using System;
using System.Numerics;

namespace AcDream.Core.World;

/// <summary>
/// Client-local atmospheric regime. Retail AC has no server weather
/// opcode (r12 §6) — the client picks a state per in-game day via a
/// deterministic seeded RNG so all players on the same server see the
/// same weather without any packets. Transitions take ~10 seconds.
///
/// <para>
/// The rendering side reads <see cref="Kind"/> to decide whether to
/// spawn rain/snow particles and which cloud mesh override to select.
/// The <see cref="Intensity"/> field lets the fog / particle rate /
/// cloud-darkness terms ease in and out smoothly rather than popping.
/// </para>
/// </summary>
public enum WeatherKind
{
    Clear    = 0,
    Overcast = 1,
    Rain     = 2,
    Snow     = 3,
    Storm    = 4,
}

/// <summary>
/// Server-forced fog override (retail <c>EnvironChangeType</c>). When
/// the server sends <c>AdminEnvirons</c> (0xEA60) with one of the
/// non-<see cref="None"/> values, the client overrides its locally-computed
/// fog color and density with the tint shown below. See r12 §5.2 and
/// <c>references/ACE/Source/ACE.Entity/Enum/EnvironChangeType.cs</c>.
/// </summary>
public enum EnvironOverride
{
    None       = 0x00,   // clear override, revert to dat-driven fog
    RedFog     = 0x01,
    BlueFog    = 0x02,
    WhiteFog   = 0x03,
    GreenFog   = 0x04,
    BlackFog   = 0x05,
    BlackFog2  = 0x06,
}

/// <summary>
/// Full per-frame atmosphere state consumed by the shader + particle
/// systems. Built by <see cref="WeatherSystem"/> from
/// <list type="bullet">
///   <item><description>the interpolated <see cref="SkyKeyframe"/>,</description></item>
///   <item><description>the current <see cref="WeatherKind"/>,</description></item>
///   <item><description>a possibly-active <see cref="EnvironOverride"/>,</description></item>
///   <item><description>a transient lightning-flash bump.</description></item>
/// </list>
/// </summary>
public readonly record struct AtmosphereSnapshot(
    WeatherKind   Kind,
    float         Intensity,          // 0..1, eases on state transitions
    Vector3       FogColor,           // final fog color (may be overridden)
    float         FogStart,
    float         FogEnd,
    FogMode       FogMode,
    float         LightningFlash,     // 0..1, decays from strike moment
    EnvironOverride Override);

/// <summary>
/// Weather state machine — deterministic per-day RNG picks the weather
/// kind; a 10-second ease blends fog + particle density between old
/// and new states. Also owns the lightning-flash timer for storms.
///
/// <para>
/// Algorithm (r12 §6.1–6.2):
/// <list type="number">
///   <item><description>
///     Derive a deterministic <c>Random(dayIndex)</c> per in-game day.
///     Roll a weighted pick from a table matching retail's rough
///     70/15/10/5 distribution (Clear dominates).
///   </description></item>
///   <item><description>
///     When the kind changes, store a <c>transitionStart</c> timestamp
///     and tween <see cref="AtmosphereSnapshot.Intensity"/> from 0 → 1
///     over <see cref="TransitionSeconds"/>.
///   </description></item>
///   <item><description>
///     Storm kind only: every 8–30 seconds fire a lightning flash; the
///     shader reads <see cref="AtmosphereSnapshot.LightningFlash"/> as
///     an additive scene bump that decays with a 200 ms time constant.
///   </description></item>
///   <item><description>
///     Any server <see cref="EnvironOverride"/> beats the local picks —
///     stick the override fog color and density in the snapshot until
///     the server sends <see cref="EnvironOverride.None"/>.
///   </description></item>
/// </list>
/// </para>
/// </summary>
public sealed class WeatherSystem
{
    /// <summary>
    /// Kept as a public constant because a handful of callers / tests
    /// reference it, but unused internally post-Phase-7: retail does
    /// not cross-fade between <see cref="WeatherKind"/>s (no such
    /// concept in the decompile). The SkyTimeOfDay keyframe interp
    /// does all time-based fog/light blending directly.
    /// </summary>
    public const float TransitionSeconds = 10f;

    // Flash decay kept so TriggerFlash() is still a usable test hook;
    // production code (PlayScript-driven lightning, Phase 6) does NOT
    // drive the flash uniform — it spawns particle emitters directly.
    private const float FlashDecay     = 1f / 0.200f;   // 1 / τ sec
    private const float FlashPeakHoldS = 0.05f;

    private WeatherKind _kind         = WeatherKind.Clear;
    private WeatherKind _previousKind = WeatherKind.Clear;

    private float _flashLevel;
    private float _flashAge;

    private EnvironOverride _override;

    private int _rolledDayIndex = int.MinValue;

    // Phase 3e — when GameWindow pushes the retail DayGroup name via
    // SetKindFromDayGroupName, the internal RollKind hash is disabled.
    private bool _externallyDriven;

    public WeatherSystem(Random? rng = null)
    {
        // The random-seed ctor argument remains for test API compat,
        // but no longer drives any production behaviour (Phase 7: the
        // Storm-kind random lightning timer was deleted — retail is
        // server-driven via PlayScript; see Agents #3 and #5).
        _ = rng;
    }

    /// <summary>Current active weather.</summary>
    public WeatherKind Kind => _kind;

    /// <summary>Last-known server fog override (sticky between sync packets).</summary>
    public EnvironOverride Override
    {
        get => _override;
        set => _override = value;
    }

    /// <summary>
    /// Debug / test hook — force a specific weather kind, ignoring the
    /// per-day roll. Passing <see cref="WeatherKind.Clear"/> returns to
    /// normal behavior starting on the next day-roll.
    /// </summary>
    public void ForceWeather(WeatherKind kind)
    {
        BeginTransition(kind);
        _rolledDayIndex = int.MaxValue;  // "forced" sentinel — don't re-roll
    }

    /// <summary>
    /// Drive the weather kind from the active retail DayGroup name
    /// (see <c>SkyDesc::PickCurrentDayGroup</c> port at
    /// <c>LoadedSkyDesc.SelectDayGroupIndex</c>). Retail has ONE source
    /// of truth for weather — the DayGroup roll — so this replaces the
    /// internal <see cref="RollKind"/> hash once the real DayGroup picker
    /// is live. Cases are loose substring matches (Dereth day groups use
    /// names like "Sunny", "Clear", "Cloudy", "Rainy", "Stormy", "Snowy"
    /// per the region dat dump 2026-04-23).
    ///
    /// <para>
    /// Once called at least once, the internal auto-roll in
    /// <see cref="Tick"/> is DISABLED for the rest of the session —
    /// control is now external. Tests that drive <see cref="Tick"/>
    /// directly without calling this method remain on the legacy hash
    /// roll unchanged.
    /// </para>
    /// </summary>
    public void SetKindFromDayGroupName(string? dayGroupName)
    {
        _externallyDriven = true;
        WeatherKind mapped = MapDayGroupNameToKind(dayGroupName);
        if (mapped != _kind) BeginTransition(mapped);
    }

    private static WeatherKind MapDayGroupNameToKind(string? name)
    {
        if (string.IsNullOrWhiteSpace(name)) return WeatherKind.Clear;
        string lc = name.ToLowerInvariant();
        // Retail DOES NOT spawn rain/snow/storm particles based on the
        // DayGroup's NAME. Parallel decompile research 2026-04-23
        // (docs/research/2026-04-23-sky-pes-wiring.md +
        //  docs/research/2026-04-23-physicsscript.md) verified:
        //
        //   1. FUN_00508010 (the sky render loop) never reads
        //      SkyObject.DefaultPesObjectId — the field is dead at
        //      render time.
        //   2. The PhysicsScript runtime (FUN_0051bed0 → FUN_0051bfb0)
        //      has no callers from the sky-render tree.
        //   3. r12 deepdive claim that retail spawns rain from a sky
        //      SkyObject's PES was not corroborated by the decompile.
        //
        // Weather particle emission in retail therefore belongs to a
        // SEPARATE camera-attached subsystem, not yet located. Until we
        // find and port that subsystem, we must NOT invent our own
        // "Rainy DayGroup name → spawn rain particles" path — it produced
        // the user-observed regression 2026-04-23 (acdream rained on a
        // DayGroup that retail rendered without any rain particles).
        //
        // Therefore ALL weathery names map to Overcast — they get the
        // correct keyframe-driven fog/cloud tone, without the particle
        // emitter. Clear names stay Clear. No Rain / Snow / Storm is
        // ever returned from name matching. Tests kept for Storm/Rain
        // constants since ForceWeather still supports them for debug.
        if (lc.Contains("storm")
         || lc.Contains("snow")
         || lc.Contains("rain")
         || lc.Contains("cloud")
         || lc.Contains("overcast")
         || lc.Contains("dark")
         || lc.Contains("fog"))      return WeatherKind.Overcast;
        return WeatherKind.Clear;    // "Sunny", "Clear", anything else
    }

    /// <summary>
    /// Advance the state machine. Call once per frame from the render
    /// loop. <paramref name="dayIndex"/> is the in-game day (derived
    /// from <see cref="DerethDateTime"/>); when it changes we re-roll
    /// the weather kind.
    /// </summary>
    public void Tick(double nowSeconds, int dayIndex, float dtSeconds)
    {
        // Phase 7 — dropped:
        //   - per-Kind cross-fade (_transitionT drove the now-removed
        //     FogForKind lerp; retail has no such machinery).
        //   - Storm-kind random lightning timer (retail lightning is
        //     server-driven via PlayScript per Agent #5 — purely visual
        //     through the particle system, no UBO flash channel).
        //
        // What remains: day-index auto-roll as a TEST-ONLY fallback
        // (externally driven callers set _externallyDriven=true through
        // SetKindFromDayGroupName and this block never fires), plus
        // flash-level decay so the TriggerFlash() test hook still works.

        // Day changed → re-roll (fallback only — disabled when externally driven).
        if (!_externallyDriven
            && dayIndex != _rolledDayIndex
            && _rolledDayIndex != int.MaxValue)
        {
            _rolledDayIndex = dayIndex;
            var newKind = RollKind(dayIndex);
            if (newKind != _kind) BeginTransition(newKind);
        }

        // Flash decay — 50ms hold then exponential decay (~200ms τ).
        // Production never TriggerFlashes; this exists for tests that
        // exercise the UBO channel.
        if (_flashLevel > 0f)
        {
            _flashAge += dtSeconds;
            if (_flashAge < FlashPeakHoldS)
                _flashLevel = 1f;
            else
                _flashLevel = MathF.Exp(-(_flashAge - FlashPeakHoldS) * FlashDecay);
            if (_flashLevel < 1e-3f) _flashLevel = 0f;
        }
    }

    /// <summary>
    /// Trigger a lightning flash manually (server-forced or test hook).
    /// </summary>
    public void TriggerFlash()
    {
        _flashLevel = 1f;
        _flashAge = 0f;
    }

    /// <summary>
    /// Produce the per-frame atmosphere snapshot from the sky keyframe.
    ///
    /// <para>
    /// <b>Retail-faithful since Phase 7 (2026-04-23):</b> fog is the
    /// keyframe's fog, passed through directly (color + distances).
    /// The only override channel is <see cref="EnvironOverride"/> set
    /// by the server's <c>AdminEnvirons</c> packet (opcode 0xEA60) —
    /// in that case we substitute the fog COLOR with the preset tint
    /// and keep the keyframe's distances untouched. There is no
    /// per-<see cref="WeatherKind"/> fog manipulation: retail's
    /// decompile (Agent #3, 2026-04-23) contains no such logic. The
    /// <see cref="WeatherKind"/> enum is now purely informational — it
    /// labels the current sky style for debug overlays but doesn't
    /// drive any rendering.
    /// </para>
    /// </summary>
    public AtmosphereSnapshot Snapshot(in SkyKeyframe kf)
    {
        // Fog passthrough from the keyframe (retail semantics).
        Vector3 fogColor = kf.FogColor;
        float   fogStart = kf.FogStart;
        float   fogEnd   = kf.FogEnd;

        // AdminEnvirons server override: replace fog COLOR only.
        // Keyframe distances unchanged until we find evidence retail
        // changes those too (Agent #3 notes the in-game crossfade
        // lerps distances via SkyTimeOfDay keyframe interp, NOT via
        // AdminEnvirons directly).
        if (_override != EnvironOverride.None)
            fogColor = EnvironOverrideColor(_override);

        return new AtmosphereSnapshot(
            Kind:            _kind,             // informational
            Intensity:       1f,                // no per-Kind easing in retail
            FogColor:        fogColor,
            FogStart:        fogStart,
            FogEnd:          fogEnd,
            FogMode:         kf.FogMode,
            LightningFlash:  _flashLevel,       // 0 in production; TriggerFlash hook for tests
            Override:        _override);
    }

    // ----------------------------------------------------------------
    // Internal machinery
    // ----------------------------------------------------------------

    private void BeginTransition(WeatherKind newKind)
    {
        _previousKind = _kind;
        _kind = newKind;
    }

    /// <summary>
    /// Deterministic per-day weighted roll. Seeded with <paramref name="dayIndex"/>
    /// alone so every client running the same day sees the same weather —
    /// retail's mechanism for "synchronized weather without any packets"
    /// (r12 §6.1).
    /// </summary>
    private static WeatherKind RollKind(int dayIndex)
    {
        // Mix the day index so consecutive days aren't adjacent in PRNG
        // state space (avoids tiny-seed correlation issues).
        int seed = unchecked((int)((uint)dayIndex * 0x9E3779B1u));
        var rng = new Random(seed);
        double r = rng.NextDouble();
        // Retail weights (approximate): 60% clear, 20% overcast, 12% rain,
        // 5% snow, 3% storm. Tuned for "most days are fine, some are bad."
        if (r < 0.60) return WeatherKind.Clear;
        if (r < 0.80) return WeatherKind.Overcast;
        if (r < 0.92) return WeatherKind.Rain;
        if (r < 0.97) return WeatherKind.Snow;
        return WeatherKind.Storm;
    }

    private static Vector3 EnvironOverrideColor(EnvironOverride o) => o switch
    {
        EnvironOverride.RedFog    => new Vector3(0.60f, 0.05f, 0.05f),
        EnvironOverride.BlueFog   => new Vector3(0.08f, 0.15f, 0.60f),
        EnvironOverride.WhiteFog  => new Vector3(0.90f, 0.90f, 0.92f),
        EnvironOverride.GreenFog  => new Vector3(0.08f, 0.55f, 0.12f),
        EnvironOverride.BlackFog  => new Vector3(0.02f, 0.02f, 0.02f),
        EnvironOverride.BlackFog2 => new Vector3(0.04f, 0.01f, 0.01f),
        _                         => new Vector3(1f, 1f, 1f),
    };
}