using System.Collections.Generic;
using System.Numerics;
using DatReaderWriter.DBObjs;
using DatReaderWriter.Enums;
using DatReaderWriter.Types;

namespace AcDream.Core.Meshing;

/// <summary>
/// Compute the per-part static transforms for a Setup using its
/// PlacementFrames. For each part <c>i</c>, the returned matrix takes a
/// point in part-local space and yields a point in setup-local space at
/// the Setup's resting pose.
///
/// <para>
/// Mirrors <see cref="SetupMesh.Flatten"/>'s pose-source priority —
/// <c>PlacementFrames[Resting]</c> → <c>[Default]</c> → first available
/// — so that a particle anchor at part <c>i</c> matches the part's
/// visible rest position. If renderer and resolver ever drift on this
/// priority, particles will visibly drift relative to their parent
/// mesh; keep them in lockstep.
/// </para>
///
/// <para>
/// Returns an empty list when the Setup has no PlacementFrames. The
/// caller (e.g. <c>ParticleHookSink.SpawnFromHook</c>) should then fall
/// back to <see cref="Matrix4x4.Identity"/> per part, which is the
/// pre-C.1.5b behavior.
/// </para>
///
/// <para>
/// For animated entities, per-part transforms vary per animation frame
/// and live in <c>AnimatedEntityState</c>; a future "animated
/// DefaultScript" path would publish those each tick via the same
/// <c>SetEntityPartTransforms</c> seam. Out of scope here.
/// </para>
/// </summary>
public static class SetupPartTransforms
{
    public static IReadOnlyList<Matrix4x4> Compute(Setup setup)
    {
        AnimationFrame? source = null;
        if (setup.PlacementFrames.TryGetValue(Placement.Resting, out var resting))
        {
            source = resting;
        }
        else if (setup.PlacementFrames.TryGetValue(Placement.Default, out var def))
        {
            source = def;
        }
        else
        {
            foreach (var kvp in setup.PlacementFrames)
            {
                source = kvp.Value;
                break;
            }
        }

        if (source is null) return System.Array.Empty<Matrix4x4>();

        int partCount = setup.Parts.Count;
        var result = new Matrix4x4[partCount];
        for (int i = 0; i < partCount; i++)
        {
            Frame frame = i < source.Frames.Count
                ? source.Frames[i]
                : new Frame { Origin = Vector3.Zero, Orientation = Quaternion.Identity };
            Vector3 scale = i < setup.DefaultScale.Count ? setup.DefaultScale[i] : Vector3.One;
            result[i] = Matrix4x4.CreateScale(scale)
                      * Matrix4x4.CreateFromQuaternion(frame.Orientation)
                      * Matrix4x4.CreateTranslation(frame.Origin);
        }
        return result;
    }
}