zeichen-core/src/data/datetime.ts

export enum TimeDivision {
  MILLISECONDS,
  SECONDS,
  MINUTES,
  HOURS,
  DAYS
}

type GetTimeDifference = (a: Date, b: Date) => (c: TimeDivision) => number

export const getTimeDifference: GetTimeDifference = (a, b) => {
  const ms = b.getTime() - a.getTime()
  const absoluteMs = ms < 0 ? -ms : ms

  return c => {
    let divisionDifference = absoluteMs
    if (c === TimeDivision.MILLISECONDS) {
      return divisionDifference
    }

    divisionDifference /= 1000
    if (c === TimeDivision.SECONDS) {
      return divisionDifference
    }

    divisionDifference /= 60
    if (c === TimeDivision.MINUTES) {
      return divisionDifference
    }

    divisionDifference /= 60
    if (c === TimeDivision.HOURS) {
      return divisionDifference
    }

    divisionDifference /= 24
    if (c === TimeDivision.DAYS) {
      return divisionDifference
    }

    throw new Error('Unknown time division.')
  }
}

type AddTime = (refDate: Date, increment: number) => (c: TimeDivision) => Date

export const addTime: AddTime = (refDate, increment) => {
  const futureDate = new Date(refDate.getTime())
  return c => {
    let msIncrement = increment
    if (c === TimeDivision.MILLISECONDS) {
      futureDate.setMilliseconds(futureDate.getMilliseconds() + msIncrement)
      return futureDate
    }

    msIncrement *= 1000
    if (c === TimeDivision.SECONDS) {
      futureDate.setMilliseconds(futureDate.getMilliseconds() + msIncrement)
      return futureDate
    }

    msIncrement *= 60
    if (c === TimeDivision.MINUTES) {
      futureDate.setMilliseconds(futureDate.getMilliseconds() + msIncrement)
      return futureDate
    }

    msIncrement *= 60
    if (c === TimeDivision.HOURS) {
      futureDate.setMilliseconds(futureDate.getMilliseconds() + msIncrement)
      return futureDate
    }

    msIncrement *= 24
    if (c === TimeDivision.DAYS) {
      futureDate.setMilliseconds(futureDate.getMilliseconds() + msIncrement)
      return futureDate
    }
    throw new Error('Unknown time division.')
  }
}