import { parse, format, addDays, subDays, differenceInDays } from 'date-fns'
import { utcToZonedTime } from 'date-fns-tz'

const FRONTEND_FORMAT = 'dd.MM.yyyy'
const BACKEND_FORMAT = 'yyyy-MM-dd'

export function daysFormatFrontend(days: string[]): string {
  if (days.length === 0) {
    return ''
  }

  if (days.length === 1) {
    return dateFormatFrontend(new Date(days[0]))
  }

  return [days[0], days.slice(-1)]
    .map((day: string) => dateFormatFrontend(new Date(day)))
    .join('-')
}

export function getDays({
  startDate,
  endDate,
  endDateExclusive = false,
}: {
  startDate: string,
  endDate: string,
  endDateExclusive?: boolean
}): string[] {
  let currentDay = new Date(startDate);
  const days = [dateFormatBackend(currentDay)]

  if (!endDate) {
    return days
  }

  const inclusiveEndDate = endDateExclusive ? subDays(new Date(endDate), 1) : new Date(endDate)

  while (currentDay < inclusiveEndDate) {
    currentDay = addDays(currentDay, 1)
    days.push(dateFormatBackend(currentDay))
  }

  return days
}

function dateFormat(date: Date, formatString: string): string {
  if (!date) {
    return null
  }
  return format(date, formatString)
}

export function dateFormatBackend(date: Date): string {
  return dateFormat(date, BACKEND_FORMAT)
}

export function dateFormatFrontend(date: Date): string {
  return dateFormat(date, FRONTEND_FORMAT)
}

function dateParse(input: string, formatString: string): Date {
  const date = parse(input, formatString, new Date())
  if (date.getTime() !== NaN) {
    return date
  }

  return null
}

export function dateParseFrontend(input: string): Date {
  return dateParse(input, FRONTEND_FORMAT)
}

export function dateParseBackend(input: string): Date {
  return dateParse(input, BACKEND_FORMAT)
}

export function nowInTz(timezone = 'Europe/Berlin'): Date {
  const now = new Date()
  return utcToZonedTime(now, timezone)
}

export function getNextDay(date: Date) {
  return addDays(date, 1)
}

export function getDayCount({ startDate, endDate }: { startDate: string, endDate: string }) {
  // TODO: check if this actually works as expected
  return differenceInDays(new Date(startDate), new Date(endDate)) + 1 // add one as it only counts full days;
}