using System;
using ytLive.Models;
using ytLive.Services.Encoder;

namespace ytLive.Services;

/// <summary>
/// TASK 33 slice 2: the session's adaptor between the pump's evidence and the pure policy. One planner
/// per broadcast, because the policy is stateful (budget, cooldown, current tier) and a step must
/// survive across windows.
///
/// <para>It deliberately does not act. It answers "should this session change something, and why"; the
/// view model owns the restart, the toast and the session-report line. Keeping the effects out is what
/// makes the whole ladder testable without an encoder running.</para>
/// </summary>
public sealed class StepDownPlanner
{
    private readonly StepDownPolicy _policy;
    private readonly DateTime _startedUtc;

    public StepDownPlanner(StepDownPolicy policy, DateTime startedUtc)
    {
        _policy = policy;
        _startedUtc = startedUtc;
    }

    public StepDownPolicy Policy => _policy;

    /// <summary>Current tier after any applied steps, in the shape the encoder restart needs.</summary>
    public (int Width, int Height, int Fps, double BitrateMbps) Current =>
        (_policy.Width, _policy.Height, _policy.Fps, _policy.BitrateMbps);

    public StepDownDecision Observe(in FramePumpStats stats, DateTime nowUtc)
    {
        var sample = new StepDownSample(
            WindowSeconds: stats.WindowSeconds,
            PumpStalls: stats.Stalls,
            PumpDrops: 0,                 // the pump pads with the last frame; FramesRendered is ambiguous
            EncoderDrops: stats.Dropped,
            AvgRenderMs: stats.AvgRenderMs,
            AvgSubmitWaitMs: stats.AvgSubmitMs);

        return _policy.Observe(sample, nowUtc - _startedUtc);
    }

    /// <summary>The plain sentence the creator sees, chosen by cause. Never a mechanism string: "ffmpeg
    /// exited", "dropped 8", an HTTP code are log findings, not an explanation of what happened to them.
    /// </summary>
    public static string Describe(in StepDownDecision decision, (int Width, int Height, int Fps, double BitrateMbps) landed)
        => decision.Action switch
        {
            StepAction.LowerFrameRate =>
                "LlamaCasty couldn't keep up with your scene, so it lowered the frame rate to " +
                $"{landed.Fps}. Your connection wasn't the problem.",
            StepAction.LowerResolution =>
                "LlamaCasty couldn't keep up with your scene, so it lowered the picture to " +
                $"{landed.Width}×{landed.Height}. Your connection wasn't the problem.",
            StepAction.LowerBitrate =>
                $"Your upload couldn't hold the bitrate, so LlamaCasty lowered it to {landed.BitrateMbps:0.0} Mbps " +
                "to stop the freezing. It will stay there for this stream.",
            StepAction.RaiseFrameRate or StepAction.RaiseResolution or StepAction.RaiseBitrate =>
                "Conditions were clean for a while, so LlamaCasty restored one step of quality.",
            _ => string.Empty,
        };

    /// <summary>One line for the session report, so a later question is answered from data.</summary>
    public static string ReportLine(DateTime whenLocal, in StepDownDecision decision) =>
        decision.Action switch
        {
            StepAction.None => string.Empty,
            StepAction.LowerFrameRate or StepAction.LowerResolution =>
                $"{whenLocal:HH:mm} quality stepped down — encoder ({decision.Why})",
            StepAction.LowerBitrate =>
                $"{whenLocal:HH:mm} quality stepped down — upload ({decision.Why})",
            _ => $"{whenLocal:HH:mm} quality restored ({decision.Why})",
        };
}