/// <summary>
/// 从缓冲区中某个起始位置读取一个记录;
/// </summary>
/// <param name="buffer"></param>
/// <param name="startIndex">可能的起始位置</param>
/// <returns></returns>
private static UsnRecordV2 ReadFromBuffer(byte[] buffer, ref int startIndex)
{
if (buffer == null)
{
throw new ArgumentNullException(nameof(buffer));
}
while (startIndex < buffer.Length && buffer[startIndex] == 0)
{
startIndex++;
}
//记录余下的长度;
var leftLength = buffer.Length - startIndex;
if (leftLength < StructWithoutFileName_Size)
{
return(null);
}
var usnRecord = new UsnRecordV2 {
RecordLength = buffer.ToUInt32LittleEndian(startIndex),
FileReferenceNumber = buffer.ToUInt64LittleEndian(FR_OFFSET + startIndex),
ParentFileReferenceNumber = buffer.ToUInt64LittleEndian(PFR_OFFSET + startIndex),
Usn = (long)buffer.ToUInt64LittleEndian(USN_OFFSET),
DateTime = System.DateTime.FromFileTime(buffer.ToInt64LittleEndian(DateTime_OFFSET + startIndex)),
Reason = buffer.ToUInt32LittleEndian(REASON_OFFSET + startIndex),
FileAttributes = buffer.ToUInt32LittleEndian(FA_OFFSET + startIndex),
FileNameLength = buffer.ToUInt16LittleEndian(FNL_OFFSET + startIndex),
FileNameOffset = buffer.ToUInt16LittleEndian(FN_OFFSET + startIndex)
};
leftLength = buffer.Length - startIndex - StructWithoutFileName_Size;
if (leftLength >= usnRecord.FileNameLength)
{
//读取名称;
try {
usnRecord.FileName = Encoding.Unicode.GetString(buffer, startIndex + StructWithoutFileName_Size, usnRecord.FileNameLength);
}
catch (Exception ex) {
LoggerService.WriteException(ex);
}
}
startIndex += (int)usnRecord.RecordLength;
return(usnRecord);
}
/// <summary>
/// 从流中读取所有可能的Usn序列,注意,此方法延迟返回;
/// </summary>
/// <param name="stream"></param>
/// <returns></returns>
public static IEnumerable <UsnRecordV2> ReadRecordsFromStream(Stream stream)
{
if (stream == null)
{
throw new ArgumentNullException(nameof(stream));
}
stream.Position = 0;
//To-Do,根据观察,usn存储在文件中的位置以簇大小(大多为4096,由于尚未遇见非4096的情况,所以未做其它考虑)取整
//即在上一个usn记录存储完后,所在簇的剩余空间
//不足以存入下下一个记录时，那么，下一条记录将以下一个簇的起始位置为起始被存储,本簇的余下部分将被零填充;
//利用如上特性,将缓冲区的大小设置为簇大小的倍数,可以将流(文件)切分为多个块进行内存内解析,以提高处理速度;
UsnRecordV2 record = null;
var traverseBuffer = new byte[TraverseBufferLength];
byte[] buffer = null;
while (stream.Position < stream.Length)
{
var bufferPosition = stream.Position;
var startIndex = 0;
if (stream.Length - stream.Position > TraverseBufferLength)
{
stream.Read(traverseBuffer, 0, TraverseBufferLength);
buffer = traverseBuffer;
}
else
{
buffer = new byte[stream.Length - stream.Position];
stream.Read(buffer, 0, buffer.Length);
}
while ((record = ReadFromBuffer(buffer, ref startIndex)) != null)
{
record.RecordPosition = bufferPosition + startIndex;
yield return(record);
}
}
}
/// <summary>
/// 从流的当前位置读取一个Usn记录;
/// </summary>
/// <param name="stream"></param>
/// <returns></returns>
public static UsnRecordV2 ReadFromStream(Stream stream)
{
if (stream == null)
{
throw new ArgumentNullException(nameof(stream));
}
if (stream.Position >= stream.Length)
{
return(null);
}
var buffer = new byte[ClusterSize];
stream.Position = stream.Position / UnitLength * UnitLength;
var readLength = 0;
var nonZeroIndex = -1;
while ((readLength = stream.Read(buffer, 0, ClusterSize)) != 0)
{
for (int i = 0; i < readLength; i++)
{
if (buffer[i] != 0)
{
nonZeroIndex = i;
break;
}
}
if (nonZeroIndex != -1)
{
break;
}
}
if (nonZeroIndex == -1)
{
return(null);
}
stream.Position -= (readLength - nonZeroIndex);
while (stream.ReadByte() == 0)
{
;
}
stream.Position--;
var startPosition = stream.Position;
var leftCount = stream.Length - stream.Position;
if (leftCount < StructWithoutFileName_Size)
{
return(null);
}
var usnRecord = new UsnRecordV2();
var bts = stream.ReadExact(StructWithoutFileName_Size);
usnRecord.RecordLength = bts.ToUInt32LittleEndian(0);
usnRecord.FileReferenceNumber = bts.ToUInt64LittleEndian(FR_OFFSET);
usnRecord.ParentFileReferenceNumber = bts.ToUInt64LittleEndian(PFR_OFFSET);
usnRecord.Usn = (long)bts.ToUInt64LittleEndian(USN_OFFSET);
usnRecord.DateTime = System.DateTime.FromFileTime(bts.ToInt64LittleEndian(DateTime_OFFSET));
usnRecord.Reason = bts.ToUInt32LittleEndian(REASON_OFFSET);
usnRecord.FileAttributes = bts.ToUInt32LittleEndian(FA_OFFSET);
usnRecord.FileNameLength = bts.ToUInt16LittleEndian(FNL_OFFSET);
usnRecord.FileNameOffset = bts.ToUInt16LittleEndian(FN_OFFSET);
usnRecord.RecordPosition = startPosition;
leftCount = stream.Length - stream.Position;
if (leftCount < usnRecord.FileNameLength)
{
return(usnRecord);
}
var nameBts = stream.ReadExact(usnRecord.FileNameLength);
usnRecord.FileName = Encoding.Unicode.GetString(nameBts);
stream.Position = startPosition + usnRecord.RecordLength;
return(usnRecord);
}