//DoubleBuffering.cpp Source File

#include "stdafx.h"
#include "DoubleBuffering.h"
#include <cassert>
#include <exception>

using namespace std;

CDoubleBuffering::CDoubleBuffering(ifstream& in, char* pcBuff, int iSize, int iDataLen) : m_rin(in),
	m_iDataLen(iDataLen), m_bEOF(false), m_iSize(iSize), m_iSize2(iSize >> 1), m_pcBuff(pcBuff)
{
	//m_iSize should be even
	if(m_iSize%2 != 0)
		throw runtime_error("CDoubleBuffering: m_iSize should be Even Number!");
	//Check file
	if(!in.is_open() || in.bad())
		throw runtime_error("CDoubleBuffering: Referenced File not Opened or in Bad State!");
	//Check construction data
	if(m_iDataLen<1 || m_iSize2<m_iDataLen)
		throw runtime_error("CDoubleBuffering: Illegal Construction Data!");
	in.read(m_pcBuff, m_iSize2);
	m_iEnd = m_rin.gcount();
	m_iCurPos = 0;
	m_iBuf = 0;
}

int CDoubleBuffering::GetData(char* pszDataBuf, int iDataLen)
{
	if(-1 == iDataLen)
		iDataLen = m_iDataLen;
	if(iDataLen<1 || m_iSize2<iDataLen)
		throw runtime_error("CDoubleBuffering::GetData(): Illegal iDataLen!");
	if(true == m_bEOF)
		return 0;
	//Estimate the next position
	int iCurPos = m_iCurPos + iDataLen;
	if(0 == m_iBuf) //First Buffer
	{
		if(iCurPos >= m_iEnd)
		{
			//Read the next buffer
			if(m_rin.eof())
			{
				m_bEOF = true;
				//Take everything remained
				int iRead = m_iEnd-m_iCurPos;
				memcpy(pszDataBuf, m_pcBuff+m_iCurPos, iRead);
				return iRead;
			}
			else
			{
				m_rin.read(m_pcBuff+m_iEnd, m_iSize2);
				m_iEnd += m_rin.gcount();
				if(iCurPos > m_iEnd) //Still greater, then EOF attained
				{
					assert(m_rin.eof());
					m_bEOF = true;
					//Take everything remained
					int iRead = m_iEnd-m_iCurPos;
					memcpy(pszDataBuf, m_pcBuff+m_iCurPos, iRead);
					return iRead;
				}
				else
				{
					memcpy(pszDataBuf, m_pcBuff+m_iCurPos, iDataLen);
					m_iCurPos = iCurPos;
					assert(m_iCurPos >= m_iSize2);
					m_iBuf = 1;
					return iDataLen;
				}
			}
		}
		else
		{
			memcpy(pszDataBuf, m_pcBuff+m_iCurPos, iDataLen);
			m_iCurPos = iCurPos;
			return iDataLen;
		}
	}
	else //1 == m_iBuf, Second Buffer
	{
		if(iCurPos >= m_iEnd)
		{
			//Read the next buffer
			if(m_rin.eof())
			{
				m_bEOF = true;
				//Take everything remained
				int iRead = m_iEnd-m_iCurPos;
				memcpy(pszDataBuf, m_pcBuff+m_iCurPos, iRead);
				return iRead;
			}
			else
			{
				m_rin.read(m_pcBuff, m_iSize2);
				m_iEnd = m_rin.gcount();
				iCurPos %= m_iSize;
				if(iCurPos > m_iEnd) //Still greater, then EOF attained
				{
					assert(m_rin.eof());
					m_bEOF = true;
					//Take everything remained
					int iRead = m_iSize-m_iCurPos;
					memcpy(pszDataBuf, m_pcBuff+m_iCurPos, iRead);
					memcpy(pszDataBuf+iRead, m_pcBuff, m_iEnd);
					return iRead + m_iEnd;
				}
				else
				{
					int iRead = m_iSize-m_iCurPos;
					memcpy(pszDataBuf, m_pcBuff+m_iCurPos, iRead);
					memcpy(pszDataBuf+iRead, m_pcBuff, iDataLen-iRead);
					m_iCurPos = iCurPos;
					assert(m_iCurPos < m_iSize2);
					m_iBuf = 0;
					return iDataLen;
				}
			}
		}
		else
		{
			memcpy(pszDataBuf, m_pcBuff+m_iCurPos, iDataLen);
			m_iCurPos = iCurPos;
			return iDataLen;
		}
	}
}