/* 全站背景图定时轮换（每 6 秒切换一张，交叉淡入淡出） */
(function () {
  var bgList = [
    'https://image.987119.xyz/uploads/2026/07/18c591a3f567fc7a381.webp',
    'https://image.987119.xyz/uploads/2026/07/18c5917f198ecf88481.webp',
    'https://image.987119.xyz/uploads/2026/07/18c5917e0bbb86bb221.webp',
    'https://image.987119.xyz/uploads/2026/07/18c591618e6be61c264.webp',
    'https://image.987119.xyz/uploads/2026/07/18c5916148d840e1468.webp',
    'https://image.987119.xyz/uploads/2026/07/18c59161282de003513.webp'
  ]
  var webBg = document.getElementById('web_bg')
  if (!webBg || bgList.length < 2) return

  // 覆盖层：用于交叉淡入，避免背景瞬间硬切
  var overlay = document.createElement('div')
  overlay.id = 'web_bg_overlay'
  overlay.style.cssText =
    'position:fixed;top:0;left:0;width:100%;height:100%;z-index:-1;' +
    'background-size:cover;background-position:center;opacity:0;' +
    'transition:opacity 1.5s ease;'
  document.body.appendChild(overlay)

  var idx = 0
  var DURATION = 1500 // 淡入淡出时长(ms)，须与 CSS transition 一致

  function swap() {
    idx = (idx + 1) % bgList.length
    var url = 'url("' + bgList[idx] + '")'
    // 新图先放到覆盖层并淡入，盖住旧图
    overlay.style.backgroundImage = url
    void overlay.offsetWidth // 强制 reflow，确保过渡生效
    overlay.style.opacity = '1'
    // 过渡结束后，把底层也换成新图，再将覆盖层淡出，实现无缝循环
    setTimeout(function () {
      webBg.style.backgroundImage = url
      webBg.style.backgroundSize = 'cover'
      webBg.style.backgroundPosition = 'center'
      overlay.style.opacity = '0'
    }, DURATION)
  }

  setInterval(swap, 6000)
})()
