// The page links of <SeoPagination> (UI-SPEC L2, D-42): five consecutive
// pages around the current one, plus page 1 and the last page when they
// fall outside, with a gap item where pages are skipped, so every page is
// within two hops of page 1 for a crawler. Pure, so `node --test` drives
// it. Names start with `seo` because app/utils is auto-imported into the
// consuming app (vue-journal-layer has its own paginationWindow).

export const SEO_PAGINATION_GAP = 'gap' as const

export type SeoPaginationItem = number | typeof SEO_PAGINATION_GAP

/**
 * The items for page `current` of `last`: page numbers in order and 'gap'
 * between page 1 or the last page and the window when pages are skipped.
 * Empty when there is nothing to paginate (last < 1).
 */
export function seoPaginationWindow(current: number, last: number, size = 5): SeoPaginationItem[] {
  if (!Number.isInteger(last) || last < 1) return []
  const cur = Math.min(Math.max(Number.isInteger(current) ? current : 1, 1), last)
  const width = Math.min(Math.max(1, size), last)
  let start = Math.max(1, cur - Math.floor(width / 2))
  if (start + width - 1 > last) start = last - width + 1
  const end = start + width - 1
  const out: SeoPaginationItem[] = []
  if (start > 1) {
    out.push(1)
    if (start > 2) out.push(SEO_PAGINATION_GAP)
  }
  for (let p = start; p <= end; p++) out.push(p)
  if (end < last) {
    if (end < last - 1) out.push(SEO_PAGINATION_GAP)
    out.push(last)
  }
  return out
}