using System;
using System.Collections.Generic;
using System.Numerics;

namespace AcDream.Core.World;

/// <summary>
/// Fog modes mirroring retail's <c>D3DFOGMODE</c>. Retail only ever uses
/// <see cref="Off"/> and <see cref="Linear"/>; the Exp variants are
/// supported by the dat schema but never appear in shipped data. See r12
/// §5 and <c>SkyTimeOfDay.WorldFog</c> (dat <c>uint</c>).
/// </summary>
public enum FogMode
{
    Off    = 0,
    Linear = 1,
    Exp    = 2,
    Exp2   = 3,
}

/// <summary>
/// One sky keyframe — the full lighting + fog state for a specific
/// day-fraction. Multiple keyframes across <c>[0, 1)</c> interpolate
/// linearly (with angular-shortest-arc wrap on sun direction) to produce
/// the current sky state.
///
/// <para>
/// Retail's <c>SkyTimeOfDay</c> dat struct carries this exact data plus
/// references to sky objects (sun mesh, moon mesh, cloud layer) which
/// belong to the renderer. This record exposes the shader-relevant
/// subset — sun direction, sun color, ambient color, linear fog. See
/// <c>references/DatReaderWriter/DatReaderWriter/Generated/Types/SkyTimeOfDay.generated.cs</c>
/// and r12 §4 + §5.
/// </para>
///
/// <para>
/// Colors are stored RAW (NOT pre-multiplied by brightness) in
/// <see cref="DirColor"/> / <see cref="AmbColor"/> with the brightness
/// scalars in <see cref="DirBright"/> / <see cref="AmbBright"/>. Retail's
/// <c>SkyDesc::GetLighting</c> at <c>0x00500ac9</c> (decomp lines
/// 261317-261331) lerps each channel separately and lerps brightness
/// separately, then multiplies post-lerp. Lerping the pre-multiplied
/// product gives mathematically different results when both color and
/// brightness change between adjacent keyframes — the cause of subtle
/// brightness discrepancies vs retail observed in dual-client
/// comparisons (Issue #3 visual sub-bug, 2026-04-27).
/// </para>
/// <para>
/// The computed properties <see cref="SunColor"/> and
/// <see cref="AmbientColor"/> return the post-multiplied product, so
/// downstream shader uniform plumbing (sky.vert / mesh.vert /
/// SceneLightingUbo) is unchanged.
/// </para>
/// </summary>
public readonly record struct SkyKeyframe(
    float   Begin,              // [0, 1] day-fraction this keyframe kicks in
    float   SunHeadingDeg,      // compass heading (0=N, 90=E, 180=S, 270=W)
    float   SunPitchDeg,        // elevation above horizon (-90=below, +90=zenith)
    Vector3 DirColor,           // RGB linear, RAW (NOT × DirBright)
    float   DirBright,          // sun brightness multiplier
    Vector3 AmbColor,           // RGB linear, RAW (NOT × AmbBright)
    float   AmbBright,          // ambient brightness multiplier
    Vector3 FogColor,
    float   FogDensity,         // retained for tests; derive from FogStart/End
    float   FogStart = 80f,     // meters (retail default ~120 clear, ~40 storm)
    float   FogEnd   = 350f,    // meters (retail default ~350 clear, ~150 storm)
    FogMode FogMode  = FogMode.Linear)
{
    /// <summary>
    /// Final directional sun color used by the shader =
    /// <see cref="DirColor"/> × <see cref="DirBright"/>. Computed property
    /// so the storage stays as separate channels (for retail-faithful
    /// keyframe interpolation) while the shader interface stays simple.
    /// </summary>
    public Vector3 SunColor => DirColor * DirBright;

    /// <summary>
    /// Final ambient color used by the shader =
    /// <see cref="AmbColor"/> × <see cref="AmbBright"/>. See
    /// <see cref="SunColor"/> for the rationale.
    /// </summary>
    public Vector3 AmbientColor => AmbColor * AmbBright;
}

/// <summary>
/// Sky keyframe interpolator — given a day fraction in [0, 1), returns
/// the blended lighting state between the surrounding keyframes.
///
/// <para>
/// Math (r12 §4):
/// <list type="number">
///   <item><description>
///     Pick the two keyframes bracketing <c>t</c>: <c>k1</c> = last
///     keyframe with <c>Begin &lt;= t</c>, <c>k2</c> = next keyframe
///     (wraps: if <c>k1</c> is last, <c>k2</c> is first).
///   </description></item>
///   <item><description>
///     Local blend <c>u = (t - k1.Begin) / (k2.Begin - k1.Begin)</c>
///     with wrap handling.
///   </description></item>
///   <item><description>
///     Lerp every vector component; use shortest-arc lerp for the sun
///     heading so k1=350° → k2=10° doesn't sweep backwards across the sky.
///   </description></item>
/// </list>
/// </para>
/// </summary>
public sealed class SkyStateProvider
{
    private readonly List<SkyKeyframe> _keyframes;

    public SkyStateProvider(IReadOnlyList<SkyKeyframe> keyframes)
    {
        if (keyframes is null || keyframes.Count == 0)
            throw new ArgumentException("At least one keyframe required", nameof(keyframes));
        // Sort by Begin so the walk is deterministic regardless of input order.
        var sorted = new List<SkyKeyframe>(keyframes);
        sorted.Sort((a, b) => a.Begin.CompareTo(b.Begin));
        _keyframes = sorted;
    }

    public int KeyframeCount => _keyframes.Count;
    public IReadOnlyList<SkyKeyframe> Keyframes => _keyframes;

    /// <summary>
    /// Default keyframe set based on retail observations — sunrise at 6am,
    /// noon at 12pm, sunset at 6pm. Used when the dat-loaded set isn't
    /// available yet or the player is in a region whose Region dat
    /// doesn't override it.
    ///
    /// <para>
    /// Fog values approximate retail clear-weather defaults: ~80m..~350m
    /// linear fog with color matching the horizon band so mountains at
    /// distance fade into the sky instead of popping at the clip plane.
    /// See r12 §5.1.
    /// </para>
    /// </summary>
    public static SkyStateProvider Default()
    {
        // Day fractions: 0.0=midnight, 0.25=dawn, 0.5=noon, 0.75=dusk.
        return new SkyStateProvider(new[]
        {
            // Default factory: brightness scalars are 1.0 here — the colors
            // ARE the final intended values. Live Dereth keyframes loaded
            // from the dat have separate non-1.0 DirBright/AmbBright values
            // and the renderer multiplies them post-lerp.
            new SkyKeyframe(
                Begin: 0.0f,
                SunHeadingDeg: 0f,     // below horizon (north)
                SunPitchDeg:   -30f,
                DirColor:      new Vector3(0.02f, 0.02f, 0.08f), // deep blue
                DirBright:     1.0f,
                AmbColor:      new Vector3(0.05f, 0.05f, 0.12f),
                AmbBright:     1.0f,
                FogColor:      new Vector3(0.02f, 0.02f, 0.05f),
                FogDensity:    0.004f,
                FogStart:      30f,
                FogEnd:        180f,
                FogMode:       FogMode.Linear),
            new SkyKeyframe(
                Begin: 0.25f,
                SunHeadingDeg: 90f,    // east at dawn
                SunPitchDeg:   0f,
                DirColor:      new Vector3(1.0f, 0.7f, 0.4f),    // sunrise warm
                DirBright:     1.0f,
                AmbColor:      new Vector3(0.4f, 0.35f, 0.3f),
                AmbBright:     1.0f,
                FogColor:      new Vector3(0.8f, 0.55f, 0.4f),
                FogDensity:    0.002f,
                FogStart:      60f,
                FogEnd:        260f,
                FogMode:       FogMode.Linear),
            new SkyKeyframe(
                Begin: 0.5f,
                SunHeadingDeg: 180f,   // south at noon
                SunPitchDeg:   70f,
                DirColor:      new Vector3(1.0f, 0.98f, 0.95f),  // bright white-ish
                DirBright:     1.0f,
                AmbColor:      new Vector3(0.5f, 0.5f, 0.55f),
                AmbBright:     1.0f,
                FogColor:      new Vector3(0.7f, 0.75f, 0.85f),
                FogDensity:    0.0008f,
                FogStart:      120f,
                FogEnd:        500f,
                FogMode:       FogMode.Linear),
            new SkyKeyframe(
                Begin: 0.75f,
                SunHeadingDeg: 270f,   // west at dusk
                SunPitchDeg:   0f,
                DirColor:      new Vector3(0.95f, 0.4f, 0.25f),  // sunset red
                DirBright:     1.0f,
                AmbColor:      new Vector3(0.35f, 0.25f, 0.25f),
                AmbBright:     1.0f,
                FogColor:      new Vector3(0.85f, 0.45f, 0.35f),
                FogDensity:    0.002f,
                FogStart:      60f,
                FogEnd:        260f,
                FogMode:       FogMode.Linear),
        });
    }

    /// <summary>
    /// Current interpolated sky state at day fraction <paramref name="t"/>.
    /// Wraps correctly across the day boundary (1.0 → 0.0).
    /// </summary>
    public SkyKeyframe Interpolate(float t)
    {
        t = (float)(t - Math.Floor(t));   // wrap to [0, 1)

        // Find k1: last keyframe with Begin <= t.
        int k1Index = _keyframes.Count - 1;
        for (int i = 0; i < _keyframes.Count; i++)
        {
            if (_keyframes[i].Begin <= t)
                k1Index = i;
            else
                break;
        }
        int k2Index = (k1Index + 1) % _keyframes.Count;

        var k1 = _keyframes[k1Index];
        var k2 = _keyframes[k2Index];

        // Compute blend weight, handling wrap (k1 is last, k2 is first).
        float k1Begin = k1.Begin;
        float k2Begin = k2.Begin;
        if (k2Begin <= k1Begin) k2Begin += 1.0f;  // unroll wrap
        float tWrapped = t;
        if (tWrapped < k1Begin) tWrapped += 1.0f;

        float span = Math.Max(1e-6f, k2Begin - k1Begin);
        float u = (tWrapped - k1Begin) / span;
        u = Math.Clamp(u, 0f, 1f);

        // Angular lerp for sun heading: pick shortest arc.
        float heading = ShortestAngleLerp(k1.SunHeadingDeg, k2.SunHeadingDeg, u);

        // Retail-faithful interpolation: lerp DirColor / DirBright /
        // AmbColor / AmbBright as SEPARATE CHANNELS, not as the
        // pre-multiplied product. Mirrors SkyDesc::GetLighting at
        // 0x00500ac9 (decomp lines 261317-261331). The post-multiplied
        // SunColor / AmbientColor are computed properties on the result.
        // Fog mode doesn't interpolate — pick k1's mode (retail uses
        // Linear everywhere).
        return new SkyKeyframe(
            Begin:         t,
            SunHeadingDeg: heading,
            SunPitchDeg:   Lerp(k1.SunPitchDeg, k2.SunPitchDeg, u),
            DirColor:      Vector3.Lerp(k1.DirColor, k2.DirColor, u),
            DirBright:     Lerp(k1.DirBright, k2.DirBright, u),
            AmbColor:      Vector3.Lerp(k1.AmbColor, k2.AmbColor, u),
            AmbBright:     Lerp(k1.AmbBright, k2.AmbBright, u),
            FogColor:      Vector3.Lerp(k1.FogColor, k2.FogColor, u),
            FogDensity:    Lerp(k1.FogDensity, k2.FogDensity, u),
            FogStart:      Lerp(k1.FogStart, k2.FogStart, u),
            FogEnd:        Lerp(k1.FogEnd, k2.FogEnd, u),
            FogMode:       k1.FogMode);
    }

    private static float Lerp(float a, float b, float u) => a + (b - a) * u;

    /// <summary>
    /// Shortest-arc heading lerp: r12 §4. If <c>a=350</c> and <c>b=10</c>
    /// the lerp walks 20° forward through 0° rather than 340° backward.
    /// </summary>
    public static float ShortestAngleLerp(float aDeg, float bDeg, float u)
    {
        float delta = bDeg - aDeg;
        while (delta > 180f)  delta -= 360f;
        while (delta < -180f) delta += 360f;
        return aDeg + delta * u;
    }

    /// <summary>
    /// World-space sun direction unit vector pointing FROM the surface
    /// TOWARDS the sun. Derived from heading + pitch in the returned
    /// keyframe — shader sunDir uniform should use -this so lighting
    /// math (N·L) works correctly for the side facing the sun.
    /// </summary>
    public static Vector3 SunDirectionFromKeyframe(SkyKeyframe kf)
    {
        float yaw = kf.SunHeadingDeg * (MathF.PI / 180f);
        float pit = kf.SunPitchDeg * (MathF.PI / 180f);
        // Heading 0 = +Y (north), +X=east. Pitch up from horizon.
        float cosP = MathF.Cos(pit);
        return new Vector3(
            MathF.Sin(yaw) * cosP,
            MathF.Cos(yaw) * cosP,
            MathF.Sin(pit));
    }
}

/// <summary>
/// Service that turns server-delivered tick counts into live sky state.
/// Owns the "current time" clock (seeded from server sync, advanced by
/// real-time elapsed between syncs).
///
/// <para>
/// Supports a debug "time override" (slash-command <c>/time 0.5</c>) that
/// forces a specific day fraction regardless of server sync — used for
/// screenshots and visual debugging. The override is transient and gets
/// cleared on the next TimeSync packet.
/// </para>
/// </summary>
public sealed class WorldTimeService
{
    private SkyStateProvider _sky;
    private double _lastSyncedTicks;
    private DateTime _lastSyncedWallClockUtc = DateTime.UtcNow;

    private float? _debugDayFractionOverride;

    /// <summary>
    /// Rate at which in-game time advances relative to real time. Retail
    /// default is 1.0 (one wall-clock second = one in-game tick). Server
    /// config can override via <c>SkyDesc.TickSize</c>; see r12 §1.2.
    /// </summary>
    public double TickSize { get; set; } = 1.0;

    public WorldTimeService(SkyStateProvider sky)
    {
        _sky = sky ?? throw new ArgumentNullException(nameof(sky));
    }

    /// <summary>
    /// Hot-swap the keyframe source — typically called once at world-load
    /// time after the Region dat has been parsed by <see cref="SkyDescLoader"/>.
    /// </summary>
    public void SetProvider(SkyStateProvider sky)
    {
        _sky = sky ?? throw new ArgumentNullException(nameof(sky));
    }

    /// <summary>
    /// Set the authoritative tick count from a server TimeSync packet.
    /// Clears any debug override.
    /// </summary>
    public void SyncFromServer(double serverTicks)
    {
        _lastSyncedTicks = serverTicks;
        _lastSyncedWallClockUtc = System.DateTime.UtcNow;
        _debugDayFractionOverride = null;

        if (System.Environment.GetEnvironmentVariable("ACDREAM_DUMP_SKY") == "1")
        {
            var df = DerethDateTime.DayFraction(serverTicks);
            var cal = DerethDateTime.ToCalendar(serverTicks);
            System.Console.WriteLine(
                $"[sky-dump] SyncFromServer: ticks={serverTicks:F1} dayFraction={df:F4} " +
                $"calendar=PY{cal.Year} {cal.Month} {cal.Day} {cal.Hour}");
        }
    }

    /// <summary>
    /// Debug-only: force a specific day fraction in [0, 1). Overrides
    /// server-synced time until cleared by <see cref="SyncFromServer"/>
    /// or <see cref="ClearDebugTime"/>.
    /// </summary>
    public void SetDebugTime(float dayFraction)
    {
        _debugDayFractionOverride = dayFraction;
    }

    public void ClearDebugTime() => _debugDayFractionOverride = null;

    /// <summary>
    /// Current ticks at <see cref="DateTime.UtcNow"/>, advanced from the
    /// last sync by real-time elapsed seconds times <see cref="TickSize"/>.
    /// </summary>
    public double NowTicks
    {
        get
        {
            double elapsed = (DateTime.UtcNow - _lastSyncedWallClockUtc).TotalSeconds;
            return _lastSyncedTicks + elapsed * TickSize;
        }
    }

    /// <summary>Current day fraction in [0, 1).</summary>
    public double DayFraction
    {
        get
        {
            if (_debugDayFractionOverride.HasValue)
                return _debugDayFractionOverride.Value;
            return DerethDateTime.DayFraction(NowTicks);
        }
    }

    /// <summary>Current sky lighting state.</summary>
    public SkyKeyframe CurrentSky => _sky.Interpolate((float)DayFraction);

    /// <summary>Convenience: current sun direction from derived sky state.</summary>
    public Vector3 CurrentSunDirection =>
        SkyStateProvider.SunDirectionFromKeyframe(CurrentSky);

    public DerethDateTime.Calendar CurrentCalendar =>
        DerethDateTime.ToCalendar(NowTicks);

    public bool IsDaytime => DerethDateTime.IsDaytime(NowTicks);
}