//LRU算法返回该表所包含的所有文件中最老的块的指针（最近最少使用的块）
BlockInfo* FileHandle::LRUAlgorithm()
{
FileInfo* fp = first_file_;
//最老的块的前一块
BlockInfo* oldestbefore = NULL;
//最老的块
BlockInfo* oldest = fp->GetFirstBlock();
//找到最老的块
while (fp != NULL)
{
BlockInfo* bpbefore = NULL;
BlockInfo* bp = fp->GetFirstBlock();
while (bp != NULL)
{
if (bp->get_age() > oldest->get_age())
{
oldestbefore = bpbefore;
oldest = bp;
}
bpbefore = bp;
bp = bp->GetNext();
}
fp = fp->GetNext();
}
//如果最老的块被修改过，则把它的内容写回文件
if (oldest->get_dirty())
oldest->WriteInfo(path_);
//如果最老的块的前一位置是空块，则说明第一块就是最老的块。这时因为要移除oldest，所有该文件第一块就变成了oldest->next
if (oldestbefore == NULL) oldest->GetFile()->SetFirstBlock(oldest->GetNext());
else oldestbefore->SetNext(oldest->GetNext());
//将最老的块年龄重置为0
oldest->ResetAge();
//oldest的next置为null
oldest->SetNext(NULL);
//返回最老的块的指针
return oldest;
}
