blog/frontend/shared/utils/pagination.ts

export type PaginationItem = number | 'ellipsis'

function range(start: number, end: number): number[] {
  const length = end - start + 1
  return Array.from({ length }, (_, i) => start + i)
}

/**
 * Windowed page list for numbered pagination: always shows the first and
 * last page, the current page and its immediate siblings, collapsing any
 * gap into a single 'ellipsis' marker. Returns every page when the total
 * is small enough that no collapsing is needed.
 */
export function paginationRange(current: number, total: number, siblingCount = 1): PaginationItem[] {
  if (total <= 0) return []

  const totalVisible = siblingCount * 2 + 5 // first + last + current + 2 siblings + 2 ellipses
  if (totalVisible >= total) return range(1, total)

  const leftSibling = Math.max(current - siblingCount, 1)
  const rightSibling = Math.min(current + siblingCount, total)

  const showLeftEllipsis = leftSibling > 2
  const showRightEllipsis = rightSibling < total - 1

  if (!showLeftEllipsis && showRightEllipsis) {
    const leftRange = range(1, 3 + siblingCount * 2)
    return [...leftRange, 'ellipsis', total]
  }

  if (showLeftEllipsis && !showRightEllipsis) {
    const rightRange = range(total - (3 + siblingCount * 2) + 1, total)
    return [1, 'ellipsis', ...rightRange]
  }

  return [1, 'ellipsis', ...range(leftSibling, rightSibling), 'ellipsis', total]
}