GetFreeSpace
if (GetFreeSpace() + sizeof(block_head) > GetBlockSize()) {
GetFreeSpace(), GetBlockSize() - sizeof(block_head)));
if (size + GetFreeSpace() > GetBlockSize()) {
GetFreeSpace()));
if (size + GetFreeSpace() > GetBlockSize()) {
GetNumber(), size, GetBlockSize(), GetFreeSpace()));
uint16 GetFreeSpace() const;