import { computed } from 'vue'
import { useData } from 'vitepress'
import { getFlatSideBarLinks, getSidebar, isActive } from './sidebar'

export function usePrevNext() {
  const { page, theme, frontmatter } = useData()

  return computed(() => {
    const sidebar = getSidebar(theme.value.sidebar, page.value.relativePath)
    const links = getFlatSideBarLinks(sidebar)

    // ignore inner-page links with hashes
    const candidates = uniqBy(links, link => link.link.replace(/[?#].*$/, ''))

    const index = candidates.findIndex((link) => {
      return isActive(page.value.relativePath, link.link)
    })

    const hidePrev
      = (theme.value.docFooter?.prev === false && !frontmatter.value.prev)
      || frontmatter.value.prev === false

    const hideNext
      = (theme.value.docFooter?.next === false && !frontmatter.value.next)
      || frontmatter.value.next === false

    return {
      prev: hidePrev
        ? undefined
        : {
            text:
              (typeof frontmatter.value.prev === 'string'
                ? frontmatter.value.prev
                : typeof frontmatter.value.prev === 'object'
                  ? frontmatter.value.prev.text
                  : undefined)
                  ?? candidates[index - 1]?.docFooterText
                  ?? candidates[index - 1]?.text,
            link:
              (typeof frontmatter.value.prev === 'object'
                ? frontmatter.value.prev.link
                : undefined) ?? candidates[index - 1]?.link,
          },
      next: hideNext
        ? undefined
        : {
            text:
              (typeof frontmatter.value.next === 'string'
                ? frontmatter.value.next
                : typeof frontmatter.value.next === 'object'
                  ? frontmatter.value.next.text
                  : undefined)
                  ?? candidates[index + 1]?.docFooterText
                  ?? candidates[index + 1]?.text,
            link:
              (typeof frontmatter.value.next === 'object'
                ? frontmatter.value.next.link
                : undefined) ?? candidates[index + 1]?.link,
          },
    } as {
      prev?: { text?: string, link?: string }
      next?: { text?: string, link?: string }
    }
  })
}

function uniqBy<T>(array: T[], keyFn: (item: T) => any): T[] {
  const seen = new Set()
  return array.filter((item) => {
    const k = keyFn(item)
    return seen.has(k) ? false : seen.add(k)
  })
}