using System.Collections.Generic;
using System.Numerics;
using AcDream.Core.Physics;
using AcDream.Core.World;

namespace AcDream.App.Rendering;

/// <summary>
/// Owns the chargen preview's per-frame idle-loop ↔ rest-pose playback,
/// mirroring <c>gmCG3DView::StartAnimation</c>/<c>StopAnimation</c>'s swap
/// (<c>0x004EE600</c>/<c>0x004EE640</c>) and
/// <c>gmCGAppearancePage::ZoomIn</c>/<c>ZoomOut</c>'s immediate call into it
/// (<c>0x0047D024</c>/<c>0x0047D160</c> — the swap happens the instant the
/// button is pressed, NOT once the camera's own 0.6s tween finishes).
///
/// <para>
/// <b>Retail default is idle-PLAYING, not frozen</b> — see
/// <see cref="ChargenPreviewEntityBuilder"/>'s class doc for the decomp
/// citations. This class's own default (<see cref="IsZoomedIn"/> starts
/// <c>false</c>) reproduces that: its constructor immediately plays the
/// idle animation's frame 0 when one resolved, matching
/// <c>gmCGAppearancePage::Update</c>'s own trailing
/// <c>if (m_bZoomedIn == 0) StartAnimation()</c> gate
/// (~0x0047EF01-0x0047EF12), which re-fires on every heritage/gender/
/// appearance change too — <see cref="SetZoomedIn"/> restarts the idle loop
/// at frame 0 on every transition INTO the playing state for the same
/// reason: <c>set_sequence_animation</c>'s <c>arg3=1</c> clears the sequence
/// before appending, so every <c>StartAnimation</c> call restarts the clip.
/// The DEFAULT-false claim itself rests on <c>gmCGAppearancePage::InitializePage
/// @ 0x0047FDD0</c>'s explicit <c>this-&gt;m_bZoomedIn = 0;</c> at
/// <c>0x004802C3</c> — written immediately after that same function sets the
/// camera to the zoomed-IN per-heritage eye (<c>0x00480286-0x0048029E</c>),
/// not from the ctor simply never touching the field (heap <c>operator new</c>
/// memory is indeterminate, not zero — that argument doesn't hold on its
/// own). One retail quirk this implies: the character starts framed close-up
/// AND not-zoomed-in at the same time, so the FIRST Zoom In click (once
/// mounted) tweens close-eye→close-eye — visually null — while still
/// freezing the animation; the port reproduces this faithfully rather than
/// treating it as a bug.
/// </para>
///
/// <para>
/// The page-mount half (CC6b, after CC4 merges) wires the Zoom In/Out
/// buttons to <see cref="SetZoomedIn"/> and the render loop to
/// <see cref="Tick"/>; nothing in this repository calls either yet.
/// </para>
/// </summary>
internal sealed class ChargenPreviewAnimator
{
    /// <summary>
    /// <c>gmCG3DView::StartAnimation</c>'s literal framerate argument
    /// (<c>set_sequence_animation(this->m_pPlayerObject,
    /// this->m_didAnimation.id, 1, 0, 30f)</c>, pseudo-C ~0x004ee61b).
    /// </summary>
    public const float IdleFramerate = 30f;

    private readonly ChargenPreviewAnimatedBuild _build;
    private float _currFrame;
    private bool _zoomedIn;

    // Double-buffered so a 30fps Tick doesn't allocate a fresh List<MeshRef>
    // every frame: one buffer is whatever Entity.MeshRefs currently points
    // at (potentially still being read by the renderer's own Render() call
    // for this frame), the other is safe to Clear()+refill for the NEXT
    // tick and only gets published once fully populated.
    private readonly List<MeshRef> _meshRefsBufferA = [];
    private readonly List<MeshRef> _meshRefsBufferB = [];
    private bool _nextBufferIsA = true;

    public ChargenPreviewAnimator(ChargenPreviewAnimatedBuild build)
    {
        _build = build ?? throw new ArgumentNullException(nameof(build));
        _currFrame = build.IdleLowFrame;
        if (build.IdleAnimation is not null)
            ApplyIdleFrame(); // retail's true default: idle playing, frame 0.
        // Else: Entity.MeshRefs already holds RestMeshRefs (set by
        // TryBuildAnimated) as the best available fallback.
    }

    /// <summary>The live preview entity — mutated in place by <see cref="Tick"/>
    /// and <see cref="SetZoomedIn"/>; the renderer never needs to re-call
    /// <c>SetPreview</c> after the first assignment (<c>WorldEntity.MeshRefs</c>
    /// is read fresh every draw — see its own doc comment).</summary>
    public WorldEntity Entity => _build.Entity;

    public bool IsZoomedIn => _zoomedIn;

    /// <summary>
    /// <c>gmCGAppearancePage::ZoomIn</c>/<c>ZoomOut</c>'s
    /// <c>StopAnimation</c>/<c>StartAnimation</c> call, applied immediately
    /// (retail does not wait for the camera tween to finish before swapping
    /// animation state — see this class's own doc comment). No-op if
    /// already in the requested state, matching retail's own early-return
    /// guards (<c>ZoomIn</c>'s <c>if (m_bZoomedIn != 0) return</c>,
    /// <c>ZoomOut</c>'s mirror).
    /// </summary>
    public void SetZoomedIn(bool zoomedIn)
    {
        if (_zoomedIn == zoomedIn)
            return;
        _zoomedIn = zoomedIn;
        if (zoomedIn)
        {
            _build.Entity.MeshRefs = _build.RestMeshRefs;
        }
        else
        {
            _currFrame = _build.IdleLowFrame;
            if (_build.IdleAnimation is not null)
                ApplyIdleFrame();
        }
    }

    /// <summary>
    /// Advances the idle loop by <paramref name="elapsedSeconds"/>. No-op
    /// while zoomed in (the rest pose is frozen — retail's framerate-0
    /// <c>set_sequence_animation</c> call never advances) or when no idle
    /// Animation resolved (heritage/DID gap; the entity keeps whatever pose
    /// the constructor seeded).
    /// </summary>
    public void Tick(float elapsedSeconds)
    {
        if (_zoomedIn || _build.IdleAnimation is null || elapsedSeconds <= 0f)
            return;

        _currFrame = RetailAnimationCyclePlayback.Advance(
            _currFrame, _build.IdleLowFrame, _build.IdleHighFrame, IdleFramerate, elapsedSeconds);
        ApplyIdleFrame();
    }

    private void ApplyIdleFrame()
    {
        DatReaderWriter.DBObjs.Animation animation = _build.IdleAnimation!;
        IReadOnlyList<ChargenPreviewDrawablePart> parts = _build.DrawableParts;
        List<MeshRef> meshRefs = _nextBufferIsA ? _meshRefsBufferA : _meshRefsBufferB;
        _nextBufferIsA = !_nextBufferIsA;
        meshRefs.Clear();
        foreach (ChargenPreviewDrawablePart part in parts)
        {
            bool resolved = RetailAnimationCyclePlayback.TryInterpolatePart(
                animation, _currFrame, _build.IdleLowFrame, _build.IdleHighFrame,
                part.SetupPartIndex, out Vector3 origin, out Quaternion orientation);
            // Same defensive default as ApplyHeldPoseTransforms: a part
            // index the bracketing frame doesn't cover (a Setup/Animation
            // part-count mismatch, never expected in practice) keeps
            // identity rather than a degenerate zero quaternion.
            if (!resolved)
            {
                origin = Vector3.Zero;
                orientation = Quaternion.Identity;
            }
            Matrix4x4 transform = RetailHeldPose.ComposePartTransform(part.DefaultScale, origin, orientation);
            meshRefs.Add(new MeshRef(part.GfxObjId, transform) { SurfaceOverrides = part.SurfaceOverrides });
        }
        _build.Entity.MeshRefs = meshRefs;
    }
}