* array = m_array) {
            for (size_t i = 0; i < array->size; ++i)
                array->data[i].~T();
        }
        for (Array* array : m_allArrays)
            fastFree(array);
    }

    // Growing is not concurrent. This assumes you are holding some other lock before you do this.
    void growExact(size_t newSize)
    {
        Array* array = m_array;
        if (array && newSize <= array->size)
            return;
        Array* newArray = createArray(newSize);
        // This allows us to do ConcurrentBuffer<std::unique_ptr<>>.
        if (array)
            memcpy(newArray->data, array->data, sizeof(T) * array->size);
        for (size_t i = array ? array->size : 0; i < newSize; ++i)
            new (newArray->data + i) T();
        WTF::storeStoreFence();
        m_array = newArray;
        WTF::storeStoreFence();
        m_allArrays.append(newArray);
    }
    
    void grow(size_t newSize)
    {
        size_t size = m_array ? m_array->size : 0;
        if (newSize <= size)
            return;
        growExact(std::max(newSize, size * 2));
    }
    
    struct Array {
        size_t size; // This is an immutable size.
        T data[1];
    };
    
    Array* array() const { return m_array; }
    
    T& operator[](size_t index) { return m_array->data[index]; }
    const T& operator[](size_t index) const { return m_array->data[index]; }
    
private:
    Array* createArray(size_t size)
    {
        Checked<size_t> objectSize = sizeof(T);
        objectSize *= size;
        objectSize += static_cast<size_t>(OBJECT_OFFSETOF(Array, data));
        Array* result = static_cast<Array*>(fastMalloc(objectSize.unsafeGet()));
        result->size = size;
        return result;
    }
    
    Array* m_array { nullptr };
    Vector<Array*> m_allArrays;
};

} // namespace WTF