export const BUSINESS_TIME_ZONE = "Asia/Shanghai";
const BUSINESS_OFFSET = "+08:00";
const DATE_ONLY_PATTERN = /^\d{4}-\d{2}-\d{2}$/;
const EXPLICIT_OFFSET_PATTERN = /(?:Z|[+-]\d{2}:?\d{2})$/i;

export function parseApiDateTime(value?: string | Date | null): Date | null {
  if (!value) return null;
  if (value instanceof Date) return Number.isNaN(value.getTime()) ? null : value;
  const text = value.trim();
  if (!text) return null;
  const isoText = text.includes(" ") ? text.replace(" ", "T") : text;
  const normalized = DATE_ONLY_PATTERN.test(isoText)
    ? `${isoText}T00:00:00${BUSINESS_OFFSET}`
    : EXPLICIT_OFFSET_PATTERN.test(isoText)
      ? isoText
      : `${isoText}Z`;
  const parsed = new Date(normalized);
  return Number.isNaN(parsed.getTime()) ? null : parsed;
}

function businessParts(value?: string | Date | null) {
  const parsed = parseApiDateTime(value);
  if (!parsed) return null;
  const parts = new Intl.DateTimeFormat("zh-CN", {
    timeZone: BUSINESS_TIME_ZONE,
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    second: "2-digit",
    hour12: false,
    hourCycle: "h23",
  }).formatToParts(parsed);
  return Object.fromEntries(parts.map((part) => [part.type, part.value]));
}

export function formatDate(value?: string | Date | null, fallback = "-") {
  const parts = businessParts(value);
  return parts ? `${parts.year}-${parts.month}-${parts.day}` : fallback;
}

export function formatTime(value?: string | Date | null, fallback = "-") {
  const parts = businessParts(value);
  return parts ? `${parts.hour}:${parts.minute}` : fallback;
}

export function formatDateTime(value?: string | Date | null, fallback = "-") {
  const parts = businessParts(value);
  return parts
    ? `${parts.year}-${parts.month}-${parts.day} ${parts.hour}:${parts.minute}:${parts.second}`
    : fallback;
}

export function businessDateKey(value?: string | Date | null): string | null {
  const parts = businessParts(value);
  return parts ? `${parts.year}-${parts.month}-${parts.day}` : null;
}

export function formatSessionTime(
  value?: string | Date | null,
  now: Date = new Date(),
): string {
  const targetKey = businessDateKey(value);
  const todayKey = businessDateKey(now);
  const time = formatTime(value, "");
  if (!targetKey || !todayKey || !time) return "";
  const dayNumber = (key: string) => {
    const [year, month, day] = key.split("-").map(Number);
    return Date.UTC(year, month - 1, day) / 86_400_000;
  };
  const days = dayNumber(todayKey) - dayNumber(targetKey);
  if (days === 0) return time;
  if (days === 1) return `昨天 ${time}`;
  return `${targetKey.slice(5).replace("-", "/")} ${time}`;
}