import {HOUR, MINUTE} from './units.js';

const offset = /^(?:GMT|UTC)(?:(\+|\-)([0-9]{2}):([0-9]{2}))?$/;

export interface TimeZone {
  id: string;
  getOffsetMillisecondsFor(instant: number): number;
}

const timeZones = new Map<string, TimeZone>();

export class DateTimeFormatTimeZone implements TimeZone {
  dateFormat: Intl.DateTimeFormat;
  constructor(public id: string) {
    this.dateFormat = new Intl.DateTimeFormat('en-US', {
      timeZoneName: 'longOffset',
      minute: 'numeric',
      timeZone: id,
    });
  }

  getOffsetMillisecondsFor(milliseconds: number): number {
    let timeZonePart = this.dateFormat.formatToParts(new Date(milliseconds)).find(part => part.type === 'timeZoneName');
    if (timeZonePart) {
      let result = timeZonePart.value.match(offset);
      if (result) {
        let sign = result[1];
        let minute = result[2];
        let second = result[3];
        if (sign != null) {
          let multiplier = sign === '-' ? -1 : 1;
          return (parseInt(minute, 10) * HOUR + parseInt(second, 10) * MINUTE) * multiplier;
        }
        return 0;
      }
    }
    throw new RangeError('Invalid offset for instant');
  }

  static from(name: string) {
    let timeZone = timeZones.get(name);
    if (!timeZone) {
      timeZone = new DateTimeFormatTimeZone(name);
      timeZones.set(name, timeZone);
    }
    return timeZone;
  }
}