/// <summary>
/// 读取块存储头部
/// </summary>
private void ReadFileHeader()
{
byte[] header = new byte[this.FileHeaderSize];
this.Stream.Seek(this.SeekStart, SeekOrigin.Begin);
this.Stream.Read(header, 0, this.FileHeaderSize);
int index = 0;
// 魔数前缀
foreach (byte b in FileHeaderPrefix)
{
if (header[index] != b)
{
throw new LinkedFileException("invalid header prefix");
}
index++;
}
// 块的大小
this.BlockSize = StoredHelper.RetrieveInt(header, index);
index += StoredConstants.IntegerLength;
// 空闲列表头序号
this.FreeBlockHead = StoredHelper.RetrieveLong(header, index);
index += StoredConstants.LongLength;
this.IsHeaderDirty = false;
if (this.BlockSize < StoredConstants.MinBlockSize)
{
throw new LinkedFileException("linked file block size too small " + this.BlockSize);
}
}
/// <summary>
/// 读取B+树存储头
/// </summary>
private void ReadHeader()
{
// 魔数前缀 | 版本 | 节点容量 | 键大小 | 根节点块序号 | 空闲块头序号
// prefix | version | node size | key size | block number of root | block number of free list head
byte[] header = new byte[HeaderSize];
this.Stream.Seek(this.SeekStart, SeekOrigin.Begin);
this.Stream.Read(header, 0, HeaderSize);
// 验证头前缀魔数
int index = 0;
foreach (byte b in StoredConstants.TreeFileHeaderPrefix)
{
if (header[index] != b)
{
throw new BlockFileException("invalid header prefix");
}
index++;
}
// 版本
this.Version = header[index];
index += 1;
// 节点容量
this.NodeCapacity = StoredHelper.RetrieveInt(header, index);
index += StoredConstants.IntegerLength;
// 键大小
this.KeyLength = StoredHelper.RetrieveInt(header, index);
index += StoredConstants.IntegerLength;
// 根节点块序号
this.RootNodeBlockNumber = StoredHelper.RetrieveLong(header, index);
index += StoredConstants.LongLength;
// 空闲块头序号
this.FreeBlockHeadNumber = StoredHelper.RetrieveLong(header, index);
index += StoredConstants.LongLength;
if (this.NodeCapacity < 2)
{
throw new BPlusTreeException("node size must be larger than 2");
}
if (this.KeyLength < 5)
{
throw new BPlusTreeException("Key length must be larger than 5");
}
}
/// <summary>
/// 在指定的块序号之后，查找新的空闲块头序号
/// </summary>
/// <param name="blockNumber">指定的块序号</param>
/// <returns>新的空闲块头序号</returns>
private long ParseFreeBlock(long blockNumber)
{
int freeSize = 1 + StoredConstants.LongLength;
byte[] block = new byte[freeSize];
this.BlockFile.ReadBlock(blockNumber, block, 0, freeSize);
if (block[0] != (byte)BPlusTreeNodeIndicator.Free)
{
throw new BPlusTreeException("free block not marked free");
}
long newHead = StoredHelper.RetrieveLong(block, 1);
return(newHead);
}
/// <summary>
/// 读取块
/// </summary>
/// <param name="blockNumber">块序号</param>
/// <param name="blockFlag">块标记</param>
/// <param name="nextBlockNumber">链接的下一个块</param>
/// <returns>块数据</returns>
private byte[] ReadBlock(long blockNumber, out LinkedFileBlockFlag blockFlag, out long nextBlockNumber)
{
byte[] fullBuffer = new byte[LinkedBlockOverhead + this.BlockSize];
this.BlockFile.ReadBlock(blockNumber, fullBuffer, 0, fullBuffer.Length);
// 读取块标记
blockFlag = (LinkedFileBlockFlag)fullBuffer[0];
// 读取链接的下一个块的序号
nextBlockNumber = StoredHelper.RetrieveLong(fullBuffer, 1);
// 读取数据
byte[] buffer = new byte[this.BlockSize];
Array.Copy(fullBuffer, LinkedBlockOverhead, buffer, 0, this.BlockSize);
return(buffer);
}