const DDMMYYYY = /^(\d{2})-(\d{2})-(\d{4})$/;

export function parseOccurredAt(value?: string): Date {
  const m = value?.match(DDMMYYYY);
  if (!m) {
    throw new Error(`Expense occurred_at is not DD-MM-YYYY: ${value}`);
  }
  const [, dd, mm, yyyy] = m;
  const d = new Date(Number(yyyy), Number(mm) - 1, Number(dd));
  if (
    Number.isNaN(d.getTime()) ||
    d.getDate() !== Number(dd) ||
    d.getMonth() !== Number(mm) - 1 ||
    d.getFullYear() !== Number(yyyy)
  ) {
    throw new Error(`Invalid expense occurred_at date: ${value}`);
  }
  return d;
}

export function isDdmmyyyy(value?: string): boolean {
  if (!value) return true;
  const m = value.match(DDMMYYYY);
  if (!m) return false;
  const [, dd, mm, yyyy] = m;
  const d = new Date(Number(yyyy), Number(mm) - 1, Number(dd));
  return !(
    Number.isNaN(d.getTime()) ||
    d.getDate() !== Number(dd) ||
    d.getMonth() !== Number(mm) - 1 ||
    d.getFullYear() !== Number(yyyy)
  );
}

export function parseDdmmyyyy(value: string): Date {
  const m = value.match(DDMMYYYY);
  if (!m) throw new Error(`Date is not DD-MM-YYYY: ${value}`);
  const [, dd, mm, yyyy] = m;
  const d = new Date(Number(yyyy), Number(mm) - 1, Number(dd));
  if (
    Number.isNaN(d.getTime()) ||
    d.getDate() !== Number(dd) ||
    d.getMonth() !== Number(mm) - 1 ||
    d.getFullYear() !== Number(yyyy)
  ) {
    throw new Error(`Invalid date: ${value}`);
  }
  return d;
}

export function monthKey(value?: string): string {
  const d = parseOccurredAt(value);
  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}`;
}

export function currentMonthKey(): string {
  const now = new Date();
  return `${now.getFullYear()}-${String(now.getMonth() + 1).padStart(2, "0")}`;
}

export function monthLabel(key: string): string {
  const [y, m] = key.split("-").map(Number);
  if (!y || !m) return key;
  const d = new Date(y, m - 1, 1);
  return d.toLocaleDateString("en-IN", { month: "long", year: "numeric" });
}

export function dateLabel(value: string): string {
  const d = parseOccurredAt(value);
  return d.toLocaleDateString("en-IN", { day: "numeric", month: "short", year: "numeric" });
}