// Page-number rules for Journal listings. Pages use ?page=N (never a path
// segment). Anything but a positive integer, and any page past the last one,
// is a real 404.

/**
 * Parses the ?page query value: 1 when absent, the integer for a string of
 * digits greater than 0, and null (a 404) for anything else: non-numeric,
 * empty, 0, negative, decimal, repeated (array) or beyond the safe range.
 */
export function parsePageParam(raw: unknown): number | null {
  if (raw === undefined || raw === null) return 1
  if (typeof raw !== 'string' || !/^\d+$/.test(raw)) return null
  const n = Number(raw)
  if (!Number.isSafeInteger(n) || n < 1) return null
  return n
}

/**
 * True when `page` lies past the last page. Page 1 is always in range, so an
 * empty listing renders its empty state instead of a 404.
 */
export function pageOutOfRange(page: number, lastPage: number): boolean {
  return page > 1 && page > lastPage
}

/**
 * The page numbers to render around `current`: at most `size` consecutive
 * pages, clipped to 1..last and kept full-width near either end.
 */
export function paginationWindow(current: number, last: number, size: number = 5): number[] {
  if (last < 1 || size < 1) return []
  const width = Math.min(size, last)
  let start = Math.max(1, current - Math.floor(width / 2))
  if (start + width - 1 > last) start = last - width + 1
  return Array.from({ length: width }, (_, i) => start + i)
}