Using FileStream.Seek
I am trying to work with FileStream.Seek to quickly jump to a line and read it.
However, I am not getting the right results. I have tried to look at this for a while and can't understand what I am doing wrong.
Environment:
OS: Windows 7
Framework: .NET 4.0
IDE: Visual C# Express 2010
Sample Data in file location: C:\Temp\Temp.txt
0001|100!2500 0002|100!2500 0003|100!2500 0004|100!2500 0005|100!2500 0006|100!2500 0007|100!2500 0008|100!2500 0009|100!2500 0010|100!2500
The code:
class PaddedFileSearch
{
private int LineLength { get; set; }
private string FileName { get; set; }
public PaddedFileSearch()
{
FileName = @"C:\Temp\Temp.txt"; // This is a padded file. All lines are of the same length.
FindLineLength();
Debug.Print("File Line length: {0}", LineLength);
// TODO: This purely for testing. Move this code out.
SeekMethod(new int[] { 5, 3, 4 });
/* Expected Results:
* Line No Position Line
* ------- -------- -----------------
* 3 30 0003|100!2500
* 4 15 0004|100!2500
* 5 15 0005|100!2500 -- This was updated after the initial request.
*/
/* THIS DOES NOT GIVE THE EXPECTED RESULTS */
SeekMethod(new int[] { 5, 3 });
/* Expected Results:
* Line No Position Line
* ------- -------- -----------------
* 3 30 0003|100!2500
* 5 30 0005|100!2500
*/
}
private void FindLineLength()
{
string line;
// Add check for FileExists
using (StreamReader reader = new StreamReader(FileName))
{
if ((line = reader.ReadLine()) != null)
{
LineLength = line.Length + 2;
// The 2 is for NewLine(\r\n)
}
}
}
public void SeekMethod(int[] lineNos)
{
long position = 0;
string line = null;
Array.Sort(lineNos);
Debug.Print("");
Debug.Print("Line No\t\tPosition\t\tLine");
Debug.Print("-------\t\t--------\t\t-----------------");
using (FileStream fs = new FileStream(FileName, FileMode.Open, FileAccess.Read, FileShare.None))
{
using (StreamReader reader = new StreamReader(fs))
{
foreach (int lineNo in lineNos)
{
position = (lineNo - 1) * LineLength - position;
fs.Seek(position, SeekOrigin.Current);
if ((line = reader.ReadLine()) != null)
{
Debug.Print("{0}\t\t\t{1}\t\t\t\t{2}", lineNo, position, line);
}
}
}
}
}
}
The output I get:
File Line length: 15 Line No Position Line ------- -------- ----------------- 3 30 0003|100!2500 4 15 0004|100!2500 5 45 0005|100!2500 Line No Position Line ------- -------- ----------------- 3 30 0003|100!2500 5 30 0004|100!2500
My problem is with the following output:
Line No Position Line ------- -------- ----------------- 5 30 0004|100!2500
The output for Line should be: 0005|100!2500
I don't understand why this is happening.
Am I doing something wrong?
Is there a workaround?
Also are there faster ways to do this using something like seek?
(I am looking for code based options and NOT Oracle or SQL Server. For the sake of argument lets also say that the file size 1 GB.)
Any help is greatly appreciated.
Thanks.
UPDATE:
I found 4 great answers here. Thanks a lot.
Sample Timings:
Based on a few runs the following are the methods from best to good. Even the good is very close to best.
In a file that contains 10K lines, 2.28 MB. I searched for same 5000 random lines using all the options.
- Seek4: Time elapsed: 00:00:00.0398530 ms -- Ritch Melton
- Seek3: Time elapsed: 00:00:00.0446072 ms -- Valentin Kuzub
- Seek1: Time elapsed: 00:00:00.0538210 ms -- Jake
- Seek2: Time elapsed: 00:00:00.0889589 ms -- bitxwise
Shown below is the code. After saving the code you can simply call it by typing TestPaddedFileSeek.CallPaddedFileSeek();. You will also have to specify the namespace and the "using references".
`
/// <summary>
/// This class multiple options of reading a by line number in a padded file (all lines are the same length).
/// The idea is to quick jump to the file.
/// Details about the discussions is available at: http://stackoverflow.com/questions/5201414/having-a-problem-while-using-filestream-seek-in-c-solved
/// </summary>
class PaddedFileSeek
{
public FileInfo File {get; private set;}
public int LineLength { get; private set; }
#region Private methods
private static int FindLineLength(FileInfo fileInfo)
{
using (StreamReader reader = new StreamReader(fileInfo.FullName))
{
string line;
if ((line = reader.ReadLine()) != null)
{
int length = line.Length + 2; // The 2 is for NewLine(\r\n)
return length;
}
}
return 0;
}
private static void PrintHeader()
{
/*
Debug.Print("");
Debug.Print("Line No\t\tLine");
Debug.Print("-------\t\t--------------------------");
*/
}
private static void PrintLine(int lineNo, string line)
{
//Debug.Print("{0}\t\t\t{1}", lineNo, line);
}
private static void PrintElapsedTime(TimeSpan elapsed)
{
Debug.WriteLine("Time elapsed: {0} ms", elapsed);
}
#endregion
public PaddedFileSeek(FileInfo fileInfo)
{
// Possibly might have to check for FileExists
int length = FindLineLength(fileInfo);
//if (length == 0) throw new PaddedProgramException();
LineLength = length;
File = fileInfo;
}
public void CallAll(int[] lineNoArray, List<int> lineNoList)
{
Stopwatch sw = new Stopwatch();
#region Seek1
// Create new stopwatch
sw.Start();
Debug.Write("Seek1: ");
// Print Header
PrintHeader();
Seek1(lineNoArray);
// Stop timing
sw.Stop();
// Print Elapsed Time
PrintElapsedTime(sw.Elapsed);
sw.Reset();
#endregion
#region Seek2
// Create new stopwatch
sw.Start();
Debug.Write("Seek2: ");
// Print Header
PrintHeader();
Seek2(lineNoArray);
// Stop timing
sw.Stop();
// Print Elapsed Time
PrintElapsedTime(sw.Elapsed);
sw.Reset();
#endregion
#region Seek3
// Create new stopwatch
sw.Start();
Debug.Write("Seek3: ");
// Print Header
PrintHeader();
Seek3(lineNoArray);
// Stop timing
sw.Stop();
// Print Elapsed Time
PrintElapsedTime(sw.Elapsed);
sw.Reset();
#endregion
#region Seek4
// Create new stopwatch
sw.Start();
Debug.Write("Seek4: ");
// Print Header
PrintHeader();
Seek4(lineNoList);
// Stop timing
sw.Stop();
// Print Elapsed Time
PrintElapsedTime(sw.Elapsed);
sw.Reset();
#endregion
}
/// <summary>
/// Option by Jake
/// </summary>
/// <param name="lineNoArray"></param>
public void Seek1(int[] lineNoArray)
{
long position = 0;
string line = null;
Array.Sort(lineNoArray);
using (FileStream fs = new FileStream(File.FullName, FileMode.Open, FileAccess.Read, FileShare.None))
{
using (StreamReader reader = new StreamReader(fs))
{
foreach (int lineNo in lineNoArray)
{
position = (lineNo - 1) * LineLength;
fs.Seek(position, SeekOrigin.Begin);
if ((line = reader.ReadLine()) != null)
{
PrintLine(lineNo, line);
}
reader.DiscardBufferedData();
}
}
}
}
/// <summary>
/// option by bitxwise
/// </summary>
public void Seek2(int[] lineNoArray)
{
string line = null;
long step = 0;
Array.Sort(lineNoArray);
using (FileStream fs = new FileStream(File.FullName, FileMode.Open, FileAccess.Read, FileShare.None))
{
// using (StreamReader reader = new StreamReader(fs))
// If you put "using" here you will get WRONG results.
// I would like to understand why this is.
{
foreach (int lineNo in lineNoArray)
{
StreamReader reader = new StreamReader(fs);
step = (lineNo - 1) * LineLength - fs.Position;
fs.Position += step;
if ((line = reader.ReadLine()) != null)
{
PrintLine(lineNo, line);
}
}
}
}
}
/// <summary>
/// Option by Valentin Kuzub
/// </summary>
/// <param name="lineNoArray"></param>
#region Seek3
public void Seek3(int[] lineNoArray)
{
long position = 0; // totalPosition = 0;
string line = null;
int oldLineNo = 0;
Array.Sort(lineNoArray);
using (FileStream fs = new FileStream(File.FullName, FileMode.Open, FileAccess.Read, FileShare.None))
{
using (StreamReader reader = new StreamReader(fs))
{
foreach (int lineNo in lineNoArray)
{
position = (lineNo - oldLineNo - 1) * LineLength;
fs.Seek(position, SeekOrigin.Current);
line = ReadLine(fs, LineLength);
PrintLine(lineNo, line);
oldLineNo = lineNo;
}
}
}
}
#region Required Private methods
/// <summary>
/// Currently only used by Seek3
/// </summary>
/// <param name="stream"></param>
/// <param name="length"></param>
/// <returns></returns>
private static string ReadLine(FileStream stream, int length)
{
byte[] bytes = new byte[length];
stream.Read(bytes, 0, length);
return new string(Encoding.UTF8.GetChars(bytes));
}
#endregion
#endregion
/// <summary>
/// Option by Ritch Melton
/// </summary>
/// <param name="lineNoArray"></param>
#region Seek4
public void Seek4(List<int> lineNoList)
{
lineNoList.Sort();
using (var fs = new FileStream(File.FullName, FileMode.Open))
{
lineNoList.ForEach(ln => OutputData(fs, ln));
}
}
#region Required Private methods
private void OutputData(FileStream fs, int lineNumber)
{
var offset = (lineNumber - 1) * LineLength;
fs.Seek(offset, SeekOrigin.Begin);
var data = new byte[LineLength];
fs.Read(data, 0, LineLength);
var text = DecodeData(data);
PrintLine(lineNumber, text);
}
private static string DecodeData(byte[] data)
{
var encoding = new UTF8Encoding();
return encoding.GetString(data);
}
#endregion
#endregion
}
static class TestPaddedFileSeek
{
public static void CallPaddedFileSeek()
{
const int arrayLenght = 5000;
int[] lineNoArray = new int[arrayLenght];
List<int> lineNoList = new List<int>();
Random random = new Random();
int lineNo;
string fileName;
fileName = @"C:\Temp\Temp.txt";
PaddedFileSeek seeker = new PaddedFileSeek(new FileInfo(fileName));
for (int n = 0; n < 25; n++)
{
Debug.Print("Loop no: {0}", n + 1);
for (int i = 0; i < arrayLenght; i++)
{
lineNo = random.Next(1, arrayLenght);
lineNoArray[i] = lineNo;
lineNoList.Add(lineNo);
}
seeker.CallAll(lineNoArray, lineNoList);
lineNoList.Clear();
Debug.Print("");
}
}
}
`
Answer
Place this in the inner loop of SeekMethod(int[] lineNos):
position = (lineNo - 1) * LineLength;
fs.Seek(position, SeekOrigin.Begin);
reader.DiscardBufferedData();
The problem is that your position variable changes based on its previous value, and StreamReader maintains a buffer so you need to clear out buffered data when you alter the stream position.