@using Sandbox
@using Sandbox.UI
@inherits PanelComponent
@namespace Sandbox

<root>
    @if (opacity > 0f)
    {
        <div class="fade" style="opacity: @opacity"></div>
    }
</root>

@code {
    public const float DefaultDuration = 2f;

    /// <summary>Per-scene singleton so the static API can find the live instance.</summary>
    public static ScreenFade Current { get; private set; }

    private float startOpacity = 1f;
    private float targetOpacity = 0f;
    private float duration = DefaultDuration;
    private RealTimeSince startedAt;
    private float opacity = 1f;

    /// <summary>Fade from black to clear over <paramref name="duration"/> seconds.</summary>
    public static void FadeIn(float duration = DefaultDuration)
    {
        Current?.Begin(targetOpacity: 0f, duration);
    }

    /// <summary>Fade from clear to black over <paramref name="duration"/> seconds.</summary>
    public static void FadeOut(float duration = DefaultDuration)
    {
        Current?.Begin(targetOpacity: 1f, duration);
    }

    protected override void OnEnabled()
    {
        base.OnEnabled();
        Current = this;
        opacity = 1f;
        Begin(targetOpacity: 0f, DefaultDuration);
    }

    protected override void OnDisabled()
    {
        base.OnDisabled();
        if (Current == this) Current = null;
    }

    protected override void OnUpdate()
    {
        float t = System.Math.Clamp((float)startedAt / duration, 0f, 1f);
        opacity = MathX.Lerp(startOpacity, targetOpacity, t);
    }

    private void Begin(float targetOpacity, float duration)
    {
        startOpacity = opacity;
        this.targetOpacity = targetOpacity;
        this.duration = System.MathF.Max(0.0001f, duration);
        startedAt = 0f;
    }

    protected override int BuildHash() => System.HashCode.Combine((int)(opacity * 100));
}