c# - txt - ¿Cómo puedo hacer que el escaneo inverso de un archivo binario sea más rápido?

modificar una linea de un archivo en c#

Question

Tengo una especificación de archivo binario que describe una estructura de datos en paquetes. Cada paquete de datos tiene un patrón de sincronización de dos bytes, por lo que es posible buscar el comienzo de un paquete, utilizando una combinación de BinaryReader y FileStream :

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.
}

Este proceso funciona perfectamente bien en la dirección hacia adelante, pero un escaneo de código similar en la dirección inversa es al menos dos órdenes de magnitud más lento:

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.
}

He intentado algunas soluciones alternativas, como retroceder por una cantidad arbitraria (actualmente 1 megabyte) y escanear hacia adelante, pero está claro que lo que realmente necesito es un BinaryReader o FileStream que se modifique para tener características de rendimiento adecuadas al leer en ambos. Direcciones hacia adelante y hacia atrás.

Ya tengo un FastFileStream que mejora el rendimiento de la lectura hacia adelante creando una subclase de un FileStream normal y almacenando en caché las propiedades de Position y Length (también proporciona las propiedades BeginningOfFile y EndOfFile ). Eso es lo que impulsa la variable reader en el código anterior.

¿Podría hacer algo similar para mejorar el rendimiento de lectura inversa, quizás incorporando un MemoryStream como un búfer?

Answer 1

EDIT: Está bien, tengo un código. Bueno, bastante código. Le permite escanear hacia delante y hacia atrás para los encabezados de paquetes.

No garantizo que no tenga errores, y definitivamente desea ajustar el tamaño del búfer para ver cómo funciona ... pero dado el mismo archivo que me envió, al menos muestra las mismas ubicaciones de encabezado de paquete cuando se escanea hacia adelante y hacia atrás :)

Antes del código, aún sugeriría que si es posible , escanear el archivo una vez y guardar un índice de información de paquetes para su uso posterior probablemente sería un mejor enfoque.

De todos modos, aquí está el código (completo sin otras pruebas que no sea el programa de ejemplo):

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 (para pruebas):

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

LB mencionado en un comentario para usar un archivo de memoria asignada, puede que se quede impresionado con el rendimiento.

Por favor, intente algo como esto:

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);
        }
    }
}