// SPDX-License-Identifier: LGPL-3.0-only
// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®

// The session: the app's end of the debug connection.
//
//     ThreadPool::Start();
//     Debug::Session debug({ .name = "MyApp" });
//     EventListener<void> pump(&window.onBeforeUpdate, [] { Debug::Pump(); });
//
// Requests arrive on transport threads and are never answered by blocking
// one: what needs only the session's own state (status, the command list,
// the log, pause, resume) is answered there and then; what touches the app
// (calls, watches, traces, screenshots) waits in a queue for the next Pump,
// which runs it on the app's own thread between frames; waits and steps are
// answered by the Pump that sees them satisfied. A request that cannot run
// before its timeout is answered Timeout and is guaranteed not to have run.
//
// Pause and step only stop what the app gates on them:
//
//     while (acc >= dt && Debug::Advance()) { sim.Tick(); acc -= dt; }  // frame loops
//     if (Debug::Paused()) { for (auto n = Debug::TakeSteps(); n > 0; --n) sim.Tick(); acc = 0; }
//                                                                       // fixed-step loops
//
// Rendering and Pump keep running while paused, so the agent can still read
// watches, take screenshots and step.
export module Crafter.Debug:Session;
import std;
import Crafter.Debug.Core;

export namespace Crafter::Debug::Detail {
    // One request's way back to its client. A transport implements it; the
    // session calls it from transport threads, the Pump thread and whatever
    // thread logs, so it must be safe from any thread and must not block.
    class Exchange {
    public:
        virtual ~Exchange() = default;
        // The final reply, ending the exchange.
        virtual void Reply(const Value& reply) = 0;
        // One more object on a streaming exchange (`log --follow`). False
        // when the client is gone or too far behind; the session then drops it.
        virtual bool Stream(const Value& line) = 0;
        // Reply, then call `afterSent` once the reply has left: what
        // `quit --force` exits from.
        virtual void ReplyThen(const Value& reply, std::function<void()> afterSent) {
            Reply(reply);
            afterSent();
        }
    };
}

export namespace Crafter::Debug {
    struct SessionOptions {
        // What `crafter-debug ls` calls this instance; the executable's
        // file name when empty.
        std::string name;
        // 0 for any free port; the discovery file says which.
        std::uint16_t port = 0;
        // Also record what the app writes to stdout and stderr on the timeline.
        bool captureStdio = false;
        // Called on the Pump thread when an agent asks the app to quit,
        // after QuitRequested() turned true. Don't call std::exit from it:
        // close the window or end the loop, so the Session is destroyed
        // before main returns.
        std::function<void()> onQuit;
        // False: no transport and no discovery file; requests come only
        // through Handle. For tests.
        bool listen = true;
    };

    // One per process; a second throws. Destroy it before
    // ThreadPool::Stop() and before main returns: it closes its connections,
    // and msquic waits for open ones at exit.
    class Session {
    public:
        explicit Session(SessionOptions options = {});
        ~Session();
        Session(const Session&) = delete;
        Session& operator=(const Session&) = delete;

        // One request, from a transport or a test. Never blocks; the reply
        // goes through `exchange`.
        void Handle(const Value& request, std::shared_ptr<Detail::Exchange> exchange);
        // What every request must carry.
        const std::string& Token() const;

        struct State;

    private:
        std::shared_ptr<State> state;
    };

    // Once per frame, on the thread that owns the app's state: runs queued
    // requests, polls Async results, evaluates waits and conditions, records
    // traces, and counts the frame. Does nothing without a Session.
    void Pump();
    // For frame loops: false while paused, true once per step granted.
    bool Advance();
    // For fixed-step loops: whether the session is paused; see TakeSteps.
    bool Paused();
    // The steps granted since the last call, all at once.
    std::uint32_t TakeSteps();
    // Whether an agent asked the app to quit.
    bool QuitRequested();
    // Where `crafter-debug screenshot` gets a PNG: called on the Pump
    // thread, returning the bytes when they are ready (a frame or two later
    // for a GPU readback). See the README for the Crafter.Graphics version.
    void SetCapture(std::function<Async<std::vector<std::byte>>()> capture);
}