using System;
using System.Collections.Generic;

namespace AcDream.Core.Lighting;

/// <summary>
/// Packs a point-light snapshot into the flat float layout the bindless mesh
/// shader reads at SSBO binding=4 (<c>mesh_modern.vert</c> <c>GlobalLight gLights[]</c>):
/// 16 floats (4 vec4) per light — posAndKind, dirAndRange, colorAndIntensity,
/// coneAngleEtc. Pure (no GL), so both <c>WbDrawDispatcher</c> and
/// <c>EnvCellRenderer</c> share ONE layout and cannot drift.
/// </summary>
public static class GlobalLightPacker
{
    public const int FloatsPerLight = 16;

    /// <summary>
    /// Fill <paramref name="buffer"/> (grown + zero-cleared as needed) with the
    /// packed snapshot; returns the light count <c>n</c>. The buffer always has at
    /// least <see cref="FloatsPerLight"/> floats (so a zero-light frame still
    /// uploads a non-empty SSBO). Callers upload <c>max(n,1) * FloatsPerLight</c> floats.
    /// </summary>
    public static int Pack(IReadOnlyList<LightSource>? snapshot, ref float[] buffer)
    {
        int n = snapshot?.Count ?? 0;
        int floatsNeeded = Math.Max(n, 1) * FloatsPerLight;
        if (buffer.Length < floatsNeeded)
            buffer = new float[floatsNeeded + FloatsPerLight * 16];
        Array.Clear(buffer, 0, floatsNeeded);

        for (int i = 0; i < n; i++)
        {
            var L = snapshot![i];
            int o = i * FloatsPerLight;
            // posAndKind (xyz world pos, w kind)
            buffer[o + 0] = L.WorldPosition.X;
            buffer[o + 1] = L.WorldPosition.Y;
            buffer[o + 2] = L.WorldPosition.Z;
            buffer[o + 3] = (int)L.Kind;
            // dirAndRange (xyz forward, w range)
            buffer[o + 4] = L.WorldForward.X;
            buffer[o + 5] = L.WorldForward.Y;
            buffer[o + 6] = L.WorldForward.Z;
            buffer[o + 7] = L.Range;   // w = Range = Falloff × static_light_factor (1.3), pre-multiplied by LightInfoLoader — NOT the raw dat Falloff
            // colorAndIntensity (xyz linear colour, w intensity)
            buffer[o + 8]  = L.ColorLinear.X;
            buffer[o + 9]  = L.ColorLinear.Y;
            buffer[o + 10] = L.ColorLinear.Z;
            buffer[o + 11] = L.Intensity;
            // coneAngleEtc (x cone radians; y = #143 dynamic flag; zw reserved)
            buffer[o + 12] = L.ConeAngle;
            buffer[o + 13] = L.IsDynamic ? 1f : 0f;   // shader: 1/d D3D attenuation vs static 1/d³
        }
        return n;
    }
}