How can I make reverse scanning of a binary file faster?

Question

I have a binary file specification that describes a packetized data structure. Each data packet has a two-byte sync pattern, so scanning for the beginning of a packet is possible, using a BinaryReader and FileStream combination:

while(!reader.EndOfFile) {     // Check for sync pattern.     if (reader.ReadUInt16() != 0xEB25)     {         // Move to next byte.         reader.BaseStream.Seek(-1, SeekOrigin.Current);         continue;     }      // If we got here, a sync pattern was found. } 

This process works perfectly fine in the forward direction, but similar code scanning in the reverse direction is at least two orders of magnitude slower:

while(!reader.BeginningOfFile) {     // Check for sync pattern.     if (reader.ReadUInt16() != 0xEB25)     {         // Move to previous byte.         reader.BaseStream.Seek(-3, SeekOrigin.Current);         continue;     }      // If we got here, a sync pattern was found. } 

I've tried a few workarounds, like moving back by an arbitrary amount (currently 1 megabyte) and scanning forward, but it's becoming clear that what I really need is a BinaryReader or FileStream that is modified to have adequate performance characteristics when reading in both forward and reverse directions.

I already have a FastFileStream which improves forward read performance by subclassing an ordinary FileStream and caching the Position and Length properties (it also provides the BeginningOfFile and EndOfFile properties). That's what drives the reader variable in the code above.

Is there something similar I could do to improve reverse reading performance, perhaps by incorporating a MemoryStream as a buffer?

Answer 1

EDIT: Okay, I've got some code. Well, quite a lot of code. It allows you to scan forwards and backwards for packet headers.

I make no guarantee that it has no bugs, and you definitely want to tweak the buffer size to see how it performs... but given the same file you sent me, it at least shows the same packet header locations when scanning forwards and backwards :)

Before the code, I would still suggest that if you possibly can, scanning through the file once and saving an index of packet information for later use would probably be a better approach.

Anyway, here's the code (complete with no tests other than the sample program):

PacketHeader.cs:

using System;  namespace Chapter10Reader {     public sealed class PacketHeader     {         private readonly long filePosition;         private readonly ushort channelId;         private readonly uint packetLength;         private readonly uint dataLength;         private readonly byte dataTypeVersion;         private readonly byte sequenceNumber;         private readonly byte packetFlags;         private readonly byte dataType;         private readonly ulong relativeTimeCounter;          public long FilePosition { get { return filePosition; } }         public ushort ChannelId { get { return channelId; } }         public uint PacketLength { get { return packetLength; } }         public uint DataLength { get { return dataLength; } }         public byte DataTypeVersion { get { return dataTypeVersion; } }         public byte SequenceNumber { get { return sequenceNumber; } }         public byte PacketFlags { get { return packetFlags; } }         public byte DataType { get { return dataType; } }         public ulong RelativeTimeCounter { get { return relativeTimeCounter; } }          public PacketHeader(ushort channelId, uint packetLength, uint dataLength, byte dataTypeVersion,             byte sequenceNumber, byte packetFlags, byte dataType, ulong relativeTimeCounter, long filePosition)         {             this.channelId = channelId;             this.packetLength = packetLength;             this.dataLength = dataLength;             this.dataTypeVersion = dataTypeVersion;             this.sequenceNumber = sequenceNumber;             this.packetFlags = packetFlags;             this.dataType = dataType;             this.relativeTimeCounter = relativeTimeCounter;             this.filePosition = filePosition;         }          internal static PacketHeader Parse(byte[] data, int index, long filePosition)         {             if (index + 24 > data.Length)             {                 throw new ArgumentException("Packet header must be 24 bytes long; not enough data");             }             ushort syncPattern = BitConverter.ToUInt16(data, index + 0);             if (syncPattern != 0xeb25)             {                 throw new ArgumentException("Packet header must start with the sync pattern");             }             ushort channelId = BitConverter.ToUInt16(data, index + 2);             uint packetLength = BitConverter.ToUInt32(data, index + 4);             uint dataLength = BitConverter.ToUInt32(data, index + 8);             byte dataTypeVersion = data[index + 12];             byte sequenceNumber = data[index + 13];             byte packetFlags = data[index + 14];             byte dataType = data[index + 15];             // TODO: Validate this...             ulong relativeTimeCounter =                 (ulong)BitConverter.ToUInt32(data, index + 16) +                 ((ulong)BitConverter.ToUInt16(data, index + 20)) << 32;             // Assume we've already validated the checksum...             return new PacketHeader(channelId, packetLength, dataLength, dataTypeVersion, sequenceNumber,                 packetFlags, dataType, relativeTimeCounter, filePosition);         }          /// <summary>         /// Checks a packet header's checksum to see whether this *looks* like a packet header.         /// </summary>         internal static bool CheckPacketHeaderChecksum(byte[] data, int index)         {             if (index + 24 > data.Length)             {                 throw new ArgumentException("Packet header must is 24 bytes long; not enough data");             }             ushort computed = 0;             for (int i = 0; i < 11; i++)             {                 computed += BitConverter.ToUInt16(data, index + i * 2);             }             return computed == BitConverter.ToUInt16(data, index + 22);         }     } } 

PacketScanner.cs:

using System; using System.Diagnostics; using System.IO;  namespace Chapter10Reader {     public sealed class PacketScanner : IDisposable     {         // 128K buffer... tweak this.         private const int BufferSize = 1024 * 128;          /// <summary>         /// Where in the file does the buffer start?         /// </summary>         private long bufferStart;          /// <summary>         /// Where in the file does the buffer end (exclusive)?         /// </summary>         private long bufferEnd;          /// <summary>         /// Where are we in the file, logically?         /// </summary>         private long logicalPosition;          // Probably cached by FileStream, but we use it a lot, so let's         // not risk it...         private readonly long fileLength;          private readonly FileStream stream;         private readonly byte[] buffer = new byte[BufferSize];                  private PacketScanner(FileStream stream)         {             this.stream = stream;             this.fileLength = stream.Length;         }          public void MoveToEnd()         {             logicalPosition = fileLength;             bufferStart = -1; // Invalidate buffer             bufferEnd = -1;         }          public void MoveToBeforeStart()         {             logicalPosition = -1;             bufferStart = -1;             bufferEnd = -1;         }          private byte this[long position]         {             get              {                 if (position < bufferStart || position >= bufferEnd)                 {                     FillBuffer(position);                 }                 return buffer[position - bufferStart];             }         }          /// <summary>         /// Fill the buffer to include the given position.         /// If the position is earlier than the buffer, assume we're reading backwards         /// and make position one before the end of the buffer.         /// If the position is later than the buffer, assume we're reading forwards         /// and make position the start of the buffer.         /// If the buffer is invalid, make position the start of the buffer.         /// </summary>         private void FillBuffer(long position)         {             long newStart;             if (position > bufferStart)             {                 newStart = position;             }             else             {                 // Keep position *and position + 1* to avoid swapping back and forth too much                 newStart = Math.Max(0, position - buffer.Length + 2);             }             // Make position the start of the buffer.             int bytesRead;             int index = 0;             stream.Position = newStart;             while ((bytesRead = stream.Read(buffer, index, buffer.Length - index)) > 0)             {                 index += bytesRead;             }             bufferStart = newStart;             bufferEnd = bufferStart + index;         }          /// <summary>         /// Make sure the buffer contains the given positions.         ///          /// </summary>         private void FillBuffer(long start, long end)         {             if (end - start > buffer.Length)             {                 throw new ArgumentException("Buffer not big enough!");             }             if (end > fileLength)             {                 throw new ArgumentException("Beyond end of file");             }             // Nothing to do.             if (start >= bufferStart && end < bufferEnd)             {                 return;             }             // TODO: Optimize this more to use whatever bits we've actually got.             // (We're optimized for "we've got the start, get the end" but not the other way round.)             if (start >= bufferStart)             {                 // We've got the start, but not the end. Just shift things enough and read the end...                 int shiftAmount = (int) (end - bufferEnd);                 Buffer.BlockCopy(buffer, shiftAmount, buffer, 0, (int) (bufferEnd - bufferStart - shiftAmount));                 stream.Position = bufferEnd;                 int bytesRead;                 int index = (int)(bufferEnd - bufferStart - shiftAmount);                 while ((bytesRead = stream.Read(buffer, index, buffer.Length - index)) > 0)                 {                     index += bytesRead;                 }                 bufferStart += shiftAmount;                 bufferEnd = bufferStart + index;                 return;             }              // Just fill the buffer starting from start...             bufferStart = -1;             bufferEnd = -1;             FillBuffer(start);         }          /// <summary>         /// Returns the header of the next packet, or null          /// if we've reached the end of the file.         /// </summary>         public PacketHeader NextHeader()         {             for (long tryPosition = logicalPosition + 1; tryPosition < fileLength - 23; tryPosition++)             {                 if (this[tryPosition] == 0x25 && this[tryPosition + 1] == 0xEB)                 {                     FillBuffer(tryPosition, tryPosition + 24);                     int bufferPosition = (int) (tryPosition - bufferStart);                     if (PacketHeader.CheckPacketHeaderChecksum(buffer, bufferPosition))                     {                         logicalPosition = tryPosition;                         return PacketHeader.Parse(buffer, bufferPosition, tryPosition);                     }                 }             }             logicalPosition = fileLength;             return null;         }          /// <summary>         /// Returns the header of the previous packet, or null          /// if we've reached the start of the file.         /// </summary>         public PacketHeader PreviousHeader()         {             for (long tryPosition = logicalPosition - 1; tryPosition >= 0; tryPosition--)             {                 if (this[tryPosition + 1] == 0xEB && this[tryPosition] == 0x25)                 {                     FillBuffer(tryPosition, tryPosition + 24);                     int bufferPosition = (int)(tryPosition - bufferStart);                     if (PacketHeader.CheckPacketHeaderChecksum(buffer, bufferPosition))                     {                         logicalPosition = tryPosition;                         return PacketHeader.Parse(buffer, bufferPosition, tryPosition);                     }                 }             }             logicalPosition = -1;             return null;         }          public static PacketScanner OpenFile(string filename)         {             return new PacketScanner(File.OpenRead(filename));         }          public void Dispose()         {             stream.Dispose();         }     } } 

Program.cs (for testing):

using System; using System.Collections.Generic; using System.Linq;  namespace Chapter10Reader {     class Program     {         static void Main(string[] args)         {             string filename = "test.ch10";              Console.WriteLine("Forwards:");             List<long> positionsForward = new List<long>();             using (PacketScanner scanner = PacketScanner.OpenFile(filename))             {                 scanner.MoveToBeforeStart();                 PacketHeader header;                 while ((header = scanner.NextHeader()) != null)                 {                     Console.WriteLine("Found header at {0}", header.FilePosition);                     positionsForward.Add(header.FilePosition);                 }             }             Console.WriteLine();             Console.WriteLine("Backwards:");             List<long> positionsBackward = new List<long>();             using (PacketScanner scanner = PacketScanner.OpenFile(filename))             {                 scanner.MoveToEnd();                 PacketHeader header;                 while ((header = scanner.PreviousHeader()) != null)                 {                     positionsBackward.Add(header.FilePosition);                 }             }             positionsBackward.Reverse();             foreach (var position in positionsBackward)             {                 Console.WriteLine("Found header at {0}", position);             }              Console.WriteLine("Same? {0}", positionsForward.SequenceEqual(positionsBackward));         }     } } 

Answer 2

L.B mentioned in a comment to use a Memory Mapped file, you may be impressed with the performance.

Please try something like this:

var memoryMapName = Path.GetFileName(fileToRead);  using (var mapStream = new FileStream(fileToRead, FileMode.Open)) {     using (var myMap = MemoryMappedFile.CreateFromFile(                             mapStream,                              memoryMapName, mapStream.Length,                             MemoryMappedFileAccess.Read, null,                              HandleInheritability.None, false))     {                             long leftToRead = mapStream.Length;         long mapSize = Math.Min(1024 * 1024, mapStream.Length);         long bytesRead = 0;         long mapOffset = Math.Max(mapStream.Length - mapSize, 0);          while (leftToRead > 1)         {             using (var FileMap = myMap.CreateViewAccessor(mapOffset,                                   mapSize, MemoryMappedFileAccess.Read))             {                 long readAt = mapSize - 2;                 while (readAt > -1)                 {                     var int16Read = FileMap.ReadUInt16(readAt);                     //0xEB25  <--check int16Read here                                                 bytesRead += 1;                     readAt -= 1;                 }             }              leftToRead = mapStream.Length- bytesRead;             mapOffset = Math.Max(mapOffset - mapSize, 0);             mapSize = Math.Min(mapSize, leftToRead);         }     } }