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FPBencode

FreePascal Bencode implementation

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README

FreePascal Bencode Library

A comprehensive and robust Bencode encoding/decoding library for FreePascal and Lazarus. Bencode is the encoding format used by the BitTorrent protocol for storing and transmitting loosely structured data.

What is Bencode?

Bencode (pronounced "bee-encode") is a simple data serialization format used by BitTorrent for .torrent files and peer communication. It supports four data types:

  • Strings: 4:spam (length:data)
  • Integers: i42e (i + number + e)
  • Lists: l4:spam4:eggse (l + items + e)
  • Dictionaries: d3:cow3:moo4:spam4:eggse (d + key-value pairs + e)

Features

✅ Full Bencode Specification Support - Complete implementation of the BitTorrent bencode standard
✅ Type-Safe API - Comprehensive error handling with custom exception classes
✅ Memory Safe - Automatic resource management with proper destructors
✅ Stream & File I/O - Support for encoding/decoding from strings, streams, and files
✅ Dictionary Key Validation - Automatic key sorting and validation per specification
✅ Large File Support - 64-bit integer support for files >4GB
✅ Cross-Platform - Works on Windows, Linux, and macOS
✅ Well Tested - Comprehensive test suite with 60+ test cases

Installation

Requirements

  • FreePascal 3.0 or later
  • Lazarus (optional, for IDE support)

Setup

  1. Download bencode.pas from this repository
  2. Copy it to your project directory or add to your library path
  3. Add bencode to your uses clause
pascal
uses
  SysUtils, Classes, bencode;

Quick Start

Basic Usage

pascal
program SimpleExample;
uses bencode;

var
  Node: TBencodeNode;
  Encoded: string;
begin
  // Create and encode a string
  Node := BencodeString('Hello, World!');
  try
    Encoded := TBencodeCodec.Encode(Node);
    WriteLn('Encoded: ', Encoded); // Output: 13:Hello, World!
  finally
    FreeAndNil(Node);
  end;
  
  // Decode it back
  Node := TBencodeCodec.Decode('13:Hello, World!');
  try
    WriteLn('Decoded: ', Node.AsString); // Output: Hello, World!
  finally
    FreeAndNil(Node);
  end;
end.

Working with Dictionaries

pascal
var
  Root: TBencodeNode;
  Dict: TBencodeDictionary;
begin
  // Create a dictionary structure
  Root := BencodeDictionary;
  try
    Dict := Root.AsDictionary;
    
    // Add various data types
    Dict.Add('name', 'My Torrent');
    Dict.Add('size', 1048576);
    Dict.Add('private', 1);
    
    // Add nested structures
    Dict.AddList('files').AsList.Add('file1.txt');
    Dict.Values['files'].AsList.Add('file2.txt');
    
    // Encode the entire structure
    WriteLn(TBencodeCodec.Encode(Root));
    
  finally
    FreeAndNil(Root);
  end;
end;

Reading a Torrent File

pascal
var
  Torrent: TBencodeNode;
  Info: TBencodeDictionary;
begin
  // Load and parse a .torrent file
  Torrent := TBencodeCodec.DecodeFromFile('example.torrent');
  try
    WriteLn('Announce URL: ', Torrent.Values['announce'].AsString);
    
    if Torrent.Values['info'].IsDictionary then
    begin
      Info := Torrent.Values['info'].AsDictionary;
      WriteLn('Name: ', Info.Values['name'].AsString);
      
      if Info.ContainsKey('length') then
        WriteLn('Size: ', Info.Values['length'].AsInteger, ' bytes');
    end;
    
  finally
    FreeAndNil(Torrent);
  end;
end;

Creating a Simple Torrent

pascal
var
  Torrent: TBencodeNode;
  Dict, Info: TBencodeDictionary;
begin
  Torrent := BencodeDictionary;
  try
    Dict := Torrent.AsDictionary;
    
    // Basic torrent metadata
    Dict.Add('announce', 'http://tracker.example.com/announce');
    Dict.Add('comment', 'Created with FreePascal Bencode Library');
    Dict.Add('creation date', DateTimeToUnix(Now));
    
    // File information
    Info := Dict.AddDictionary('info').AsDictionary;
    Info.Add('name', 'example.txt');
    Info.Add('length', 12345);
    Info.Add('piece length', 32768);
    Info.Add('pieces', 'dummy_hash_data_goes_here');
    
    // Save the torrent file
    TBencodeCodec.EncodeToFile(Torrent, 'created.torrent');
    WriteLn('Torrent file created successfully!');
    
  finally
    FreeAndNil(Torrent);
  end;
end;

API Reference

Core Classes

TBencodeNode - Main Data Container

pascal
// Type checking
function IsString/IsInteger/IsList/IsDictionary: Boolean;

// Value access
property AsString: string;
property AsInteger: Int64;
property AsList: TBencodeList;
property AsDictionary: TBencodeDictionary;

// Collection access
property Count: Integer;
property Items[Index: Integer]: TBencodeNode;      // For lists
property Values[Key: string]: TBencodeNode;        // For dictionaries

// Utility
function Clone: TBencodeNode;
procedure Dump(Indent: Integer = 0);

TBencodeCodec - Encoding/Decoding Engine

pascal
// Decoding
class function Decode(const Data: string): TBencodeNode;
class function Decode(Stream: TStream): TBencodeNode;
class function DecodeFromFile(const FileName: string): TBencodeNode;

// Encoding  
class function Encode(Node: TBencodeNode): string;
class procedure Encode(Node: TBencodeNode; Stream: TStream);
class procedure EncodeToFile(Node: TBencodeNode; const FileName: string);

// Validation
class function IsValidBencode(const Data: string): Boolean;

Helper Classes

  • TBencodeList - Easy list manipulation with Add(), Delete(), Clear() methods
  • TBencodeDictionary - Dictionary operations with Add(), Remove(), ContainsKey() methods

Convenience Functions

pascal
function BencodeString(const Value: string): TBencodeNode;
function BencodeInteger(const Value: Int64): TBencodeNode;  
function BencodeList: TBencodeNode;
function BencodeDictionary: TBencodeNode;

Exception Handling

pascal
try
  Node := TBencodeCodec.Decode(SomeData);
  // Use node...
except
  on E: EBencodeParseError do
    WriteLn('Parse error: ', E.Message);
  on E: EBencodeTypeError do  
    WriteLn('Type mismatch: ', E.Message);
  on E: EBencodeKeyError do
    WriteLn('Key not found: ', E.Message);
end;

Testing

Run the comprehensive test suite to verify the implementation:

bash
fpc bencode_tests.pas
./bencode_tests

# Expected output:
# FreePascal Bencode Library Test Suite
# ====================================
# ...
# Passed: 79/79 tests
# SUCCESS: All tests passed!

Specification Compliance

This library implements the complete Bencode specification as used by BitTorrent:

  • ✅ String encoding with length prefixes
  • ✅ Integer encoding with proper validation (no leading zeros, no negative zero)
  • ✅ List encoding with arbitrary nesting
  • ✅ Dictionary encoding with mandatory key sorting
  • ✅ Comprehensive error detection for malformed data
  • ✅ Support for binary data in strings

License

This project is licensed under the MIT License - see the LICENSE file for details.

Made with ❤️ for the FreePascal community

If you find this library useful, please give it a ⭐ star on GitHub!