using System;
using Silk.NET.OpenGL;

namespace AcDream.App.Diagnostics;

/// <summary>
/// MP0 (2026-07-05) — whole-frame GPU time via a ring of
/// <see cref="QueryTarget.TimeElapsed"/> queries (depth 4, so results are
/// read ~3 frames late and never stall). Mirrors WbDrawDispatcher's query
/// idiom including the #125 lesson: a glGenQueries name is not a query
/// OBJECT until first glBeginQuery, so never-begun slots are skipped via
/// the Begun flags.
///
/// <para>MUST NOT be active while ACDREAM_WB_DIAG=1: GL forbids two
/// simultaneously active TimeElapsed queries and WbDrawDispatcher brackets
/// its passes with them under that flag. The caller (FrameProfiler)
/// enforces the exclusion; this class just does the ring.</para>
/// </summary>
internal sealed class GpuFrameTimer : IDisposable
{
    private const int RingDepth = 4;

    private readonly GL _gl;
    private readonly uint[] _queries = new uint[RingDepth];
    private readonly bool[] _begun = new bool[RingDepth];
    private int _frameIndex;
    private bool _queryActive;

    public GpuFrameTimer(GL gl)
    {
        _gl = gl ?? throw new ArgumentNullException(nameof(gl));
        for (int i = 0; i < RingDepth; i++)
            _queries[i] = _gl.GenQuery();
    }

    /// <summary>
    /// Call once per frame at the frame boundary. Ends the previous
    /// frame's query, polls the oldest slot non-blocking, begins this
    /// frame's query. Returns the completed GPU time in microseconds for
    /// a ~RingDepth-frames-old frame, or null when no result is ready.
    /// </summary>
    public long? FrameBoundary()
    {
        if (_queryActive)
        {
            _gl.EndQuery(QueryTarget.TimeElapsed);
            _queryActive = false;
        }

        long? completedUs = null;
        int readSlot = _frameIndex % RingDepth;   // about to be reused — oldest
        if (_begun[readSlot])
        {
            _gl.GetQueryObject(_queries[readSlot], QueryObjectParameterName.ResultAvailable, out int avail);
            if (avail != 0)
            {
                _gl.GetQueryObject(_queries[readSlot], QueryObjectParameterName.Result, out ulong ns);
                completedUs = (long)(ns / 1000UL);
            }
            // Not available ⇒ sample silently dropped (same policy as
            // WbDrawDispatcher) — percentiles tolerate missing samples.
        }

        _gl.BeginQuery(QueryTarget.TimeElapsed, _queries[readSlot]);
        _begun[readSlot] = true;
        _queryActive = true;
        _frameIndex++;
        return completedUs;
    }

    /// <summary>End any active query without beginning a new one (used when the profiler is toggled off mid-session).</summary>
    public void Stop()
    {
        if (_queryActive)
        {
            _gl.EndQuery(QueryTarget.TimeElapsed);
            _queryActive = false;
        }
    }

    public void Dispose()
    {
        Stop();
        for (int i = 0; i < RingDepth; i++)
            _gl.DeleteQuery(_queries[i]);
    }
}