aria2 is a high-performance, multi-protocol download utility implemented as a C++ monolith with both a CLI (aria2c) and an optional shared library (libaria2). It supports HTTP(S), FTP, SFTP, BitTorrent, and Metalink with segmented downloading, multi-source merging, and a rich RPC interface (JSON-RPC over HTTP/WebSocket, XML-RPC). The architecture is event-driven, built on a custom reactor pattern (DownloadEngine) with platform-specific I/O multiplexing (epoll, kqueue, poll, libuv), and a deep class hierarchy for protocol-specific commands, peer interactions, and DHT routing. External dependencies are abstracted behind internal interfaces (e.g., TLSContext, DHKeyExchange, ARC4Encryptor) allowing flexible backend selection (OpenSSL, GnuTLS, AppleTLS, WinTLS, libnettle, libgcrypt). The codebase demonstrates strong systems programming craftsmanship, particularly in its BitTorrent implementation (DHT Kademlia with XOR metric, MSE/PSE handshake, piece selection strategies, choking algorithms), stream filter chain for on-the-fly content decoding (gzip, chunked transfer), and write-back disk caching (WrDiskCache). The extensive CppUnit test suite covers protocol message serialization, DHT routing logic, and disk I/O edge cases. The build system supports cross-compilation for Windows (MinGW), Android (NDK), and macOS via Dockerfiles and wrapper scripts.