/** Pure pagination math shared by server components, query helpers and tests (§295 URL state). */

export interface PageInfo {
  /** 1-based current page, clamped into [1, pageCount]. */
  page: number;
  pageSize: number;
  total: number;
  pageCount: number;
  /** SQL OFFSET for the clamped page. */
  offset: number;
  /** 1-based index of the first row shown (0 when total = 0). */
  from: number;
  /** 1-based index of the last row shown (0 when total = 0). */
  to: number;
  hasPrev: boolean;
  hasNext: boolean;
}

export function pageCount(total: number, pageSize: number): number {
  if (!Number.isFinite(total) || total <= 0) return 1;
  const size = Math.max(1, Math.floor(pageSize));
  return Math.max(1, Math.ceil(total / size));
}

/** Clamp a requested page into the valid range for `total` rows (always ≥ 1). */
export function clampPage(page: number, total: number, pageSize: number): number {
  const n = Number.isFinite(page) ? Math.floor(page) : 1;
  return Math.min(pageCount(total, pageSize), Math.max(1, n));
}

export function offsetFor(page: number, pageSize: number): number {
  return (Math.max(1, Math.floor(page)) - 1) * Math.max(1, Math.floor(pageSize));
}

export function pageInfo(page: number, pageSize: number, total: number): PageInfo {
  const size = Math.max(1, Math.floor(pageSize));
  const count = pageCount(total, size);
  const p = clampPage(page, total, size);
  const from = total === 0 ? 0 : (p - 1) * size + 1;
  const to = total === 0 ? 0 : Math.min(total, p * size);
  return { page: p, pageSize: size, total, pageCount: count, offset: (p - 1) * size, from, to, hasPrev: p > 1, hasNext: p < count };
}

/**
 * Compact list of page numbers to show as links: first, last, current ±1, with `null` gaps.
 * e.g. page 7 of 20 → [1, null, 6, 7, 8, null, 20].
 */
export function pageWindow(page: number, count: number, radius = 1): Array<number | null> {
  if (count <= 1) return [1];
  const want = new Set<number>([1, count]);
  for (let i = page - radius; i <= page + radius; i++) if (i >= 1 && i <= count) want.add(i);
  const sorted = [...want].sort((a, b) => a - b);
  const out: Array<number | null> = [];
  for (let i = 0; i < sorted.length; i++) {
    const cur = sorted[i]!;
    const prev = sorted[i - 1];
    if (prev != null && cur - prev === 2) out.push(prev + 1);
    else if (prev != null && cur - prev > 2) out.push(null);
    out.push(cur);
  }
  return out;
}