import { ISO_DATE_LENGTH, ISO_DATE_START } from '../../common/time';

const UTC_MONTH_INDEX_OFFSET = 1;
const NEXT_DAY_OFFSET = 1;

const MOSCOW_UTC_OFFSET_HOURS = 3;
export type DateOnlyRange = { from: string; to: string };
const moscow = 'Europe/Moscow';
const moscowFormatter = new Intl.DateTimeFormat('en-CA', {
  timeZone: moscow,
  year: 'numeric',
  month: '2-digit',
  day: '2-digit',
});
export function moscowDate(value: Date): string {
  const parts = moscowFormatter.formatToParts(value);
  const get = (type: string) => parts.find((part) => part.type === type)?.value;
  return `${get('year')}-${get('month')}-${get('day')}`;
}

// Inclusive Moscow calendar dates become a half-open instant range, capped at now.
export function dateOnlyBounds(
  range: DateOnlyRange,
  now = new Date(),
): { from: Date; to: Date } {
  const from = dateParts(range.from);
  const end = dateParts(range.to);
  const to = new Date(
    Date.UTC(end[0], end[1] - UTC_MONTH_INDEX_OFFSET, end[2] + NEXT_DAY_OFFSET, -MOSCOW_UTC_OFFSET_HOURS),
  );
  return {
    from: new Date(
      Date.UTC(from[0], from[1] - UTC_MONTH_INDEX_OFFSET, from[2], -MOSCOW_UTC_OFFSET_HOURS),
    ),
    to: to > now ? now : to,
  };
}

export function dateParts(value: string): [number, number, number] {
  const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(value);
  if (!match) throw new Error('Invalid date');
  const parts = [Number(match[1]), Number(match[2]), Number(match[3])] as [
    number,
    number,
    number,
  ];
  if (utcDate(value).toISOString().slice(ISO_DATE_START, ISO_DATE_LENGTH) !== value) {
    throw new Error('Invalid date');
  }
  return parts;
}

export function utcDate(value: string): Date {
  const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(value);
  if (!match) return new Date(NaN);
  return new Date(
    Date.UTC(Number(match[1]), Number(match[2]) - UTC_MONTH_INDEX_OFFSET, Number(match[3])),
  );
}