export type AgenticRectificationDatePrecision = "day" | "month" | "quarter" | "year";

export type AgenticRectificationDateRange = Readonly<{
  start: string;
  end: string;
  precision: AgenticRectificationDatePrecision;
  label: string;
}>;

function isoDate(year: number, month: number, day: number): string {
  const value = new Date(Date.UTC(year, month - 1, day));
  if (value.getUTCFullYear() !== year || value.getUTCMonth() !== month - 1 || value.getUTCDate() !== day) {
    throw new Error("invalid_calendar_date");
  }
  return value.toISOString().slice(0, 10);
}

function monthEnd(year: number, month: number): string {
  return new Date(Date.UTC(year, month, 0)).toISOString().slice(0, 10);
}

export function dateRangeFromDeclared(
  value: string,
  precision: AgenticRectificationDatePrecision,
): AgenticRectificationDateRange {
  if (precision === "day") {
    if (!/^\d{4}-\d{2}-\d{2}$/.test(value)) throw new Error("invalid_day");
    const [year, month, day] = value.split("-").map(Number);
    const date = isoDate(year!, month!, day!);
    return { start: date, end: date, precision, label: value };
  }
  if (precision === "month") {
    if (!/^\d{4}-\d{2}$/.test(value)) throw new Error("invalid_month");
    const [year, month] = value.split("-").map(Number);
    const start = isoDate(year!, month!, 1);
    return { start, end: monthEnd(year!, month!), precision, label: value };
  }
  if (precision === "quarter") {
    const matched = /^(\d{4})-Q([1-4])$/.exec(value);
    if (!matched) throw new Error("invalid_quarter");
    const year = Number(matched[1]);
    const firstMonth = (Number(matched[2]) - 1) * 3 + 1;
    return {
      start: isoDate(year, firstMonth, 1),
      end: monthEnd(year, firstMonth + 2),
      precision,
      label: value,
    };
  }
  if (!/^\d{4}$/.test(value)) throw new Error("invalid_year");
  const year = Number(value);
  return { start: isoDate(year, 1, 1), end: isoDate(year, 12, 31), precision, label: value };
}