using Microsoft.Xna.Framework;
using MrGameEng.Core;

namespace MrGameEng.Lighting;

/// <summary>Colors for the day/night ambient: the tint at deep night and at midday.</summary>
public readonly record struct DayNightSettings
{
    /// <summary>Ambient tint at midnight (a dim, cool night).</summary>
    public Color NightColor { get; init; }

    /// <summary>Ambient tint at noon (white = no tint).</summary>
    public Color DayColor { get; init; }

    /// <summary>Sensible defaults: a dim blue night, untinted day.</summary>
    public static DayNightSettings Default =>
        new() { NightColor = new Color(45, 55, 95), DayColor = Color.White };
}

/// <summary>
/// Day/night ambient driven by <see cref="Calendar.DayProgress"/>: a daylight factor (0 at midnight,
/// 1 at noon) and an ambient color lerped from night to day. <see cref="SampleAt"/> is the sampleable
/// light interface read by both the renderer (scene darkening) and the simulation (plant light);
/// it returns the global daylight today and will return locally-shadowed light once point lights and
/// occlusion land. Pure read-side, GPU-free and deterministic.
/// </summary>
public sealed class DayNight
{
    private readonly Calendar _calendar;
    private readonly DayNightSettings _settings;

    /// <summary>Creates the cycle reading <paramref name="calendar"/> with the given <paramref name="settings"/>.</summary>
    public DayNight(Calendar calendar, DayNightSettings settings)
    {
        _calendar = calendar ?? throw new ArgumentNullException(nameof(calendar));
        _settings = settings;
    }

    /// <summary>Daylight factor in <c>[0, 1]</c> — 0 at midnight, 1 at noon, 0 again at midnight.</summary>
    public float Daylight
    {
        get
        {
            var value = -MathF.Cos(MathF.Tau * _calendar.DayProgress);
            return value > 0f ? value : 0f;
        }
    }

    /// <summary>Global light intensity in <c>[0, 1]</c>; same as <see cref="Daylight"/> today.</summary>
    public float Intensity => Daylight;

    /// <summary>Light intensity at a world point in <c>[0, 1]</c>. Global today; local (with shadows) later.</summary>
    public float SampleAt(Vector2 world) => Daylight;

    /// <summary>
    /// Offset, in grid cells, of the shadow an occluder casts under the current sun: opposite the
    /// sun's east–west position and longest near sunrise/sunset (low sun), shrinking to zero at noon
    /// and at night. Feeds the lightmap's directional shadow pass; <paramref name="maxLength"/> caps
    /// the dawn/dusk shadow length. Tilted slightly "south" (down) so shadows fall in front of objects.
    /// </summary>
    public Vector2 SunShadow(float maxLength)
    {
        var day = (_calendar.DayProgress - 0.25f) / 0.5f; // daytime fraction over [06:00, 18:00]
        if (day <= 0f || day >= 1f)
        {
            return Vector2.Zero; // night — no sun; the ambient floor handles darkness
        }

        var altitude = MathF.Sin(day * MathF.PI); // 0 at dawn/dusk, 1 at noon
        var direction = new Vector2(2f * day - 1f, 0.4f); // sun east→west ⇒ shadow west→east, tilted south
        direction.Normalize();
        return direction * (maxLength * (1f - altitude));
    }

    /// <summary>Ambient tint for the scene: night color at night, day color at noon, eased between.</summary>
    public Color Ambient
    {
        get
        {
            var t = Smoothstep(Daylight);
            return Color.Lerp(_settings.NightColor, _settings.DayColor, t);
        }
    }

    private static float Smoothstep(float x) => x * x * (3f - 2f * x);
}