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Advanced File Processing Techniques: Using Chunks for Handling Large Files

Published June 10, 2024 by Martin T.

A recent blog post, A Beginner’s Guide to Handling Text Files in Xojo, covered the basics of text file handling in Xojo. This post delves into advanced techniques for reading and writing large files in chunks. This method is crucial for managing large datasets efficiently, minimizing memory usage, and maintaining application performance.

Why Read and Write in Chunks?

Loading or writing a large file in one go can overwhelm your application’s memory and degrade performance. Processing files in smaller, manageable chunks allows for better memory management and more responsive applications.

Reading Files in Chunks

To read a large file in chunks, you can repeatedly read smaller portions of the file until you reach the end. Here’s an example:

Var file As FolderItem = FolderItem.ShowOpenFileDialog("text/plain")
If file <> Nil Then
  Const kChunkSize As Integer = 1024  ' 1 KB chunks

  Var inputStream As TextInputStream = TextInputStream.Open(file)
  Var buffer As String

  While Not inputStream.EndOfFile
    buffer = inputStream.Read(kChunkSize)
    // Process the buffer (for demonstration, we'll just print it)
    System.DebugLog(buffer)
 Wend

  inputStream.Close
Else
  MessageBox("No file selected.")
End If

In this example:

  • A TextInputStream is created to read the file.
  • The kChunkSize defines how much data is read at a time.
  • The file is read in a loop until the end is reached, processing each chunk.

Writing Files in Chunks

Writing large files in chunks involves writing smaller portions incrementally. Here’s an example using String.Bytes and String.MiddleBytes for better performance:

Var file As FolderItem = FolderItem.ShowSaveFileDialog("text/plain", "example.txt")
If file <> Nil Then
  Const kChunkSize As Integer = 1024  ' 1 KB chunks

  Var outputStream As TextOutputStream = TextOutputStream.Create(file)
  Var data As String = "Large data string here..." // Your data source
  Var totalBytes As Integer = data.Bytes

  For i As Integer = 0 To totalBytes Step kChunkSize
    Var chunk As String = data.MiddleBytes(i, kChunkSize)
    outputStream.Write(chunk)
  Next

  // Write any remaining data
  If totalBytes Mod kChunkSize <> 0 Then
    Var remainingBytes As Integer = totalBytes Mod kChunkSize
    Var chunk As String = data.MiddleBytes(totalBytes - remainingBytes, remainingBytes)
    outputStream.Write(chunk)
  End If

  outputStream.Close
  MessageBox("File written successfully.")
Else
  MessageBox("No file specified.")
End If

In this example:

  • A TextOutputStream is created to write to a file.
  • The total bytes of the string are calculated for iteration.
  • Data is written in chunks defined by kChunkSize using String.MiddleBytes for improved performance.
  • After the loop, any remaining data that didn’t fit into a full chunk is written.

Practical Example: Processing Large Log Files

Processing large log files line by line in chunks can be done as follows:

Var file As FolderItem = FolderItem.ShowOpenFileDialog("text/plain")
If file <> Nil Then
  Const kChunkSize As Integer = 4096 ' 4 KB chunks
  Var inputStream As TextInputStream = TextInputStream.Open(file)
  Var buffer, remaining As String
  While Not inputStream.EndOfFile
    buffer = inputStream.Read(kChunkSize)
    buffer = remaining + buffer
    Var lines() As String = buffer.ToArray(EndOfLine)
    // Process all but the last line
    For i As Integer = lines.FirstIndex To lines.LastIndex - 1
      System.DebugLog(lines(i))
    Next
    // Save the last line for the next chunk
    remaining = lines(lines.LastIndex)
  Wend
  // Process any remaining content
  If Not remaining.IsEmpty Then
    System.DebugLog(remaining)
  End If
  inputStream.Close
Else
  MessageBox("No file selected.")
End If

In this example:

  • The file is read in larger chunks (4 KB).
  • The buffer is split into lines, and all but the last line are processed.
  • The last line is saved and appended to the next chunk to ensure no data is lost.

Advanced Techniques: Handling Binary Files

Reading and writing binary files also benefits from chunk processing. Here’s a basic example of reading binary files in chunks:

Var file As FolderItem = FolderItem.ShowOpenFileDialog("")
If file <> Nil Then
  Const kChunkSize As Integer = 1024 ' 1 KB chunks
  Var binaryStream As BinaryStream = BinaryStream.Open(file, False)
  Var buffer As MemoryBlock
  While Not binaryStream.EndOfFile
    buffer = binaryStream.Read(kChunkSize)
    // Process the buffer (for demonstration, we'll just print its size)
    System.DebugLog(buffer.Size.ToString)
  Wend
  binaryStream.Close
Else
  MessageBox("No file selected.")
End If

In this example:

  • A BinaryStream is created to read the file.
  • Data is read in chunks and processed accordingly.

Writing Binary Files in Chunks

You can also write binary files in chunks. Here’s a basic example:

Var file As FolderItem = FolderItem.ShowSaveFileDialog("text/plain", "example.txt")
If file <> Nil Then
  Var binaryStream As BinaryStream = BinaryStream.Create(file, True)
  Const kChunkSize As Integer = 1024 ' 1 KB chunks
  Var data As MemoryBlock = ...  ' Your binary data source
  Var totalBytes As Integer = data.Size
  For i As Integer = 0 To totalBytes Step kChunkSize
    Var chunk As MemoryBlock = data.MidB(i, kChunkSize)
    binaryStream.Write(chunk)
  Next
  // Write any remaining data
  If totalBytes Mod kChunkSize <> 0 Then
    Var remainingBytes As Integer = totalBytes Mod kChunkSize
    Var chunk As MemoryBlock = data.MidB(totalBytes - remainingBytes, remainingBytes)
    binaryStream.Write(chunk)
  End If
  binaryStream.Close
  MessageBox("Binary file written successfully.")
Else
  MessageBox("No file specified.")
End If

In this example:

  • A BinaryStream is created to write to a file.
  • Data is written in chunks using MemoryBlock.MidB to handle binary data.
  • After the loop, any remaining data that didn’t fit into a full chunk is written.

Using Threads for Large File Operations

When working with large files, consider using a Thread to perform read and write operations. This keeps the user interface responsive while the file operations are running in the background.

Example of reading a large file in a thread: 

Class FileReadThread Inherits Thread
  Private mFile As FolderItem
  Sub Constructor(file As FolderItem)
    mFile = file
  End Sub
  Sub Run()
    Const kChunkSize As Integer = 1024 ' 1 KB chunks
    Var inputStream As TextInputStream = TextInputStream.Open(mFile)
    Var buffer As String
    While Not inputStream.EndOfFile
      buffer = inputStream.Read(kChunkSize)
      // Process the buffer (for demonstration, we'll just print it)
      System.DebugLog(buffer)
    Wend
    inputStream.Close
  End Sub
End Class

// Usage
Var file As FolderItem = FolderItem.ShowOpenFileDialog("text/plain")
If file <> Nil Then
  Var fileThread As New FileReadThread(file)
  fileThread.Run
Else
  MessageBox("No file selected.")
End If

In this example:

  • A Thread subclass is created to handle file reading.
  • The main UI remains responsive while the thread processes the file in the background.

Conclusion

Handling large files efficiently is crucial for developing robust Xojo applications. By reading and writing files in chunks, you can manage memory usage better and ensure your application remains responsive even when dealing with large datasets. Experiment with these techniques in your projects to experience the benefits.

Happy coding!

Martin T. is a Xojo MVP and has been very involved in testing Android support.

Published in Guest Post, Learning and Tips