c++ 求2两点延长线上的点

//坐标系为左上角为（0，0）右边为正方向，下方为正方向//qtWindow界面的坐标系

int nStartPointX = 10;//开始点的x坐标
int nStartPointY = 10;//开始点的Y坐标
int nEndPointX = 30;//终止点的x坐标
int nEndPointY = 40;//终止点的y坐标

int nExtendedLineLenth = 10;//延长线的长度

double dExtendedLinePointX,dExtendedLinePointY;//延长线的点的坐标

int nOldLineLenth = (nEndPointX - nStartPointX);
int nOldLineHeight = (nEndPointY - nStartPointY);

if(nOldLineLenth == 0 && nOldLineHeight == 0)
{//线段长度为0
return;
}
else if(nOldLineLenth == 0 && nOldLineHeight != 0)
{//垂直的线
dExtendedLinePointY = nOldLineHeight < 0 ? nEndPointY - nExtendedLineLenth : nEndPointY + nExtendedLineLenth;
dExtendedLinePointX = nStartPointX;
return;
}
else if(nOldLineLenth != 0 && nOldLineHeight == 0)
{//横向的线
dExtendedLinePointX = nOldLineLenth < 0 ? nEndPointX - nExtendedLineLenth : nEndPointX + nExtendedLineLenth;
dExtendedLinePointY = nStartPointY;
return;
}

double dAbsOldLineLenth = std::abs(nOldLineLenth);
double dAbsOldLineHeight = std::abs(nOldLineHeight);

double dAbsOldLineLenth2Pow = dAbsOldLineLenth * dAbsOldLineLenth;
double dAbsOldLineHeight2Pow = dAbsOldLineHeight * dAbsOldLineHeight;

double dNewLineLenth = std::sqrt(dAbsOldLineLenth2Pow + dAbsOldLineHeight2Pow) + nExtendedLineLenth;

double dExtendedLinePointLenth,dExtendedLinePointHeight;

dExtendedLinePointLenth = std::sqrt(dNewLineLenth * dNewLineLenth / ((dAbsOldLineLenth / dAbsOldLineHeight) * (dAbsOldLineLenth / dAbsOldLineHeight) + 1.0));
dExtendedLinePointHeight = (dAbsOldLineHeight / dAbsOldLineHeight) * dExtendedLinePointLenth;

dExtendedLinePointX = dExtendedLinePointLenth + nStartPointX;
dExtendedLinePointY = dExtendedLinePointHeight + nStartPointY;

if(nOldLineLenth < 0)
{
dExtendedLinePointX = dExtendedLinePointLenth - nStartPointX;
}

if(nOldLineHeight < 0)
{
dExtendedLinePointY = dExtendedLinePointLenth - nStartPointY;
}