/**
 * 客户月度接口固定返回“截至查询结束日的近 12 个月”。
 * 键保留完整 YYYY-MM，避免跨年时把不同年份的同月合并。
 */
export function customerMonthRange(asOfYmd: string, count = 12): string[] {
  const matched = /^(\d{4})-(\d{2})-\d{2}$/.exec(asOfYmd);
  if (!matched || count < 1) return [];

  const endYear = Number(matched[1]);
  const endMonth = Number(matched[2]);
  if (endMonth < 1 || endMonth > 12) return [];

  const months: string[] = [];
  for (let offset = count - 1; offset >= 0; offset -= 1) {
    const date = new Date(Date.UTC(endYear, endMonth - 1 - offset, 1));
    months.push(`${date.getUTCFullYear()}-${String(date.getUTCMonth() + 1).padStart(2, '0')}`);
  }
  return months;
}

export function customerMonthLabel(monthKey: string): string {
  const matched = /^(\d{4})-(\d{2})$/.exec(monthKey);
  return matched ? `${matched[1]}年${Number(matched[2])}月` : monthKey;
}

/** 闭区间日数；用于让卡片副标题与实际查询口径保持一致。 */
export function inclusiveDayCount(start: string, end: string): number {
  const startTime = Date.parse(`${start}T00:00:00Z`);
  const endTime = Date.parse(`${end}T00:00:00Z`);
  if (!Number.isFinite(startTime) || !Number.isFinite(endTime) || endTime < startTime) return 0;
  return Math.floor((endTime - startTime) / 86_400_000) + 1;
}

export function dateRangeLabel(start: string, end: string): string {
  const days = inclusiveDayCount(start, end);
  return days > 0 ? `${start} 至 ${end}（共 ${days} 天）` : `${start} 至 ${end}`;
}