voice-cat/docs/tech-stack.md

# Tech Stack & Dependencies

## Initial .NET rewrite

The parallel rewrite under `dotnet/` targets .NET 10. Its initial dependencies are
Google.Protobuf 3.36.1 (BSD-3-Clause), build-only Grpc.Tools 2.83.0 (Apache-2.0), and
BouncyCastle.Cryptography 2.6.2 (MIT). Media AEAD prefers the platform implementation;
BouncyCastle provides the managed fallback and is the planned TLS/exporter provider.
No managed server or audio replacement is shipped yet.

Project files and NuGet lock files pin versions. `dotnet/check-licenses.ps1` checks
all restored direct/transitive packages against a permissive license allowlist in CI;
unknown or copyleft licenses fail. See `dotnet/README.md` for build and test commands.
The existing implementation's dependency choices follow below.

Concrete library choices with versions and rationale. Everything in the **core** is C++
(C++20). UIs are Swift and C#. Build is CMake + vcpkg.

## 1. Core library (`libvoicecat`, C++20)

| Concern | Choice | Version (as of 2026-06) | Why / notes |
|---------|--------|-------------------------|-------------|
| Sockets, timers, async | **Standalone Asio** | 1.30.x | Header-only, no Boost dependency, cross-platform TCP+UDP+timers, one reactor for client and server. (Boost.Asio is interchangeable if we already pull Boost.) |
| TLS 1.3 (control) | **mbedTLS 3.6 LTS** | 3.6.x (LTS ≥ Mar 2027) | **Apache-2.0** (permissive — clean for eventual closed-source distribution). TLS 1.3 client+server, plus `mbedtls_ssl_export_keying_material()` to seed the media AEAD. **Static-links cleanly → single self-host binary.** OpenSSL 3.x (Apache-2.0) is an interchangeable alternative. No DTLS/wolfSSL (GPL) — see [security.md](security.md) §2. |
| Crypto primitives + password hashing + media AEAD | **libsodium** | 1.0.20 | **ISC.** Argon2id (`crypto_pwhash`), ChaCha20-Poly1305 (per-frame media encryption), Ed25519 server identity, X25519, CSPRNG. Audited, hard to misuse. |
| Audio codec | **libopus** | **1.6** (2025-12) | Per-channel mono/stereo, bitrate, frame size; in-band FEC, DTX, PLC, and optional **DRED** deep redundancy; Opus HD/96 kHz available. The whole reason the design is codec-flexible. |
| Audio capture/playback | **miniaudio** | 0.11.x | Single-header, public-domain, backends for **WASAPI / CoreAudio / ALSA / PulseAudio**. One real-time abstraction across all desktop targets; keeps the RT path identical. |
| Audio DSP — noise suppression (NS) | **RNNoise** (vendored, `third_party/rnnoise/`) | xiph @ `70f1d25` (2026-06) | **BSD-3-Clause + CC0-1.0** (model). Hybrid DSP/RNN speech denoiser, mono/48 kHz, ~60× real time, no deps. The shipped NS backend behind `ApmProcessor` (`RnnoiseProcessor`), used by both send-side mic NR (`vc_set_input_noise_reduction`) and per-listener receive NR (`vc_set_remote_stream`). Vendored (not vcpkg) because the vcpkg port is `!windows !arm`. See [voice.md](voice.md) §10. |
| Audio DSP — AEC/AGC/VAD | **webrtc-audio-processing** (APM) — **planned, not built** | 1.x (standalone APM) | **BSD-3**, but has no working Windows/MSVC build upstream (GCC-only Meson, MinGW support unfinished, hard `abseil-cpp` dep — see roadmap.md §2). v1 ships a lightweight, dependency-free energy/RMS VAD (`EnergyVadProcessor`, `core/src/audio/apm_processor.cpp`); **NS now exists via RNNoise (row above)**, but there is still **no AEC or AGC** (iOS gets AEC/NS/AGC natively from VPIO). Real APM stays a tracked future swap behind the same `ApmProcessor` interface. |
| Resampling + jitter ref | **speexdsp** | 1.2.x | BSD. Resampler for non-48 kHz devices; lightweight jitter-buffer reference. (No longer the NS/AGC/VAD source — APM replaces it.) |
| Control serialization | **Protocol Buffers** (protobuf-lite) | 5.x (proto3) | Codegen for C++/C#/Swift; additive, forward/backward compatible; `oneof` envelopes. `nanopb` is a fallback if footprint matters. |
| Server persistence | **SQLite** | 3.4x | Accounts, channels, bans, config. Zero-admin, single file, ships everywhere. |
| Logging | **spdlog** | 1.14.x | Fast, async-capable; off the RT path. |

Resampling note: Opus runs internally at 48 kHz; miniaudio can deliver 48 kHz directly, so
explicit resampling (speexdsp/libsamplerate) is only needed when a device can't do 48 kHz.

## 2. Clients

### Apple clients — .NET native UI with a Swift ReplayKit exception

| Concern | Choice | Notes |
|---------|--------|-------|
| Language | **Swift 5.9+** | Direct **Swift↔C interop** — the C ABI (`voicecat.h`) is imported as a Clang module (`import VoiceCatC`) via a module map in the XCFramework headers; no manual struct/function redeclaration (unlike the C# P/Invoke layer). A Swift wrapper (`VoiceCatCore` package) provides Swift-idiomatic types on top. |
| UI — macOS | **AppKit** | Chosen over SwiftUI for the most mature, granular **VoiceOver** accessibility story (per-control `accessibilityLabel`/`accessibilityHelp`/`accessibilityRole`, `NSAccessibility.post(.announcement)` for live announcements) — the same rationale that drove the Windows client to WinForms over WinUI 3 for screen-reader (NVDA/JAWS/Narrator) UIA support (resolved decision in `docs/roadmap.md`). macOS 14 (Sonoma) deployment target. |
| UI — iOS | **C# / UIKit (`net10.0-ios`)** | Native UIKit keeps direct lifecycle/audio control and predictable VoiceOver semantics. MAUI was rejected because a cross-platform abstraction provides no benefit for this platform-specific client. iOS 18.0 deployment target. |
| Shared core | **VoiceCatCore** Swift Package | One Swift library wrapping the C ABI, consumed by both the macOS AppKit app and the iOS SwiftUI app. Mirrors the C# `VoiceCat.Interop` layer. Events delivered on `@MainActor` via a coalesced `DispatchQueue.main` drain (the Swift analog of C#'s `Channel<VoiceCatEvent>` + 30ms WinForms Timer pump). |
| Audio session (iOS) | **AVAudioSession + AVAudioEngine from C#** | The UIKit host owns category, permission, interruptions, routes, VPIO capture and planar playback. Converted PCM crosses bounded managed rings; callbacks allocate no managed objects, lock, or block. |
| Packaging | .NET Apple workloads + Xcode appex | The managed iOS host statically links merged Opus/RNNoise archives. MSBuild invokes Xcode to build/embed the small Swift ReplayKit extension, which communicates through the versioned App Group PCM ring. |
| Future | CallKit / PushKit | For background VoIP + incoming-call UX on iOS. Post-v1. |

### Windows — C# (shipped in M4, 2026-06-17)

| Concern | Choice | Notes |
|---------|--------|-------|
| Runtime | **.NET 10 LTS** (`net10.0-windows`) | In-service until 2028. |
| Interop | **`[LibraryImport]`** (source-gen P/Invoke) over the C ABI | `[UnmanagedCallersOnly]` static methods for `on_event`/`on_level`; `VoiceCatClientHandle : SafeHandle` owns the `vc_client*` lifetime. |
| Event delivery | **`System.Threading.Channels.Channel<VoiceCatEvent>`** | Single-writer/reader, unbounded; drained by a 30ms `System.Windows.Forms.Timer` on the UI thread. Simpler than a message-only HWND with no meaningful latency cost. |
| UI | **WinForms** | Chosen over WinUI 3 / Avalonia for mature, predictable NVDA/JAWS/Narrator UIA support. Win32 HWND controls have the most complete accessibility story on .NET 10 today. See roadmap.md §2. |
| Persistence | **`System.Text.Json`** (`servers.json`), **`ProtectedData`** (DPAPI) | Saved-server list in `%AppData%\VoiceCat\`; passwords DPAPI-encrypted at rest, opt-in, `CurrentUser` scope. |
| Audio | Handled by the core (miniaudio/WASAPI) | C# only drives device selection + meters. |

## 3. Server (`voicecat-server`)

- Pure C++ linking the core; **no GUI**. Runs on **Linux** (primary), **macOS**, **Windows**.
- Config via a `server.toml` (`allow_guests`, ports, channel defaults, Opus policy, TLS cert
  paths or auto-self-signed + Ed25519 identity, Argon2id cost params, rate limits).
- SQLite for state. Single process for v1; interfaces drawn so a multi-node build is
  *possible* later but explicitly out of scope.
- Packaging: static-ish binary per OS; systemd unit + Docker image for Linux.

## 4. Build & tooling

| Tool | Use |
|------|-----|
| **CMake** (3.25+) | One build graph for core + server + test CLI; UI projects consume the built core. |
| **vcpkg** (manifest mode) | Pin C/C++ deps (opus, libsodium, mbedtls, protobuf, sqlite3, spdlog, asio, miniaudio — see `vcpkg.json`). `webrtc-audio-processing`/`speexdsp` are **not** in the manifest: no working vcpkg port / no working Windows/MSVC build exists upstream for the former; the latter was never actually wired up (the lightweight VAD needs no resampler). Reproducible across OSes. Triplet auto-resolved from the host platform by [`cmake/voicecat-toolchain.cmake`](../cmake/voicecat-toolchain.cmake) — `x64-mingw-static` on Windows, `x64-linux` on Linux, `arm64-osx` on Apple Silicon. Apple platform scaffolding presets (`apple-dev`/`apple-ios`/`apple-ios-sim`) produce static `libvoicecat.a` slices for XCFramework consumption. vcpkg itself is bundled as a git submodule at `vcpkg/`, pinned to `vcpkg.json`'s `builtin-baseline` commit — `VCPKG_ROOT` overrides it for an external checkout. See [building.md §2](building.md#2-one-time-setup-for-the-real-deps-presets). |
| **protoc** | Generate C++/C#/Swift from `core/proto/*.proto` (single source of truth). |
| **clang-format / clang-tidy** | Style + static analysis on the core. |
| **CTest + a fuzz target** | Unit/integration tests; fuzz the frame parser and protobuf boundary (security-sensitive). |
| **GitHub Actions** (or similar) | Matrix CI: Linux/macOS/Windows core+server; Xcode build for Apple; `dotnet` build for Windows. |

## 5. Licensing — permissive only (hard rule)

The code will eventually be distributed in **closed-source** form, so **no GPL/LGPL
dependencies are permitted.** Every dependency below is BSD / MIT / ISC / Apache-2.0 /
public-domain:

- **mbedTLS** — Apache-2.0 ✅ · **libsodium** — ISC ✅ · **libopus** — BSD ✅ ·
  **miniaudio** — public domain / MIT-0 ✅ · **protobuf** — BSD ✅ ·
  **SQLite** — public domain ✅ · **Asio** (standalone) — Boost ✅ · **spdlog** — MIT ✅ ·
  **RNNoise** — BSD-3-Clause (code) + CC0-1.0 (model) ✅, vendored in `third_party/rnnoise/`
  (not vcpkg — the port is `!windows !arm`; see [`third_party/README.md`](../third_party/README.md)).
  **webrtc-audio-processing** would be BSD-3 ✅ if/when it's actually built in (see §1) —
  not a live dependency today, so not part of the resolved vcpkg graph the license scanner
  below checks.
- **Explicitly rejected:** **wolfSSL** (GPLv2/commercial) and any DTLS stack that would drag
  in copyleft. The exported-keys + AEAD media design (security.md §2) removes the need for
  one entirely.
- CI runs a license scanner over the resolved vcpkg graph and **fails the build on any
  GPL/LGPL transitive dependency**, so this rule can't silently regress.

## 6. Why not the obvious alternatives

- **WebRTC** — explicitly rejected: ICE/SDP/TURN complexity, huge dependency, opaque. We
  want plain TCP+UDP we fully control.
- **QUIC** — capable (reliable streams + datagrams + TLS 1.3 in one), but heavier and drifts
  toward the complexity we're avoiding. Revisit only if NAT traversal/multiplexing pain
  appears.
- **gRPC** for control — pulls HTTP/2 and a lot of surface for what is a simple framed
  message stream over TLS. Plain protobuf-over-framed-TLS is enough.
- **A Rust core** — viable and memory-safe, but the user prefers C++ and the Swift/C#
  binding story is marginally simpler from C++ (Swift can even consume C++ directly).