buffer.cc

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/*
 * Copyright (c) 2005,2006,2007 INRIA
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation;
 *
 * This program 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 * Author: Mathieu Lacage <mathieu.lacage@sophia.inria.fr>
 */
#include "buffer.h"
 
#include "ns3/assert.h"
#include "ns3/log.h"
 
#define LOG_INTERNAL_STATE(y)                                                                      \
    NS_LOG_LOGIC(y << "start=" << m_start << ", end=" << m_end                                     \
                   << ", zero start=" << m_zeroAreaStart << ", zero end=" << m_zeroAreaEnd         \
                   << ", count=" << m_data->m_count << ", size=" << m_data->m_size                 \
                   << ", dirty start=" << m_data->m_dirtyStart                                     \
                   << ", dirty end=" << m_data->m_dirtyEnd)
 
namespace
{
 
struct Zeroes
{
    Zeroes()
        : size(1000)
    {
        memset(buffer, 0, size);
    }
 
    char buffer[1000];   
    const uint32_t size; 
} g_zeroes;              
 
} // namespace
 
namespace ns3
{
 
NS_LOG_COMPONENT_DEFINE("Buffer");
 
uint32_t Buffer::g_recommendedStart = 0;
#ifdef BUFFER_FREE_LIST
/* The following macros are pretty evil but they are needed to allow us to
 * keep track of 3 possible states for the g_freeList variable:
 *  - uninitialized means that no one has created a buffer yet
 *    so no one has created the associated free list (it is created
 *    on-demand when the first buffer is created)
 *  - initialized means that the free list exists and is valid
 *  - destroyed means that the static destructors of this compilation unit
 *    have run so, the free list has been cleared from its content
 * The key is that in destroyed state, we are careful not re-create it
 * which is a typical weakness of lazy evaluation schemes which use
 * '0' as a special value to indicate both un-initialized and destroyed.
 * Note that it is important to use '0' as the marker for un-initialized state
 * because the variable holding this state information is initialized to zero
 * which the compiler assigns to zero-memory which is initialized to _zero_
 * before the constructors run so this ensures perfect handling of crazy
 * constructor orderings.
 */
#define MAGIC_DESTROYED (~(long)0)
#define IS_UNINITIALIZED(x) (x == (Buffer::FreeList*)0)
#define IS_DESTROYED(x) (x == (Buffer::FreeList*)MAGIC_DESTROYED)
#define IS_INITIALIZED(x) (!IS_UNINITIALIZED(x) && !IS_DESTROYED(x))
#define DESTROYED ((Buffer::FreeList*)MAGIC_DESTROYED)
#define UNINITIALIZED ((Buffer::FreeList*)0)
uint32_t Buffer::g_maxSize = 0;
Buffer::FreeList* Buffer::g_freeList = nullptr;
struct Buffer::LocalStaticDestructor Buffer::g_localStaticDestructor;
 
Buffer::LocalStaticDestructor::~LocalStaticDestructor()
{
    NS_LOG_FUNCTION(this);
    if (IS_INITIALIZED(g_freeList))
    {
        for (Buffer::FreeList::iterator i = g_freeList->begin(); i != g_freeList->end(); i++)
        {
            Buffer::Deallocate(*i);
        }
        delete g_freeList;
        g_freeList = DESTROYED;
    }
}
 
void
Buffer::Recycle(struct Buffer::Data* data)
{
    NS_LOG_FUNCTION(data);
    NS_ASSERT(data->m_count == 0);
    NS_ASSERT(!IS_UNINITIALIZED(g_freeList));
    g_maxSize = std::max(g_maxSize, data->m_size);
    /* feed into free list */
    if (data->m_size < g_maxSize || IS_DESTROYED(g_freeList) || g_freeList->size() > 1000)
    {
        Buffer::Deallocate(data);
    }
    else
    {
        NS_ASSERT(IS_INITIALIZED(g_freeList));
        g_freeList->push_back(data);
    }
}
 
Buffer::Data*
Buffer::Create(uint32_t dataSize)
{
    NS_LOG_FUNCTION(dataSize);
    /* try to find a buffer correctly sized. */
    if (IS_UNINITIALIZED(g_freeList))
    {
        g_freeList = new Buffer::FreeList();
    }
    else if (IS_INITIALIZED(g_freeList))
    {
        while (!g_freeList->empty())
        {
            struct Buffer::Data* data = g_freeList->back();
            g_freeList->pop_back();
            if (data->m_size >= dataSize)
            {
                data->m_count = 1;
                return data;
            }
            Buffer::Deallocate(data);
        }
    }
    struct Buffer::Data* data = Buffer::Allocate(dataSize);
    NS_ASSERT(data->m_count == 1);
    return data;
}
#else  /* BUFFER_FREE_LIST */
void
Buffer::Recycle(struct Buffer::Data* data)
{
    NS_LOG_FUNCTION(data);
    NS_ASSERT(data->m_count == 0);
    Deallocate(data);
}
 
Buffer::Data*
Buffer::Create(uint32_t size)
{
    NS_LOG_FUNCTION(size);
    return Allocate(size);
}
#endif /* BUFFER_FREE_LIST */
 
struct Buffer::Data*
Buffer::Allocate(uint32_t reqSize)
{
    NS_LOG_FUNCTION(reqSize);
    if (reqSize == 0)
    {
        reqSize = 1;
    }
    NS_ASSERT(reqSize >= 1);
    uint32_t size = reqSize - 1 + sizeof(struct Buffer::Data);
    uint8_t* b = new uint8_t[size];
    struct Buffer::Data* data = reinterpret_cast<struct Buffer::Data*>(b);
    data->m_size = reqSize;
    data->m_count = 1;
    return data;
}
 
void
Buffer::Deallocate(struct Buffer::Data* data)
{
    NS_LOG_FUNCTION(data);
    NS_ASSERT(data->m_count == 0);
    uint8_t* buf = reinterpret_cast<uint8_t*>(data);
    delete[] buf;
}
 
Buffer::Buffer()
{
    NS_LOG_FUNCTION(this);
    Initialize(0);
}
 
Buffer::Buffer(uint32_t dataSize)
{
    NS_LOG_FUNCTION(this << dataSize);
    Initialize(dataSize);
}
 
Buffer::Buffer(uint32_t dataSize, bool initialize)
{
    NS_LOG_FUNCTION(this << dataSize << initialize);
    if (initialize == true)
    {
        Initialize(dataSize);
    }
}
 
bool
Buffer::CheckInternalState() const
{
    NS_LOG_FUNCTION(this);
#if 0
  // If you want to modify any code in this file, enable this checking code.
  // Otherwise, there is not much point is enabling it because the
  // current implementation has been fairly seriously tested and the cost
  // of this constant checking is pretty high, even for a debug build.
  bool offsetsOk =
    m_start <= m_zeroAreaStart &&
    m_zeroAreaStart <= m_zeroAreaEnd &&
    m_zeroAreaEnd <= m_end;
  bool dirtyOk =
    m_start >= m_data->m_dirtyStart &&
    m_end <= m_data->m_dirtyEnd;
  bool internalSizeOk = m_end - (m_zeroAreaEnd - m_zeroAreaStart) <= m_data->m_size &&
    m_start <= m_data->m_size &&
    m_zeroAreaStart <= m_data->m_size;
 
  bool ok = m_data->m_count > 0 && offsetsOk && dirtyOk && internalSizeOk;
  if (!ok)
    {
      LOG_INTERNAL_STATE ("check " << this <<
                          ", " << (offsetsOk ? "true" : "false") <<
                          ", " << (dirtyOk ? "true" : "false") <<
                          ", " << (internalSizeOk ? "true" : "false") << " ");
    }
  return ok;
#else
    return true;
#endif
}
 
void
Buffer::Initialize(uint32_t zeroSize)
{
    NS_LOG_FUNCTION(this << zeroSize);
    m_data = Buffer::Create(0);
    m_start = std::min(m_data->m_size, g_recommendedStart);
    m_maxZeroAreaStart = m_start;
    m_zeroAreaStart = m_start;
    m_zeroAreaEnd = m_zeroAreaStart + zeroSize;
    m_end = m_zeroAreaEnd;
    m_data->m_dirtyStart = m_start;
    m_data->m_dirtyEnd = m_end;
    NS_ASSERT(CheckInternalState());
}
 
Buffer&
Buffer::operator=(const Buffer& o)
{
    NS_ASSERT(CheckInternalState());
    if (m_data != o.m_data)
    {
        // not assignment to self.
        m_data->m_count--;
        if (m_data->m_count == 0)
        {
            Recycle(m_data);
        }
        m_data = o.m_data;
        m_data->m_count++;
    }
    g_recommendedStart = std::max(g_recommendedStart, m_maxZeroAreaStart);
    m_maxZeroAreaStart = o.m_maxZeroAreaStart;
    m_zeroAreaStart = o.m_zeroAreaStart;
    m_zeroAreaEnd = o.m_zeroAreaEnd;
    m_start = o.m_start;
    m_end = o.m_end;
    NS_ASSERT(CheckInternalState());
    return *this;
}
 
Buffer::~Buffer()
{
    NS_LOG_FUNCTION(this);
    NS_ASSERT(CheckInternalState());
    g_recommendedStart = std::max(g_recommendedStart, m_maxZeroAreaStart);
    m_data->m_count--;
    if (m_data->m_count == 0)
    {
        Recycle(m_data);
    }
}
 
uint32_t
Buffer::GetInternalSize() const
{
    NS_LOG_FUNCTION(this);
    return m_zeroAreaStart - m_start + m_end - m_zeroAreaEnd;
}
 
uint32_t
Buffer::GetInternalEnd() const
{
    NS_LOG_FUNCTION(this);
    return m_end - (m_zeroAreaEnd - m_zeroAreaStart);
}
 
void
Buffer::AddAtStart(uint32_t start)
{
    NS_LOG_FUNCTION(this << start);
    NS_ASSERT(CheckInternalState());
    bool isDirty = m_data->m_count > 1 && m_start > m_data->m_dirtyStart;
    if (m_start >= start && !isDirty)
    {
        /* enough space in the buffer and not dirty.
         * To add: |..|
         * Before: |*****---------***|
         * After:  |***..---------***|
         */
        NS_ASSERT(m_data->m_count == 1 || m_start == m_data->m_dirtyStart);
        m_start -= start;
        // update dirty area
        m_data->m_dirtyStart = m_start;
    }
    else
    {
        uint32_t newSize = GetInternalSize() + start;
        struct Buffer::Data* newData = Buffer::Create(newSize);
        memcpy(newData->m_data + start, m_data->m_data + m_start, GetInternalSize());
        m_data->m_count--;
        if (m_data->m_count == 0)
        {
            Buffer::Recycle(m_data);
        }
        m_data = newData;
 
        int32_t delta = start - m_start;
        m_start += delta;
        m_zeroAreaStart += delta;
        m_zeroAreaEnd += delta;
        m_end += delta;
        m_start -= start;
 
        // update dirty area
        m_data->m_dirtyStart = m_start;
        m_data->m_dirtyEnd = m_end;
    }
    m_maxZeroAreaStart = std::max(m_maxZeroAreaStart, m_zeroAreaStart);
    LOG_INTERNAL_STATE("add start=" << start << ", ");
    NS_ASSERT(CheckInternalState());
}
 
void
Buffer::AddAtEnd(uint32_t end)
{
    NS_LOG_FUNCTION(this << end);
    NS_ASSERT(CheckInternalState());
    bool isDirty = m_data->m_count > 1 && m_end < m_data->m_dirtyEnd;
    if (GetInternalEnd() + end <= m_data->m_size && !isDirty)
    {
        /* enough space in buffer and not dirty
         * Add:    |...|
         * Before: |**----*****|
         * After:  |**----...**|
         */
        NS_ASSERT(m_data->m_count == 1 || m_end == m_data->m_dirtyEnd);
        m_end += end;
        // update dirty area.
        m_data->m_dirtyEnd = m_end;
    }
    else
    {
        uint32_t newSize = GetInternalSize() + end;
        struct Buffer::Data* newData = Buffer::Create(newSize);
        memcpy(newData->m_data, m_data->m_data + m_start, GetInternalSize());
        m_data->m_count--;
        if (m_data->m_count == 0)
        {
            Buffer::Recycle(m_data);
        }
        m_data = newData;
 
        int32_t delta = -m_start;
        m_zeroAreaStart += delta;
        m_zeroAreaEnd += delta;
        m_end += delta;
        m_start += delta;
        m_end += end;
 
        // update dirty area
        m_data->m_dirtyStart = m_start;
        m_data->m_dirtyEnd = m_end;
    }
    m_maxZeroAreaStart = std::max(m_maxZeroAreaStart, m_zeroAreaStart);
    LOG_INTERNAL_STATE("add end=" << end << ", ");
    NS_ASSERT(CheckInternalState());
}
 
void
Buffer::AddAtEnd(const Buffer& o)
{
    NS_LOG_FUNCTION(this << &o);
 
    if (m_data->m_count == 1 && (m_end == m_zeroAreaEnd || m_zeroAreaStart == m_zeroAreaEnd) &&
        m_end == m_data->m_dirtyEnd && o.m_start == o.m_zeroAreaStart &&
        o.m_zeroAreaEnd - o.m_zeroAreaStart > 0)
    {
        if (m_zeroAreaStart == m_zeroAreaEnd)
        {
            m_zeroAreaStart = m_end;
        }
        uint32_t zeroSize = o.m_zeroAreaEnd - o.m_zeroAreaStart;
        m_zeroAreaEnd = m_end + zeroSize;
        m_end = m_zeroAreaEnd;
        m_data->m_dirtyEnd = m_zeroAreaEnd;
        uint32_t endData = o.m_end - o.m_zeroAreaEnd;
        AddAtEnd(endData);
        Buffer::Iterator dst = End();
        dst.Prev(endData);
        Buffer::Iterator src = o.End();
        src.Prev(endData);
        dst.Write(src, o.End());
        NS_ASSERT(CheckInternalState());
        return;
    }
 
    *this = CreateFullCopy();
    AddAtEnd(o.GetSize());
    Buffer::Iterator destStart = End();
    destStart.Prev(o.GetSize());
    destStart.Write(o.Begin(), o.End());
    NS_ASSERT(CheckInternalState());
}
 
void
Buffer::RemoveAtStart(uint32_t start)
{
    NS_LOG_FUNCTION(this << start);
    NS_ASSERT(CheckInternalState());
    uint32_t newStart = m_start + start;
    if (newStart <= m_zeroAreaStart)
    {
        /* only remove start of buffer
         */
        m_start = newStart;
    }
    else if (newStart <= m_zeroAreaEnd)
    {
        /* remove start of buffer _and_ start of zero area
         */
        uint32_t delta = newStart - m_zeroAreaStart;
        m_start = m_zeroAreaStart;
        m_zeroAreaEnd -= delta;
        m_end -= delta;
    }
    else if (newStart <= m_end)
    {
        /* remove start of buffer, complete zero area, and part
         * of end of buffer
         */
        NS_ASSERT(m_end >= start);
        uint32_t zeroSize = m_zeroAreaEnd - m_zeroAreaStart;
        m_start = newStart - zeroSize;
        m_end -= zeroSize;
        m_zeroAreaStart = m_start;
        m_zeroAreaEnd = m_start;
    }
    else
    {
        /* remove all buffer */
        m_end -= m_zeroAreaEnd - m_zeroAreaStart;
        m_start = m_end;
        m_zeroAreaEnd = m_end;
        m_zeroAreaStart = m_end;
    }
    m_maxZeroAreaStart = std::max(m_maxZeroAreaStart, m_zeroAreaStart);
    LOG_INTERNAL_STATE("rem start=" << start << ", ");
    NS_ASSERT(CheckInternalState());
}
 
void
Buffer::RemoveAtEnd(uint32_t end)
{
    NS_LOG_FUNCTION(this << end);
    NS_ASSERT(CheckInternalState());
    uint32_t newEnd = m_end - std::min(end, m_end - m_start);
    if (newEnd > m_zeroAreaEnd)
    {
        /* remove part of end of buffer */
        m_end = newEnd;
    }
    else if (newEnd > m_zeroAreaStart)
    {
        /* remove end of buffer, part of zero area */
        m_end = newEnd;
        m_zeroAreaEnd = newEnd;
    }
    else if (newEnd > m_start)
    {
        /* remove end of buffer, zero area, part of start of buffer */
        m_end = newEnd;
        m_zeroAreaEnd = newEnd;
        m_zeroAreaStart = newEnd;
    }
    else
    {
        /* remove all buffer */
        m_end = m_start;
        m_zeroAreaEnd = m_start;
        m_zeroAreaStart = m_start;
    }
    m_maxZeroAreaStart = std::max(m_maxZeroAreaStart, m_zeroAreaStart);
    LOG_INTERNAL_STATE("rem end=" << end << ", ");
    NS_ASSERT(CheckInternalState());
}
 
Buffer
Buffer::CreateFragment(uint32_t start, uint32_t length) const
{
    NS_LOG_FUNCTION(this << start << length);
    NS_ASSERT(CheckInternalState());
    Buffer tmp = *this;
    tmp.RemoveAtStart(start);
    tmp.RemoveAtEnd(GetSize() - (start + length));
    NS_ASSERT(CheckInternalState());
    return tmp;
}
 
Buffer
Buffer::CreateFullCopy() const
{
    NS_LOG_FUNCTION(this);
    NS_ASSERT(CheckInternalState());
    if (m_zeroAreaEnd - m_zeroAreaStart != 0)
    {
        Buffer tmp;
        tmp.AddAtStart(m_zeroAreaEnd - m_zeroAreaStart);
        tmp.Begin().WriteU8(0, m_zeroAreaEnd - m_zeroAreaStart);
        uint32_t dataStart = m_zeroAreaStart - m_start;
        tmp.AddAtStart(dataStart);
        tmp.Begin().Write(m_data->m_data + m_start, dataStart);
        uint32_t dataEnd = m_end - m_zeroAreaEnd;
        tmp.AddAtEnd(dataEnd);
        Buffer::Iterator i = tmp.End();
        i.Prev(dataEnd);
        i.Write(m_data->m_data + m_zeroAreaStart, dataEnd);
        NS_ASSERT(tmp.CheckInternalState());
        return tmp;
    }
    NS_ASSERT(CheckInternalState());
    return *this;
}
 
uint32_t
Buffer::GetSerializedSize() const
{
    NS_LOG_FUNCTION(this);
    uint32_t dataStart = (m_zeroAreaStart - m_start + 3) & (~0x3);
    uint32_t dataEnd = (m_end - m_zeroAreaEnd + 3) & (~0x3);
 
    // total size 4-bytes for dataStart length
    // + X number of bytes for dataStart
    // + 4-bytes for dataEnd length
    // + X number of bytes for dataEnd
    uint32_t sz = sizeof(uint32_t) + sizeof(uint32_t) + dataStart + sizeof(uint32_t) + dataEnd;
 
    return sz;
}
 
uint32_t
Buffer::Serialize(uint8_t* buffer, uint32_t maxSize) const
{
    NS_LOG_FUNCTION(this << &buffer << maxSize);
    uint32_t* p = reinterpret_cast<uint32_t*>(buffer);
    uint32_t size = 0;
 
    // Add the zero data length
    if (size + 4 <= maxSize)
    {
        size += 4;
        *p++ = m_zeroAreaEnd - m_zeroAreaStart;
    }
    else
    {
        return 0;
    }
 
    // Add the length of actual start data
    uint32_t dataStartLength = m_zeroAreaStart - m_start;
    if (size + 4 <= maxSize)
    {
        size += 4;
        *p++ = dataStartLength;
    }
    else
    {
        return 0;
    }
 
    // Add the actual data
    if (size + ((dataStartLength + 3) & (~3)) <= maxSize)
    {
        size += (dataStartLength + 3) & (~3);
        memcpy(p, m_data->m_data + m_start, dataStartLength);
        p += (((dataStartLength + 3) & (~3)) / 4); // Advance p, insuring 4 byte boundary
    }
    else
    {
        return 0;
    }
 
    // Add the length of the actual end data
    uint32_t dataEndLength = m_end - m_zeroAreaEnd;
    if (size + 4 <= maxSize)
    {
        size += 4;
        *p++ = dataEndLength;
    }
    else
    {
        return 0;
    }
 
    // Add the actual data
    if (size + ((dataEndLength + 3) & (~3)) <= maxSize)
    {
        // The following line is unnecessary.
        // size += (dataEndLength + 3) & (~3);
        memcpy(p, m_data->m_data + m_zeroAreaStart, dataEndLength);
        // The following line is unnecessary.
        // p += (((dataEndLength + 3) & (~3))/4); // Advance p, insuring 4 byte boundary
    }
    else
    {
        return 0;
    }
 
    // Serialized everything successfully
    return 1;
}
 
uint32_t
Buffer::Deserialize(const uint8_t* buffer, uint32_t size)
{
    NS_LOG_FUNCTION(this << &buffer << size);
    const uint32_t* p = reinterpret_cast<const uint32_t*>(buffer);
    uint32_t sizeCheck = size - 4;
 
    NS_ASSERT(sizeCheck >= 4);
    uint32_t zeroDataLength = *p++;
    sizeCheck -= 4;
 
    // Create zero bytes
    Initialize(zeroDataLength);
 
    // Add start data
    NS_ASSERT(sizeCheck >= 4);
    uint32_t dataStartLength = *p++;
    sizeCheck -= 4;
    AddAtStart(dataStartLength);
 
    NS_ASSERT(sizeCheck >= dataStartLength);
    Begin().Write(reinterpret_cast<uint8_t*>(const_cast<uint32_t*>(p)), dataStartLength);
    p += (((dataStartLength + 3) & (~3)) / 4); // Advance p, insuring 4 byte boundary
    sizeCheck -= ((dataStartLength + 3) & (~3));
 
    // Add end data
    NS_ASSERT(sizeCheck >= 4);
    uint32_t dataEndLength = *p++;
    sizeCheck -= 4;
    AddAtEnd(dataEndLength);
 
    NS_ASSERT(sizeCheck >= dataEndLength);
    Buffer::Iterator tmp = End();
    tmp.Prev(dataEndLength);
    tmp.Write(reinterpret_cast<uint8_t*>(const_cast<uint32_t*>(p)), dataEndLength);
    // The following line is unnecessary.
    // p += (((dataEndLength+3)&(~3))/4); // Advance p, insuring 4 byte boundary
    sizeCheck -= ((dataEndLength + 3) & (~3));
 
    NS_ASSERT(sizeCheck == 0);
    // return zero if buffer did not
    // contain a complete message
    return (sizeCheck != 0) ? 0 : 1;
}
 
void
Buffer::TransformIntoRealBuffer() const
{
    NS_LOG_FUNCTION(this);
    NS_ASSERT(CheckInternalState());
    Buffer tmp = CreateFullCopy();
    *const_cast<Buffer*>(this) = tmp;
    NS_ASSERT(CheckInternalState());
}
 
const uint8_t*
Buffer::PeekData() const
{
    NS_LOG_FUNCTION(this);
    NS_ASSERT(CheckInternalState());
    TransformIntoRealBuffer();
    NS_ASSERT(CheckInternalState());
    return m_data->m_data + m_start;
}
 
void
Buffer::CopyData(std::ostream* os, uint32_t size) const
{
    NS_LOG_FUNCTION(this << &os << size);
    if (size > 0)
    {
        uint32_t tmpsize = std::min(m_zeroAreaStart - m_start, size);
        os->write((const char*)(m_data->m_data + m_start), tmpsize);
        if (size > tmpsize)
        {
            size -= m_zeroAreaStart - m_start;
            tmpsize = std::min(m_zeroAreaEnd - m_zeroAreaStart, size);
            uint32_t left = tmpsize;
            while (left > 0)
            {
                uint32_t toWrite = std::min(left, g_zeroes.size);
                os->write(g_zeroes.buffer, toWrite);
                left -= toWrite;
            }
            if (size > tmpsize)
            {
                size -= tmpsize;
                tmpsize = std::min(m_end - m_zeroAreaEnd, size);
                os->write((const char*)(m_data->m_data + m_zeroAreaStart), tmpsize);
            }
        }
    }
}
 
uint32_t
Buffer::CopyData(uint8_t* buffer, uint32_t size) const
{
    NS_LOG_FUNCTION(this << &buffer << size);
    uint32_t originalSize = size;
    if (size > 0)
    {
        uint32_t tmpsize = std::min(m_zeroAreaStart - m_start, size);
        memcpy(buffer, (const char*)(m_data->m_data + m_start), tmpsize);
        buffer += tmpsize;
        size -= tmpsize;
        if (size > 0)
        {
            tmpsize = std::min(m_zeroAreaEnd - m_zeroAreaStart, size);
            uint32_t left = tmpsize;
            while (left > 0)
            {
                uint32_t toWrite = std::min(left, g_zeroes.size);
                memcpy(buffer, g_zeroes.buffer, toWrite);
                left -= toWrite;
                buffer += toWrite;
            }
            size -= tmpsize;
            if (size > 0)
            {
                tmpsize = std::min(m_end - m_zeroAreaEnd, size);
                memcpy(buffer, (const char*)(m_data->m_data + m_zeroAreaStart), tmpsize);
                size -= tmpsize;
            }
        }
    }
    return originalSize - size;
}
 
/******************************************************
 *            The buffer iterator below.
 ******************************************************/
 
uint32_t
Buffer::Iterator::GetDistanceFrom(const Iterator& o) const
{
    NS_LOG_FUNCTION(this << &o);
    NS_ASSERT(m_data == o.m_data);
    int32_t diff = m_current - o.m_current;
    if (diff < 0)
    {
        return -diff;
    }
    else
    {
        return diff;
    }
}
 
bool
Buffer::Iterator::IsEnd() const
{
    NS_LOG_FUNCTION(this);
    return m_current == m_dataEnd;
}
 
bool
Buffer::Iterator::IsStart() const
{
    NS_LOG_FUNCTION(this);
    return m_current == m_dataStart;
}
 
bool
Buffer::Iterator::CheckNoZero(uint32_t start, uint32_t end) const
{
    NS_LOG_FUNCTION(this << &start << &end);
    return !(start < m_dataStart || end > m_dataEnd ||
             (end > m_zeroStart && start < m_zeroEnd && m_zeroEnd != m_zeroStart && start != end));
}
 
bool
Buffer::Iterator::Check(uint32_t i) const
{
    NS_LOG_FUNCTION(this << &i);
    return i >= m_dataStart && !(i >= m_zeroStart && i < m_zeroEnd) && i <= m_dataEnd;
}
 
void
Buffer::Iterator::Write(Iterator start, Iterator end)
{
    NS_LOG_FUNCTION(this << &start << &end);
    NS_ASSERT(start.m_data == end.m_data);
    NS_ASSERT(start.m_current <= end.m_current);
    NS_ASSERT(start.m_zeroStart == end.m_zeroStart);
    NS_ASSERT(start.m_zeroEnd == end.m_zeroEnd);
    NS_ASSERT(m_data != start.m_data);
    uint32_t size = end.m_current - start.m_current;
    NS_ASSERT_MSG(CheckNoZero(m_current, m_current + size), GetWriteErrorMessage());
    if (start.m_current <= start.m_zeroStart)
    {
        uint32_t toCopy = std::min(size, start.m_zeroStart - start.m_current);
        memcpy(&m_data[m_current], &start.m_data[start.m_current], toCopy);
        start.m_current += toCopy;
        m_current += toCopy;
        size -= toCopy;
    }
    if (start.m_current <= start.m_zeroEnd)
    {
        uint32_t toCopy = std::min(size, start.m_zeroEnd - start.m_current);
        memset(&m_data[m_current], 0, toCopy);
        start.m_current += toCopy;
        m_current += toCopy;
        size -= toCopy;
    }
    uint32_t toCopy = std::min(size, start.m_dataEnd - start.m_current);
    uint8_t* from = &start.m_data[start.m_current - (start.m_zeroEnd - start.m_zeroStart)];
    uint8_t* to = &m_data[m_current];
    memcpy(to, from, toCopy);
    m_current += toCopy;
}
 
void
Buffer::Iterator::WriteU16(uint16_t data)
{
    NS_LOG_FUNCTION(this << data);
    WriteU8(data & 0xff);
    data >>= 8;
    WriteU8(data & 0xff);
}
 
void
Buffer::Iterator::WriteU32(uint32_t data)
{
    NS_LOG_FUNCTION(this << data);
    WriteU8(data & 0xff);
    data >>= 8;
    WriteU8(data & 0xff);
    data >>= 8;
    WriteU8(data & 0xff);
    data >>= 8;
    WriteU8(data & 0xff);
}
 
void
Buffer::Iterator::WriteU64(uint64_t data)
{
    NS_LOG_FUNCTION(this << data);
    WriteU8(data & 0xff);
    data >>= 8;
    WriteU8(data & 0xff);
    data >>= 8;
    WriteU8(data & 0xff);
    data >>= 8;
    WriteU8(data & 0xff);
    data >>= 8;
    WriteU8(data & 0xff);
    data >>= 8;
    WriteU8(data & 0xff);
    data >>= 8;
    WriteU8(data & 0xff);
    data >>= 8;
    WriteU8(data & 0xff);
}
 
void
Buffer::Iterator::WriteHtolsbU16(uint16_t data)
{
    NS_LOG_FUNCTION(this << data);
    WriteU8((data >> 0) & 0xff);
    WriteU8((data >> 8) & 0xff);
}
 
void
Buffer::Iterator::WriteHtolsbU32(uint32_t data)
{
    NS_LOG_FUNCTION(this << data);
    WriteU8((data >> 0) & 0xff);
    WriteU8((data >> 8) & 0xff);
    WriteU8((data >> 16) & 0xff);
    WriteU8((data >> 24) & 0xff);
}
 
void
Buffer::Iterator::WriteHtolsbU64(uint64_t data)
{
    NS_LOG_FUNCTION(this << data);
    WriteU8((data >> 0) & 0xff);
    WriteU8((data >> 8) & 0xff);
    WriteU8((data >> 16) & 0xff);
    WriteU8((data >> 24) & 0xff);
    WriteU8((data >> 32) & 0xff);
    WriteU8((data >> 40) & 0xff);
    WriteU8((data >> 48) & 0xff);
    WriteU8((data >> 56) & 0xff);
}
 
void
Buffer::Iterator::WriteHtonU64(uint64_t data)
{
    NS_LOG_FUNCTION(this << data);
    WriteU8((data >> 56) & 0xff);
    WriteU8((data >> 48) & 0xff);
    WriteU8((data >> 40) & 0xff);
    WriteU8((data >> 32) & 0xff);
    WriteU8((data >> 24) & 0xff);
    WriteU8((data >> 16) & 0xff);
    WriteU8((data >> 8) & 0xff);
    WriteU8((data >> 0) & 0xff);
}
 
void
Buffer::Iterator::Write(const uint8_t* buffer, uint32_t size)
{
    NS_LOG_FUNCTION(this << &buffer << size);
    NS_ASSERT_MSG(CheckNoZero(m_current, size), GetWriteErrorMessage());
    uint8_t* to;
    if (m_current <= m_zeroStart)
    {
        to = &m_data[m_current];
    }
    else
    {
        to = &m_data[m_current - (m_zeroEnd - m_zeroStart)];
    }
    memcpy(to, buffer, size);
    m_current += size;
}
 
uint32_t
Buffer::Iterator::ReadU32()
{
    NS_LOG_FUNCTION(this);
    uint8_t byte0 = ReadU8();
    uint8_t byte1 = ReadU8();
    uint8_t byte2 = ReadU8();
    uint8_t byte3 = ReadU8();
    uint32_t data = byte3;
    data <<= 8;
    data |= byte2;
    data <<= 8;
    data |= byte1;
    data <<= 8;
    data |= byte0;
    return data;
}
 
uint64_t
Buffer::Iterator::ReadU64()
{
    NS_LOG_FUNCTION(this);
    uint8_t byte0 = ReadU8();
    uint8_t byte1 = ReadU8();
    uint8_t byte2 = ReadU8();
    uint8_t byte3 = ReadU8();
    uint8_t byte4 = ReadU8();
    uint8_t byte5 = ReadU8();
    uint8_t byte6 = ReadU8();
    uint8_t byte7 = ReadU8();
    uint64_t data = byte7;
    data <<= 8;
    data |= byte6;
    data <<= 8;
    data |= byte5;
    data <<= 8;
    data |= byte4;
    data <<= 8;
    data |= byte3;
    data <<= 8;
    data |= byte2;
    data <<= 8;
    data |= byte1;
    data <<= 8;
    data |= byte0;
 
    return data;
}
 
uint16_t
Buffer::Iterator::SlowReadNtohU16()
{
    NS_LOG_FUNCTION(this);
    uint16_t retval = 0;
    retval |= ReadU8();
    retval <<= 8;
    retval |= ReadU8();
    return retval;
}
 
uint32_t
Buffer::Iterator::SlowReadNtohU32()
{
    NS_LOG_FUNCTION(this);
    uint32_t retval = 0;
    retval |= ReadU8();
    retval <<= 8;
    retval |= ReadU8();
    retval <<= 8;
    retval |= ReadU8();
    retval <<= 8;
    retval |= ReadU8();
    return retval;
}
 
uint64_t
Buffer::Iterator::ReadNtohU64()
{
    NS_LOG_FUNCTION(this);
    uint64_t retval = 0;
    retval |= ReadU8();
    retval <<= 8;
    retval |= ReadU8();
    retval <<= 8;
    retval |= ReadU8();
    retval <<= 8;
    retval |= ReadU8();
    retval <<= 8;
    retval |= ReadU8();
    retval <<= 8;
    retval |= ReadU8();
    retval <<= 8;
    retval |= ReadU8();
    retval <<= 8;
    retval |= ReadU8();
    return retval;
}
 
uint16_t
Buffer::Iterator::ReadLsbtohU16()
{
    NS_LOG_FUNCTION(this);
    uint8_t byte0 = ReadU8();
    uint8_t byte1 = ReadU8();
    uint16_t data = byte1;
    data <<= 8;
    data |= byte0;
    return data;
}
 
uint32_t
Buffer::Iterator::ReadLsbtohU32()
{
    NS_LOG_FUNCTION(this);
    uint8_t byte0 = ReadU8();
    uint8_t byte1 = ReadU8();
    uint8_t byte2 = ReadU8();
    uint8_t byte3 = ReadU8();
    uint32_t data = byte3;
    data <<= 8;
    data |= byte2;
    data <<= 8;
    data |= byte1;
    data <<= 8;
    data |= byte0;
    return data;
}
 
uint64_t
Buffer::Iterator::ReadLsbtohU64()
{
    NS_LOG_FUNCTION(this);
    uint8_t byte0 = ReadU8();
    uint8_t byte1 = ReadU8();
    uint8_t byte2 = ReadU8();
    uint8_t byte3 = ReadU8();
    uint8_t byte4 = ReadU8();
    uint8_t byte5 = ReadU8();
    uint8_t byte6 = ReadU8();
    uint8_t byte7 = ReadU8();
    uint64_t data = byte7;
    data <<= 8;
    data |= byte6;
    data <<= 8;
    data |= byte5;
    data <<= 8;
    data |= byte4;
    data <<= 8;
    data |= byte3;
    data <<= 8;
    data |= byte2;
    data <<= 8;
    data |= byte1;
    data <<= 8;
    data |= byte0;
 
    return data;
}
 
void
Buffer::Iterator::Read(uint8_t* buffer, uint32_t size)
{
    NS_LOG_FUNCTION(this << &buffer << size);
    for (uint32_t i = 0; i < size; i++)
    {
        buffer[i] = ReadU8();
    }
}
 
uint16_t
Buffer::Iterator::CalculateIpChecksum(uint16_t size)
{
    NS_LOG_FUNCTION(this << size);
    return CalculateIpChecksum(size, 0);
}
 
uint16_t
Buffer::Iterator::CalculateIpChecksum(uint16_t size, uint32_t initialChecksum)
{
    NS_LOG_FUNCTION(this << size << initialChecksum);
    /* see RFC 1071 to understand this code. */
    uint32_t sum = initialChecksum;
 
    for (int j = 0; j < size / 2; j++)
    {
        sum += ReadU16();
    }
 
    if (size & 1)
    {
        sum += ReadU8();
    }
 
    while (sum >> 16)
    {
        sum = (sum & 0xffff) + (sum >> 16);
    }
    return ~sum;
}
 
uint32_t
Buffer::Iterator::GetSize() const
{
    NS_LOG_FUNCTION(this);
    return m_dataEnd - m_dataStart;
}
 
uint32_t
Buffer::Iterator::GetRemainingSize() const
{
    NS_LOG_FUNCTION(this);
    return m_dataEnd - m_current;
}
 
std::string
Buffer::Iterator::GetReadErrorMessage() const
{
    NS_LOG_FUNCTION(this);
    std::string str = "You have attempted to read beyond the bounds of the "
                      "available buffer space. This usually indicates that a "
                      "Header::Deserialize or Trailer::Deserialize method "
                      "is trying to read data which was not written by "
                      "a Header::Serialize or Trailer::Serialize method. "
                      "In short: check the code of your Serialize and Deserialize "
                      "methods.";
    return str;
}
 
std::string
Buffer::Iterator::GetWriteErrorMessage() const
{
    NS_LOG_FUNCTION(this);
    std::string str;
    if (m_current < m_dataStart)
    {
        str = "You have attempted to write before the start of the available "
              "buffer space. This usually indicates that Trailer::GetSerializedSize "
              "returned a size which is too small compared to what Trailer::Serialize "
              "is actually using.";
    }
    else if (m_current >= m_dataEnd)
    {
        str = "You have attempted to write after the end of the available "
              "buffer space. This usually indicates that Header::GetSerializedSize "
              "returned a size which is too small compared to what Header::Serialize "
              "is actually using.";
    }
    else
    {
        NS_ASSERT(m_current >= m_zeroStart && m_current < m_zeroEnd);
        str = "You have attempted to write inside the payload area of the "
              "buffer. This usually indicates that your Serialize method uses more "
              "buffer space than what your GetSerialized method returned.";
    }
    return str;
}
 
} // namespace ns3