import { formatInTimeZone } from 'date-fns-tz';

export class DateTimeConstants {
  static readonly DAYS_IN_WEEK: number = 7;
  static readonly HOURS_IN_DAY: number = 24;
  static readonly DAY_INTERVAL: number = 24 * 60 * 60 * 1000;
  static readonly HOUR_INTERVAL: number = 60 * 60 * 1000;
}

export function atEndOfDay(date: Date): Date {
  let endOfDay = new Date(date);
  endOfDay.setHours(23, 59, 59, 999);
  return endOfDay;
}

export function atStartOfDay(date: Date): Date {
  let startOfDay = new Date(date);
  startOfDay.setHours(0, 0, 0, 0);
  return startOfDay;
}

export function atEndOfHour(date: Date): Date {
  let endOfHour = new Date(date);
  endOfHour.setMinutes(59, 59, 999);
  return endOfHour;
}

export function atStartOfHour(date: Date): Date {
  let startOfHour = new Date(date);
  startOfHour.setMinutes(0, 0, 0);
  return startOfHour;
}

/** Clears Daylight saving time adjustment from the given time. */
export function toNormalDate(zonedDate: Date, adjustment: number): Date {
  return new Date(zonedDate.getTime() - adjustment);
}

export function adjustToMiddleOfDay(zonedDate: Date, timeZone: string): Date {
  let hourStr: string = formatInTimeZone(zonedDate, timeZone, "HH");
  let h: number = parseInt(hourStr);
  let addHours: number = 12 - h;
  return new Date(zonedDate.getTime() + (addHours * DateTimeConstants.HOUR_INTERVAL));
}

export function getDSTAdjustedDate(previousIsDST: boolean, zonedDate: Date, dstAdjustment: number): Date {
  // adjusts previously without dst adjusted date by dst
  // ((date + interval) - dst )
  // Note! intervals that are less or equal to dst are not supported
  // currently.
  let isDST: boolean = dstAdjustment > 0;
  if (previousIsDST && !isDST) {
    // previously added interval is shorter than the real interval.
    // with 24h interval and 1h dst: real interval is 25h.
    return new Date(zonedDate.getTime() + dstAdjustment);
  } else if (!previousIsDST && isDST) {
    // previously added interval is longer than the real interval.
    // with 24h interval and 1h dst: real interval is 23h.
    return new Date(zonedDate.getTime() - dstAdjustment);
  }
  return zonedDate;
}