CoopAllTheThings/host/src/audio/audio_loopback.hpp

// Mirrors a target process's audio so Steam Remote Play Together (which streams
// THIS host's audio) carries the real game's sound, re-rendering it on the
// default output endpoint.
//
// Two source paths (see docs/audio-render-hook-plan.md):
//   - Hooked: the injected render-hook copies the game's frames into a shared
//     audio ring AND silences the game locally, so there is no echo. Preferred.
//   - Loopback: WASAPI process-loopback capture of the game (the game still
//     plays locally, so the operator hears it twice). Automatic fallback when
//     the hook isn't present / doesn't publish a format in time.
#pragma once

#include <atomic>
#include <mutex>
#include <string>
#include <thread>
#include <vector>

#include <windows.h>

#include "coop/audio_ring.hpp"
#include "coop/protocol.hpp" // kMaxAudioStreams
#include "coop/shared_memory.hpp"

namespace coop
{

class AudioMirror
{
public:
	AudioMirror() = default;
	~AudioMirror();

	AudioMirror(const AudioMirror&) = delete;
	AudioMirror& operator=(const AudioMirror&) = delete;

	// Start mirroring audio from `pid` (and its child processes). Replaces any
	// running mirror. Returns false only on synchronous setup failure; capture
	// errors surface asynchronously via status().
	bool start(DWORD pid);
	void stop();

	// True once the audio thread is actively mirroring (false while starting or
	// after a failure).
	[[nodiscard]] bool running() const
	{
		return running_.load(std::memory_order_acquire);
	}

	// The process currently targeted (0 if stopped). Updated synchronously by
	// start()/stop() so the UI can detect target changes without races.
	[[nodiscard]] DWORD target_pid() const
	{
		return pid_;
	}

	[[nodiscard]] unsigned sample_rate() const
	{
		return sample_rate_.load(std::memory_order_relaxed);
	}
	[[nodiscard]] unsigned channels() const
	{
		return channels_.load(std::memory_order_relaxed);
	}

	// Audio currently buffered between capture and the output device, in ms — a
	// health/latency proxy (rises if the consumer can't keep up). 0 when stopped.
	[[nodiscard]] unsigned buffered_ms() const
	{
		return buffered_ms_.load(std::memory_order_relaxed);
	}

	// Which capture path is active, for the UI's source indicator.
	enum class Source
	{
		None,
		Hooked,   // shared audio ring from the render-hook (no echo)
		Loopback, // WASAPI process loopback (echo)
	};
	[[nodiscard]] Source source() const
	{
		return source_.load(std::memory_order_relaxed);
	}
	[[nodiscard]] const char* source_name() const
	{
		switch (source())
		{
		case Source::Hooked:
			return "Hooked"; // echo depends on the format provenance; the panel shows it
		case Source::Loopback:
			return "Loopback (echo)";
		default:
			return "—";
		}
	}

	[[nodiscard]] std::string status() const;

	// Why the loopback (echo) path is active instead of the hooked one, for the Audio
	// panel. Empty when on the hooked path or before any fallback decision.
	[[nodiscard]] std::string fallback_reason() const;

	// Post an operator command (AudioRingOp) to the hook for stream `slot` -- re-measure
	// the rate or override the format. Thread-safe; queued and applied to the ring on the
	// audio thread (which owns the ring mappings). For a re-measure the format args are 0.
	void request_op(unsigned slot, std::uint32_t kind, std::uint32_t rate = 0, std::uint32_t channels = 0,
	                std::uint32_t bits = 0, std::uint32_t format_tag = 0);

private:
	void thread_main(DWORD pid);
	// Outcome of a hooked render session.
	enum class HookedResult
	{
		Stopped, // clean stop (mirror stopping) -> done
		Failed,  // setup failed (format not renderable) -> caller falls back to loopback
		Reinit,  // the hook re-published the format (re-measure/override) -> re-read and retry
	};
	// Runs the hooked (no-echo) render path until stop, a setup failure, or a format change
	// (re-measure/override). `rings[0]` is the primary stream; additional non-null rings are
	// mixed in.
	HookedResult run_hooked(AudioRingHeader* const* rings);
	// Loopback (echo) capture. If `promote_ring` is non-null, returns true the moment
	// that ring's format becomes ready (the hook caught up -> caller promotes to hooked);
	// returns false when stopped. With a null ring it only returns false (on stop).
	bool run_loopback(DWORD pid, AudioRingHeader* promote_ring);
	bool wait_for_format(AudioRingHeader* ring, DWORD timeout_ms);
	static void enable_capture(AudioRingHeader* const* rings, bool on);
	void drain_ops(); // audio thread: post queued operator ops to the session rings
	bool stop_requested() const;
	void set_status(std::string s);
	void set_fallback_reason(std::string s);

	std::thread thread_;
	HANDLE stop_event_ = nullptr;
	DWORD pid_ = 0;

	SharedMemory audio_ring_shm_[kMaxAudioStreams]; // per-stream rings (coop_audio_<pid>[_<i>])
	AudioRingHeader* session_rings_[kMaxAudioStreams] = {}; // set on the audio thread for the session

	// Operator ops queued by request_op (any thread) and applied to the rings on the
	// audio thread (which owns the mappings). Guarded by ops_mutex_.
	struct PendingOp
	{
		unsigned slot;
		std::uint32_t kind, rate, channels, bits, format_tag;
	};
	std::mutex ops_mutex_;
	std::vector<PendingOp> pending_ops_;

	std::atomic<bool> running_{false};
	std::atomic<Source> source_{Source::None};
	std::atomic<unsigned> sample_rate_{0};
	std::atomic<unsigned> channels_{0};
	std::atomic<unsigned> buffered_ms_{0};

	mutable std::mutex status_mutex_;
	std::string status_;
	std::string fallback_reason_; // why loopback is active (shown in the panel); guarded by status_mutex_
};

} // namespace coop