namespace IfLoc.Sim;

/// <summary>
/// A bidirectional in-memory link modelling the bonded BLE GATT session between the
/// locator (peripheral) and the App (central). Frames pushed toward the App surface on
/// <see cref="ToApp"/>; frames the App writes back surface on <see cref="ToLocator"/>.
/// A transport-agnostic stand-in for the real GATT characteristics — a TCP or real-BLE
/// implementation can replace it without touching the simulator or codec.
/// </summary>
public sealed class LoopbackLink
{
    /// <summary>Locator → App (Telemetry / Event characteristics).</summary>
    public event Action<byte[]>? ToApp;
    /// <summary>App → Locator (Command / Capture Result characteristics).</summary>
    public event Action<byte[]>? ToLocator;

    public void PublishToApp(byte[] frame) => ToApp?.Invoke(frame);
    public void WriteToLocator(byte[] frame) => ToLocator?.Invoke(frame);
}

/// <summary>Result of a replayed session, for test assertions.</summary>
public sealed record SessionReport
{
    public int TelemetryFramesEmitted { get; init; }
    public int CaptureTriggersEmitted { get; init; }
    public int CaptureResultsReceived { get; init; }
    public IReadOnlyList<CaptureResult> Results { get; init; } = Array.Empty<CaptureResult>();
    /// <summary>Trigger captureSeqs still awaiting a result — must be empty (no silent-failure, SRS-LOG-7).</summary>
    public IReadOnlyList<ushort> PendingCaptures { get; init; } = Array.Empty<ushort>();
}

/// <summary>
/// Replays a canned locate session over the IF-LOC contract: emits telemetry at the
/// configured rate, raises capture-triggers at scripted frames, and consumes the App's
/// capture-results — correlating each result to its trigger by captureSeq. Built purely
/// from the frozen data dictionary so App development is unblocked without firmware
/// (Risk R-3).
/// </summary>
public sealed class LocatorSimulator
{
    private readonly LoopbackLink _link;
    private ushort _seq;
    private ushort _captureSeq;
    private readonly HashSet<ushort> _pending = new();
    private readonly List<CaptureResult> _results = new();

    public LocatorSimulator(LoopbackLink link)
    {
        _link = link;
        _link.ToLocator += OnAppFrame;
    }

    private void OnAppFrame(byte[] frame)
    {
        if (frame.Length < IfLoc.HeaderLen) return;                 // §14 ignore malformed
        if (Frames.PeekType(frame) != MessageType.CaptureResult) return;
        var result = CaptureResult.DecodeFrame(frame);
        _results.Add(result);
        _pending.Remove(result.CaptureSeq);
    }

    /// <summary>
    /// Emits <paramref name="frames"/> telemetry frames, raising a capture-trigger at each
    /// index in <paramref name="captureAtFrames"/>. When <paramref name="realTime"/> is true,
    /// paces at <paramref name="rateHz"/> for a live demo; otherwise emits as fast as possible
    /// for deterministic tests. Returns once all frames are emitted (results may still be
    /// arriving synchronously on the loopback).
    /// </summary>
    public async Task<SessionReport> ReplayAsync(
        int frames,
        IReadOnlySet<int> captureAtFrames,
        double rateHz = 5.0,
        bool realTime = false,
        CancellationToken ct = default)
    {
        int triggers = 0;
        uint uptime = 0;
        int stepMs = (int)Math.Round(1000.0 / rateHz);

        for (int i = 0; i < frames; i++)
        {
            ct.ThrowIfCancellationRequested();
            var tele = BuildTelemetry(i, uptime);
            _link.PublishToApp(tele.EncodeFrame(_seq++));

            if (captureAtFrames.Contains(i))
            {
                var trigger = new CaptureTrigger
                {
                    CaptureSeq = ++_captureSeq,          // locator-initiated: high bit clear (§5.1)
                    TriggerType = TriggerType.ButtonSingle,
                    Snapshot = tele,
                };
                _pending.Add(trigger.CaptureSeq);
                triggers++;
                _link.PublishToApp(trigger.EncodeFrame(_seq++));
            }

            uptime += (uint)stepMs;
            if (realTime) await Task.Delay(stepMs, ct).ConfigureAwait(false);
        }

        return new SessionReport
        {
            TelemetryFramesEmitted = frames,
            CaptureTriggersEmitted = triggers,
            CaptureResultsReceived = _results.Count,
            Results = _results.ToArray(),
            PendingCaptures = _pending.ToArray(),
        };
    }

    /// <summary>
    /// A deterministic, physically-plausible canned telemetry frame for step <paramref name="i"/>.
    /// Sweeps depth, current, and signal across their ranges and exercises a warning flag
    /// and a no-depth sentinel so consumers see the full dictionary.
    /// </summary>
    public static Telemetry BuildTelemetry(int i, uint uptimeMs)
    {
        bool noDepth = i % 20 == 19;                 // periodically drop depth (sentinel path)
        var warn = WarningFlags.None;
        if (i % 25 == 12) warn |= WarningFlags.Shallow;
        if (i % 40 == 30) warn |= WarningFlags.Overload;

        return new Telemetry
        {
            LocatorUptimeMs = uptimeMs,
            DepthMeters = noDepth ? null : 0.50 + 0.01 * (i % 200),   // 0.50 → 2.49 m
            SignalCurrentMa = (ushort)(50 + (i % 300)),
            FrequencyHz = 32768,
            Mode = LocateMode.Twin,
            SignalType = SignalType.Active,
            GainDb = (byte)(40 + (i % 60)),
            SignalLevel = (byte)(60 + (i % 40)),
            DistortionQualityPct = (byte)(100 - (i % 15)),
            SignalDirection = 1,
            CompassAngleDeg = (byte)(i % 181),
            GuidanceOffset = (short)(((i % 41) - 20) * 25),           // −500 → +500, negative = left
            Warnings = warn,
            Utility = UtilityType.Water,
            BatteryPercent = (byte)Math.Max(5, 100 - i / 10),
            Status = StatusFlags.Locating,
        };
    }
}