type RouterLike = { push: (to: unknown) => Promise<unknown> | unknown }

let globalRouter: RouterLike | null = null
let clickHandler: ((event: MouseEvent) => void) | null = null

const isModifiedClick = (event: MouseEvent): boolean => event.metaKey || event.ctrlKey || event.shiftKey || event.altKey

const findLinkAnchor = (event: Event): HTMLAnchorElement | null => {
  const { target } = event
  if (!(target instanceof Element)) return null

  const anchor = target.closest('a[href]')
  return anchor instanceof HTMLAnchorElement ? anchor : null
}

/**
 * Returns true when href is a relative URL suitable for in-app navigation
 * (e.g. `/dashboard`, `page`, `./page`) — not absolute, protocol-relative, or hash-only.
 */
export const isRelativeHref = (href: string): boolean => {
  if (!href || href.startsWith('#')) return false
  if (/^[a-z][a-z0-9+.-]*:/i.test(href)) return false
  if (href.startsWith('//')) return false
  return true
}

/**
 * Resolves an anchor href to a value suitable for `router.push`.
 * Returns `null` when the link should use normal browser navigation.
 */
export const resolveRouterLinkTarget = (anchor: HTMLAnchorElement): string | null => {
  if (anchor.hasAttribute('disabled') || anchor.getAttribute('aria-disabled') === 'true') {
    return null
  }
  if (anchor.hasAttribute('download')) return null
  if (anchor.target && anchor.target !== '_self') return null

  const href = anchor.getAttribute('href')
  if (!href || !isRelativeHref(href)) return null

  try {
    const url = new URL(href, window.location.href)
    return url.pathname + url.search + url.hash
  } catch {
    return href
  }
}

const handleClick = (event: MouseEvent) => {
  if (event.defaultPrevented) return
  if (event.button !== 0) return
  if (isModifiedClick(event)) return

  const anchor = findLinkAnchor(event)
  if (!anchor || !globalRouter) return

  const to = resolveRouterLinkTarget(anchor)
  if (!to) return

  event.preventDefault()
  void globalRouter.push(to)
}

/**
 * Interceptar klick på relativa `<a href>` och navigerar med `router.push`
 * i stället för full sidladdning. Användbart för HTML/JSON från t.ex. Payload CMS
 * eller Poeditor där `router-link` inte kan användas direkt.
 *
 * Anropa en gång vid app-start:
 * ```ts
 * setupRouterLinkInterceptor(router)
 * ```
 *
 * HTML-exempel:
 * ```html
 * <a href="/dashboard">Öppna dashboard</a>
 * ```
 */
export const setupRouterLinkInterceptor = (router: RouterLike) => {
  teardownRouterLinkInterceptor()
  globalRouter = router
  clickHandler = handleClick
  document.addEventListener('click', clickHandler)
}

/** Tar bort den globala router-link-interceptorn. */
export const teardownRouterLinkInterceptor = () => {
  if (clickHandler) {
    document.removeEventListener('click', clickHandler)
    clickHandler = null
  }
  globalRouter = null
}