type Props = {
  currentPage: number;
  totalPages: number;
  basePath: string;
};

export default function Pagination(
  { currentPage, totalPages, basePath }: Props,
) {
  if (totalPages <= 1) {
    return <div></div>;
  }

  const pages = generatePageNumbers(currentPage, totalPages);

  return (
    <nav className="pagination">
      <div className="pagination-prev">
        {currentPage > 1
          ? (
            <a href={pageUrlAt(basePath, currentPage - 1)}>
              前へ
            </a>
          )
          : null}
      </div>
      {pages.map((page) => {
        if (page === "...") {
          return (
            <div className="pagination-elipsis">
              …
            </div>
          );
        } else if (page === currentPage) {
          return (
            <div className="pagination-page pagination-page-current">
              <span>{String(page)}</span>
            </div>
          );
        } else {
          return (
            <div className="pagination-page">
              <a href={pageUrlAt(basePath, page)}>{String(page)}</a>
            </div>
          );
        }
      })}
      <div className="pagination-next">
        {currentPage < totalPages
          ? (
            <a href={pageUrlAt(basePath, currentPage + 1)}>
              次へ
            </a>
          )
          : null}
      </div>
    </nav>
  );
}

type PageItem = number | "...";

/**
 * Generates page numbers for pagination display.
 *
 * - Always show the first page
 * - Always show the last page
 * - Always show the current page
 * - Always show the page before and after the current page
 * - If there's only one page gap between displayed pages, fill it
 * - If there are two or more pages gap between displayed pages, show ellipsis
 */
function generatePageNumbers(
  currentPage: number,
  totalPages: number,
): PageItem[] {
  const pages = new Set<number>();
  pages.add(1);
  pages.add(Math.max(1, currentPage - 1));
  pages.add(currentPage);
  pages.add(Math.min(totalPages, currentPage + 1));
  pages.add(totalPages);

  const sorted = Array.from(pages).sort((a, b) => a - b);

  const result: PageItem[] = [];
  for (let i = 0; i < sorted.length; i++) {
    if (i > 0) {
      const gap = sorted[i] - sorted[i - 1];
      if (gap === 2) {
        result.push(sorted[i - 1] + 1);
      } else if (gap > 2) {
        result.push("...");
      }
    }
    result.push(sorted[i]);
  }

  return result;
}

function pageUrlAt(basePath: string, page: number): string {
  return page === 1 ? basePath : `${basePath}${page}/`;
}