using UnityEngine;

/// <summary>
/// gameplay 世界空间背景(SpriteRenderer，如 bg_replace)自适应覆盖。
/// 该背景由透视相机渲染、带旋转/缩放，尺寸按 16:9 设计。超宽屏(21:9/32:9)下相机
/// 水平视野扩大会看到背景边缘之外的黑边，故按屏幕宽高比等比放大其 localScale 使其覆盖。
///
/// 仅调整 localScale(表现层)，不改旋转/位置/材质/引用，不影响业务逻辑与动画。
/// 16:9 屏下缩放系数=1，与原设计逐像素一致。由 UIResolutionAutoFitter 自动附加。
///
/// 满足三要求：1) 不改业务逻辑/引用；2) 16:9 缩放=1、不动旋转位置；3) 超宽屏铺满。
/// </summary>
[RequireComponent(typeof(SpriteRenderer))]
[DisallowMultipleComponent]
public sealed class UISpriteBackgroundCover : MonoBehaviour
{
    private const float DesignAspect = 16f / 9f;

    private Transform tf;
    private Vector3 baseScale;      // 初始设计缩放(16:9 基准)，永不变
    private bool baseCaptured;
    private int lastW = -1, lastH = -1;

    private void OnEnable()
    {
        tf = transform;
        CaptureBaseScale();
        Apply();
    }

    private void CaptureBaseScale()
    {
        if (baseCaptured) return;
        baseScale = tf.localScale;
        baseCaptured = true;
    }

    private void LateUpdate()
    {
        if (Screen.width != lastW || Screen.height != lastH)
        {
            Apply();
        }
    }

    private void Apply()
    {
        if (!baseCaptured) CaptureBaseScale();
        lastW = Screen.width;
        lastH = Screen.height;

        float aspect = (float)Screen.width / Mathf.Max(1, Screen.height);
        // 更宽的屏：水平视野变宽，需按 aspect/DesignAspect 放大以覆盖两侧。
        // 更窄的屏：垂直视野相对更高，需按 DesignAspect/aspect 放大以覆盖上下。
        // 取二者较大值(cover)，保证任一方向都不露黑边。16:9 时系数=1。
        float widthFactor = aspect > DesignAspect ? aspect / DesignAspect : 1f;
        float heightFactor = aspect < DesignAspect ? DesignAspect / aspect : 1f;
        float factor = Mathf.Max(widthFactor, heightFactor);

        tf.localScale = baseScale * factor;
    }
}