using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Globalization;
using System.IO;
using System.Text;
using AcDream.Core.Rendering;
using Silk.NET.OpenGL;

namespace AcDream.App.Diagnostics;

/// <summary>Stage indices for per-frame CPU attribution.</summary>
public enum FrameStage
{
    /// <summary>Whole OnUpdate body (simulation + streaming apply).</summary>
    Update = 0,
    /// <summary>WbMeshAdapter.Tick — staged mesh/texture GPU upload drain.</summary>
    Upload = 1,
    /// <summary>ImGui Render (dev overlay).</summary>
    ImGui = 2,
    /// <summary>Software presentation deadline wait (zero under VSync or uncapped mode).</summary>
    Pacing = 3,
}

/// <summary>
/// One <c>ACDREAM_FRAME_HISTORY</c> CSV row — every field the aggregated
/// <c>[frame-prof]</c> report discards when its 5-second window resets.
/// Stage fields mirror <see cref="FrameStage"/> positionally (Update /
/// Upload / ImGui / Pacing, matching <see cref="FrameProfiler.FormatReport"/>'s
/// <c>names</c> array) — if <see cref="FrameStage"/> grows, extend this
/// record, <see cref="FrameProfiler.WriteHistoryCsv"/>, and the CSV header
/// together. <c>GpuUs</c> is <c>-1</c> for a frame with no available GPU
/// sample (warm-up, or <c>ACDREAM_WB_DIAG=1</c> self-disable).
/// </summary>
internal readonly record struct FrameHistoryRecord(
    int FrameIndex,
    double TimestampMs,
    long CpuUs,
    long GpuUs,
    long AllocBytes,
    long UpdateUs,
    long UploadUs,
    long ImGuiUs,
    long PacingUs);

/// <summary>
/// MP0 (2026-07-05) — the permanent honest frame profiler. One
/// <c>FrameBoundary</c> call at the top of the accepted render transaction
/// measures CPU frame time as the delta between consecutive boundaries
/// (captures the FULL frame including present) and samples per-frame allocated
/// bytes + GC collection counts. <see cref="BeginGpuFrame"/> and
/// <see cref="EndGpuFrame"/> separately bracket only submitted render work.
/// Stage scopes
/// (<see cref="BeginStage"/>) attribute CPU time to Update / Upload /
/// ImGui. Emits one <c>[frame-prof]</c> line every ~5 s while
/// <see cref="RenderingDiagnostics.FrameProfEnabled"/> is true; costs one
/// bool check per frame when off.
///
/// <para>Permanent apparatus — every MP-track gate reads it; do not strip.
/// Whole-frame GPU timing self-disables under <c>ACDREAM_WB_DIAG=1</c>
/// (nested TimeElapsed is illegal GL; see GpuFrameTimer).</para>
///
/// <para>2026-07-24 measurement-tooling review — the aggregated report
/// resets its ring buffers every ~5 s (<see cref="FrameStatsBuffer.Reset"/>),
/// so route-wide p50/p95/p99 distributions across a whole soak cannot be
/// reconstructed after the fact. <see cref="RenderingDiagnostics.FrameHistoryPath"/>
/// (<c>ACDREAM_FRAME_HISTORY=&lt;path&gt;</c>) opts into a SEPARATE
/// per-frame history: one <see cref="FrameHistoryRecord"/> per frame in a
/// preallocated, grow-as-needed <see cref="List{T}"/> (1 int + 1 double +
/// 7 longs ≈ 72 bytes/record; a multi-hour capture at 165 fps is roughly
/// 165 * 3600 * 72 bytes ≈ 43 MB/hour — fine for a bounded diagnostic run,
/// not for unattended day-long capture). Its initial capacity is about 9 MiB
/// and covers the canonical route above 300 FPS without a frame-thread resize.
/// ZERO frame-thread I/O: the CSV is written once, from
/// <see cref="Dispose"/>, at shutdown. Recording only takes effect while
/// <see cref="RenderingDiagnostics.FrameProfEnabled"/> is ALSO true — it
/// reuses that instrumentation rather than duplicating it. Does not
/// change the <c>[frame-prof]</c> report format or any existing metric.</para>
/// Spec: docs/superpowers/specs/2026-07-05-modern-pipeline-design.md §5.
/// </summary>
public sealed class FrameProfiler : IDisposable
{
    private const int WindowCapacity = 2048;              // ~12 s at 165 fps
    private const int HistoryInitialCapacity = 131072;
    private const long ReportIntervalTicks = 5 * TimeSpan.TicksPerSecond;
    private static readonly int StageCount = Enum.GetValues<FrameStage>().Length;

    private readonly FrameStatsBuffer _cpuUs = new(WindowCapacity);
    private readonly FrameStatsBuffer _gpuUs = new(WindowCapacity);
    private readonly FrameStatsBuffer _allocBytes = new(WindowCapacity);
    private readonly FrameStatsBuffer[] _stageUs;
    private readonly long[] _stageAccumTicks;
    private readonly long[] _lastStageUs;
    private readonly bool _wbDiagActive =
        Environment.GetEnvironmentVariable("ACDREAM_WB_DIAG") == "1";
    private readonly List<FrameHistoryRecord>? _history;
    private readonly long _profilerStartTimestamp;
    private readonly DateTime _profilerStartUtc;
    private int _currentFrameIndex = -1;

    private GpuFrameTimer? _gpuTimer;
    private long _lastBoundaryTimestamp;
    private long _lastAllocBytes;
    private long _lastReportTicks;
    private int _gc0Base, _gc1Base, _gc2Base;
    private int _framesInWindow;
    private int _ownerThreadId;
    private bool _threadWarned;
    private bool _wbDiagNoticePrinted;
    private bool _wasEnabled;

    /// <summary>Most recent immutable report line, for explicit automation checkpoints.</summary>
    public string? LastReport { get; private set; }

    public FrameProfiler()
    {
        _stageUs = new FrameStatsBuffer[StageCount];
        for (int i = 0; i < StageCount; i++) _stageUs[i] = new FrameStatsBuffer(WindowCapacity);
        _stageAccumTicks = new long[StageCount];
        _lastStageUs = new long[StageCount];
        _profilerStartTimestamp = Stopwatch.GetTimestamp();
        _profilerStartUtc = DateTime.UtcNow;
        if (RenderingDiagnostics.FrameHistoryPath is not null)
            _history = new List<FrameHistoryRecord>(HistoryInitialCapacity);
    }

    /// <summary>
    /// Call once at the accepted render-transaction boundary, before
    /// <see cref="BeginGpuFrame"/>.
    /// </summary>
    public void FrameBoundary(GL gl)
    {
        bool enabled = RenderingDiagnostics.FrameProfEnabled;
        if (!enabled)
        {
            if (_wasEnabled)
            {
                // Dispose (not just Stop) so a later re-enable rebuilds the
                // query ring fresh — a kept instance would poll slots left
                // pending from BEFORE the pause and report temporally stale
                // GPU samples. Safe here: this runs at the top of OnRender
                // with the GL context current.
                _gpuTimer?.Dispose();
                _gpuTimer = null;
                _wasEnabled = false;
                _lastBoundaryTimestamp = 0;
                _currentFrameIndex = -1;
            }
            return;
        }

        if (_ownerThreadId == 0) _ownerThreadId = Environment.CurrentManagedThreadId;
        else if (!_threadWarned && _ownerThreadId != Environment.CurrentManagedThreadId)
        {
            _threadWarned = true;
            Console.WriteLine("[frame-prof] WARNING: frame boundary crossed threads; alloc counter is per-thread and now unreliable");
        }

        long now = Stopwatch.GetTimestamp();
        long allocNow = GC.GetAllocatedBytesForCurrentThread();

        if (!_wasEnabled)
        {
            // First enabled frame (startup or runtime toggle-on): establish
            // baselines, emit nothing. Clear any stage ticks a StageScope
            // disposed after toggle-off may have accumulated mid-pause —
            // EndStage still runs on scopes that were live when the flag
            // flipped, and that partial delta must not leak into the first
            // re-enabled frame.
            _wasEnabled = true;
            _lastReportTicks = DateTime.UtcNow.Ticks;
            Array.Clear(_stageAccumTicks);
            _gc0Base = GC.CollectionCount(0); _gc1Base = GC.CollectionCount(1); _gc2Base = GC.CollectionCount(2);
            if (_gpuTimer is null && !_wbDiagActive) _gpuTimer = new GpuFrameTimer(gl);
            _currentFrameIndex = 0;
            if (_wbDiagActive && !_wbDiagNoticePrinted)
            {
                _wbDiagNoticePrinted = true;
                Console.WriteLine("[frame-prof] GPU frame timing OFF: ACDREAM_WB_DIAG=1 owns TimeElapsed queries (nested queries are illegal GL)");
            }
        }
        else
        {
            long cpuUs = (now - _lastBoundaryTimestamp) * 1_000_000L / Stopwatch.Frequency;
            _cpuUs.Push(cpuUs);
            long allocDelta = allocNow - _lastAllocBytes;
            _allocBytes.Push(allocDelta);
            for (int i = 0; i < StageCount; i++)
            {
                long stageUs = _stageAccumTicks[i] * 1_000_000L / Stopwatch.Frequency;
                _stageUs[i].Push(stageUs);
                _lastStageUs[i] = stageUs;
                _stageAccumTicks[i] = 0;
            }
            _framesInWindow++;
            if (_history is not null)
            {
                _history.Add(new FrameHistoryRecord(
                    _currentFrameIndex,
                    (now - _profilerStartTimestamp) * 1000.0 / Stopwatch.Frequency,
                    cpuUs,
                    -1L,
                    allocDelta,
                    _lastStageUs[(int)FrameStage.Update],
                    _lastStageUs[(int)FrameStage.Upload],
                    _lastStageUs[(int)FrameStage.ImGui],
                    _lastStageUs[(int)FrameStage.Pacing]));
            }
            _currentFrameIndex++;
        }

        _lastBoundaryTimestamp = now;
        _lastAllocBytes = allocNow;

        long nowTicks = DateTime.UtcNow.Ticks;
        if (nowTicks - _lastReportTicks >= ReportIntervalTicks && _framesInWindow > 0)
        {
            int gc0 = GC.CollectionCount(0) - _gc0Base;
            int gc1 = GC.CollectionCount(1) - _gc1Base;
            int gc2 = GC.CollectionCount(2) - _gc2Base;
            LastReport = FormatReport(_framesInWindow, _cpuUs, _gpuUs,
                gpuActive: _gpuTimer is not null, _allocBytes, gc0, gc1, gc2, _stageUs);
            Console.WriteLine(LastReport);
            _lastReportTicks = nowTicks;
            _gc0Base += gc0; _gc1Base += gc1; _gc2Base += gc2;
            _framesInWindow = 0;
            _cpuUs.Reset(); _gpuUs.Reset(); _allocBytes.Reset();
            for (int i = 0; i < StageCount; i++) _stageUs[i].Reset();
        }
    }

    /// <summary>
    /// Begin GPU timing immediately before render-resource, world, and private
    /// presentation submission. Completed delayed samples are associated with
    /// their owning history row rather than the frame that happened to poll
    /// them.
    /// </summary>
    public void BeginGpuFrame()
    {
        if (!_wasEnabled || _gpuTimer is null || _currentFrameIndex < 0)
            return;

        Span<GpuFrameSample> completed = stackalloc GpuFrameSample[4];
        int completedCount = _gpuTimer.BeginFrame(_currentFrameIndex, completed);
        for (int index = 0; index < completedCount; index++)
        {
            GpuFrameSample sample = completed[index];
            _gpuUs.Push(sample.ElapsedUs);
            if (_history is not null
                && (uint)sample.FrameIndex < (uint)_history.Count)
            {
                FrameHistoryRecord row = _history[sample.FrameIndex];
                _history[sample.FrameIndex] = row with { GpuUs = sample.ElapsedUs };
            }
        }
    }

    /// <summary>End GPU timing immediately after render submission.</summary>
    public void EndGpuFrame()
    {
        _gpuTimer?.EndFrame();
    }

    /// <summary>
    /// Attribute the enclosed CPU time to <paramref name="stage"/>.
    /// Usage: <c>using var _ = profiler.BeginStage(FrameStage.Update);</c>.
    /// Zero-cost (default scope) when the profiler is off.
    /// </summary>
    public StageScope BeginStage(FrameStage stage)
        => RenderingDiagnostics.FrameProfEnabled
            ? new StageScope(this, stage, Stopwatch.GetTimestamp())
            : default;

    internal void EndStage(FrameStage stage, long startTimestamp)
        => _stageAccumTicks[(int)stage] += Stopwatch.GetTimestamp() - startTimestamp;

    /// <summary>Pure report formatter — unit-tested; invariant culture.</summary>
    public static string FormatReport(
        int frameCount,
        FrameStatsBuffer cpu, FrameStatsBuffer gpu, bool gpuActive,
        FrameStatsBuffer alloc, int gc0, int gc1, int gc2,
        FrameStatsBuffer[] stages)
    {
        var ci = CultureInfo.InvariantCulture;
        var sb = new StringBuilder(256);
        sb.Append("[frame-prof] n=").Append(frameCount);
        sb.AppendFormat(ci, " | cpu_ms p50={0:0.0} p95={1:0.0} p99={2:0.0} max={3:0.0}",
            cpu.Percentile(0.50) / 1000.0, cpu.Percentile(0.95) / 1000.0,
            cpu.Percentile(0.99) / 1000.0, cpu.Max() / 1000.0);
        if (gpuActive)
            sb.AppendFormat(ci, " | gpu_ms p50={0:0.0} p95={1:0.0}",
                gpu.Percentile(0.50) / 1000.0, gpu.Percentile(0.95) / 1000.0);
        else
            sb.Append(" | gpu=off(wbdiag)");
        sb.AppendFormat(ci, " | alloc_kb p50={0:0.0} max={1:0.0} gc={2}/{3}/{4}",
            alloc.Percentile(0.50) / 1024.0, alloc.Max() / 1024.0, gc0, gc1, gc2);
        string[] names = { "upd", "upl", "imgui", "pace" };
        for (int i = 0; i < stages.Length && i < names.Length; i++)
            sb.AppendFormat(ci, " | {0} p50={1:0.0} p95={2:0.0}",
                names[i], stages[i].Percentile(0.50) / 1000.0, stages[i].Percentile(0.95) / 1000.0);
        return sb.ToString();
    }

    /// <summary>Pure CSV formatter — unit-tested; invariant culture. One header row plus one row per record.</summary>
    internal static void WriteHistoryCsv(
        IEnumerable<FrameHistoryRecord> records,
        TextWriter writer,
        DateTime profilerStartUtc)
    {
        var ci = CultureInfo.InvariantCulture;
        DateTime startUtc = profilerStartUtc.ToUniversalTime();
        writer.WriteLine(
            "frame,timestamp_ms,timestamp_utc,cpu_us,gpu_us,alloc_bytes,"
            + "update_us,upload_us,imgui_us,pacing_us");
        foreach (FrameHistoryRecord r in records)
        {
            writer.Write(r.FrameIndex.ToString(ci)); writer.Write(',');
            writer.Write(r.TimestampMs.ToString("0.000", ci)); writer.Write(',');
            writer.Write(startUtc.AddMilliseconds(r.TimestampMs).ToString("O", ci));
            writer.Write(',');
            writer.Write(r.CpuUs.ToString(ci)); writer.Write(',');
            writer.Write(r.GpuUs.ToString(ci)); writer.Write(',');
            writer.Write(r.AllocBytes.ToString(ci)); writer.Write(',');
            writer.Write(r.UpdateUs.ToString(ci)); writer.Write(',');
            writer.Write(r.UploadUs.ToString(ci)); writer.Write(',');
            writer.Write(r.ImGuiUs.ToString(ci)); writer.Write(',');
            writer.WriteLine(r.PacingUs.ToString(ci));
        }
    }

    /// <summary>
    /// Shutdown-only write of the accumulated <see cref="_history"/> as CSV
    /// (the ONLY I/O this feature performs — never from <see cref="FrameBoundary"/>).
    /// Failures are logged, not thrown: a history-export problem must never
    /// block the rest of the render-owner shutdown chain
    /// (<c>GameWindowLifetime</c>'s <c>Hard("frame profiler", ...)</c> stage).
    /// </summary>
    public void Dispose()
    {
        _gpuTimer?.Dispose();
        if (_history is { Count: > 0 } && RenderingDiagnostics.FrameHistoryPath is { } path)
        {
            try
            {
                using var writer = new StreamWriter(path, append: false);
                WriteHistoryCsv(_history, writer, _profilerStartUtc);
                Console.WriteLine($"[frame-prof] wrote {_history.Count} history record(s) to '{path}'");
            }
            catch (Exception ex)
            {
                Console.WriteLine($"[frame-prof] WARNING: failed to write frame history to '{path}': {ex.Message}");
            }
        }
    }
}

/// <summary>Disposable stage scope; default instance is a no-op.</summary>
public readonly struct StageScope : IDisposable
{
    private readonly FrameProfiler? _owner;
    private readonly FrameStage _stage;
    private readonly long _start;

    internal StageScope(FrameProfiler owner, FrameStage stage, long start)
    {
        _owner = owner; _stage = stage; _start = start;
    }

    public void Dispose() => _owner?.EndStage(_stage, _start);
}