BitTorrent Tracker Subsystem

This repository contains a C++20 implementation of a BitTorrent tracker subsystem, including:

• Bencode parser/encoder (bittorrent::bencode)
• Metainfo parser for .torrent and magnet links (bittorrent::metainfo)
• Tracker clients (bittorrent::tracker):
  • HTTP(S) tracker client (BEP 3/7)
  • UDP tracker client (BEP 15)
• TrackerManager for coordinating multiple tracker tiers, scheduling announces, and draining peers.

The implementation is modular and covered by unit tests using Catch2 v3.

UML Design:

Features

• Parse .torrent files and magnet URIs.
• Compute spec-correct info-hash (SHA-1 over exact "info" slice).
• Encode/decode bencoded values.
• Handle announce and scrape requests:
  • HTTP/HTTPS trackers via libcurl
  • UDP trackers via raw sockets
• Support BEP 12 tracker tiers (list-of-lists).
• Expose peers to client code via callbacks or drainNewPeers().
• Unit test coverage for:
  • BencodeValue / BencodeParser
  • Metainfo
  • CompactPeerCodec
  • TrackerEndpoint / TrackerTier
  • HttpTracker
  • TrackerManager

Repository Structure

include/bittorrent/
├── bencode/            # Bencode parser/encoder
│   ├── bencode.hpp
│   └── bencode.cpp
│   └── tests/
├── metainfo/           # .torrent parsing
│   ├── metainfo.hpp
│   └── metainfo.cpp
│   └── tests/
└── tracker/            # Tracker subsystem
├── include/        # Public tracker headers
│   ├── types.hpp
│   ├── endpoint.hpp
│   ├── http_tracker.hpp
│   ├── udp_tracker.hpp
│   ├── manager.hpp
│   └── http_client.hpp
│   └── compact_peer_codec.hpp
│   └── iclient.hpp
│   └── expected.hpp
├── src/            # Tracker implementation
│   ├── types.cpp
│   ├── compact_peer.cpp
│   ├── endpoint.cpp
│   ├── http_tracker.cpp
│   ├── http_client_curl.cpp
│   └── manager.cpp
└── tests/          # Unit tests + demo
├── test_types.cpp
├── test_compact_peer.cpp
├── test_endpoint.cpp
├── test_http_tracker.cpp
├── test_tracker_manager.cpp
├── demo_tracker.cpp        # Demo executable
└── sample.torrent

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Dependencies

• C++20 compiler (tested with GCC 14 / Clang 17)
• CMake ≥ 3.16
• Catch2 v3 (system-installed)
• libcurl (for HTTP/HTTPS tracker client)

On Ubuntu/Debian:

sudo apt update
sudo apt install build-essential cmake libcurl4-openssl-dev catch2

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Build & Test

cd bittorrent/tracker/tests
sudo chmod +x run_tests.sh
./run_tests.sh   #builds and runs all tests

Available Tests

• test_types
• test_compact_peer
• test_endpoint
• test_http_tracker
• test_tracker_manager

Run a single test:

./build/test_http_tracker

Demo: Running a Tracker Announce

A small demo program demo_tracker.cpp is included. It loads a .torrent file, extracts announce tiers, and performs tracker announces:

./build/demo_tracker ../archive_1.torrent (or any other torrent file to test can be placed in tests dir)

Example output:

=== Running demo_tracker ===
Announce list:
  Tier 0:
    http://bittorrent-test-tracker.codecrafters.io/announce
Peers:
  165.232.35.114:51444
  165.232.38.164:51433
  165.232.41.73:51544
  165.232.35.114:51444
  165.232.38.164:51433
  165.232.41.73:51544

Note: Peers listed are raw IP:port endpoints discovered from public trackers. No peer connections are established by this demo.

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Next Steps

• Extend TrackerManager scheduling: exponential backoff, jitter.
• Implement full UDP tracker support (connect + announce + scrape).
• Wire into TorrentSession for peer management.
• Add integration tests with real trackers.

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