time-service.ts

import { Persistence } from "https://deno.land/x/persistence@1.1.0/persistence.ts"

export class TimeService {

    public static async getAllTimeZoneEntries(): Promise<any[]> {
        const pathToTimeZonesFile = 'https://raw.githubusercontent.com/michael-spengler/time/master/timezones.json'

        const allTimeZoneEntries = JSON.parse(await Persistence.readFromRemoteFile(pathToTimeZonesFile))

        return allTimeZoneEntries
    }

    public static getTimeByOffset(offset: string) {
        const minutes = Number(TimeService.convertOffsetToMinutes(offset))

        let getDifferenceToUtcInMilisec = minutes * 60000;
        let getUTCMilisecond = new Date().getTime();

        const result = new Date(getUTCMilisecond - getDifferenceToUtcInMilisec).toISOString()


        return result.substr(11, 8)
    }


    public static async getTimeZone(countryCode: string, cityName: string): Promise<string> {

        const entry = await TimeService.getTimeZoneEntry(countryCode, cityName)

        return entry.timezone
    }

    public static async getTimeZoneOffset(countryCode: string, cityName: string, convertToMinutes: boolean = false): Promise<string> {

        const entry = await TimeService.getTimeZoneEntry(countryCode, cityName)
        if (convertToMinutes) {
            return TimeService.convertOffsetToMinutes(entry.offset).toString()
        }

        return entry.offset
    }

    public static async getTimeZoneOffsetDST(countryCode: string, cityName: string, convertToMinutes: boolean = false): Promise<string> {

        const entry = await TimeService.getTimeZoneEntry(countryCode, cityName)
        if (convertToMinutes) {
            return TimeService.convertOffsetToMinutes(entry.dayLightSavingTimeOffset).toString()
        }
        return entry.dayLightSavingTimeOffset
    }

    public static convertOffsetToMinutes(offsetString: string): number {
        const preFix = offsetString.substr(0, 1)
        const hours = Number(offsetString.substr(1, 2))
        const minutes = Number(offsetString.substr(4, 2))
        const complete = (hours * 60) + (minutes)
        if (preFix === '+') {
            return complete * -1
        }
        return complete
    }

    private static async getTimeZoneEntry(countryCode: string, cityName: string): Promise<any> {

        const allTimeZones = await TimeService.getAllTimeZoneEntries()

        const entry = allTimeZones.filter((e: any) => e.iso2 === countryCode && (e.city === cityName || e.city_ascii === cityName))[0]
        if (entry === undefined) {
            throw new Error(`I could not find the timezone for ${cityName} (${countryCode})`)
        }

        return entry
    }
}