export interface NextTimeConfig {
  /** the date from which to obtain next date according to given step */
  from: Date
  /** interval length in ms */
  step: number

  startingDate?: Date
}

// const t0 = new Date(0)

/**
 * It partitions the time line in `config.step` milliseconds and get's the next time interval start time from given date. 
 * Useful to schedule tasks or expire stuff, like, "give me the next weekly day"
 */
export function nextTime(config: NextTimeConfig): Date {
  const t0 = config.startingDate ? config.startingDate.getTime() : 0
  const t = config.from.getTime()
  if (t <= config.step) {
    throw Error('step too large')
  }
  const result = (Math.ceil(t / config.step)) * config.step + t0
  // console.log(`mod: ${t % config.step}, division: ${t/config.step}`);
  // console.log(`next time from ${t}, step ${config.step} starting from ${t0} is ${result}`);
  return new Date(result)
}