import toDate from '../toDate/index'
import requiredArgs from '../_lib/requiredArgs/index'
import { Interval } from '../types'

const MILLISECONDS_IN_DAY = 24 * 60 * 60 * 1000

/**
 * @name getOverlappingDaysInIntervals
 * @category Interval Helpers
 * @summary Get the number of days that overlap in two time intervals
 *
 * @description
 * Get the number of days that overlap in two time intervals
 *
 * ### v2.0.0 breaking changes:
 *
 * - [Changes that are common for the whole library](https://github.com/date-fns/date-fns/blob/master/docs/upgradeGuide.md#Common-Changes).
 *
 * - The function was renamed from `getOverlappingDaysInRanges` to `getOverlappingDaysInIntervals`.
 *   This change was made to mirror the use of the word "interval" in standard ISO 8601:2004 terminology:
 *
 *   ```
 *   2.1.3
 *   time interval
 *   part of the time axis limited by two instants
 *   ```
 *
 *   Also, this function now accepts an object with `start` and `end` properties
 *   instead of two arguments as an interval.
 *   This function now throws `RangeError` if the start of the interval is after its end
 *   or if any date in the interval is `Invalid Date`.
 *
 *   ```javascript
 *   // Before v2.0.0
 *
 *   getOverlappingDaysInRanges(
 *     new Date(2014, 0, 10), new Date(2014, 0, 20),
 *     new Date(2014, 0, 17), new Date(2014, 0, 21)
 *   )
 *
 *   // v2.0.0 onward
 *
 *   getOverlappingDaysInIntervals(
 *     { start: new Date(2014, 0, 10), end: new Date(2014, 0, 20) },
 *     { start: new Date(2014, 0, 17), end: new Date(2014, 0, 21) }
 *   )
 *   ```
 *
 * @param {Interval} intervalLeft - the first interval to compare. See [Interval]{@link docs/Interval}
 * @param {Interval} intervalRight - the second interval to compare. See [Interval]{@link docs/Interval}
 * @returns {Number} the number of days that overlap in two time intervals
 * @throws {TypeError} 2 arguments required
 * @throws {RangeError} The start of an interval cannot be after its end
 * @throws {RangeError} Date in interval cannot be `Invalid Date`
 *
 * @example
 * // For overlapping time intervals adds 1 for each started overlapping day:
 * getOverlappingDaysInIntervals(
 *   { start: new Date(2014, 0, 10), end: new Date(2014, 0, 20) },
 *   { start: new Date(2014, 0, 17), end: new Date(2014, 0, 21) }
 * )
 * //=> 3
 *
 * @example
 * // For non-overlapping time intervals returns 0:
 * getOverlappingDaysInIntervals(
 *   { start: new Date(2014, 0, 10), end: new Date(2014, 0, 20) },
 *   { start: new Date(2014, 0, 21), end: new Date(2014, 0, 22) }
 * )
 * //=> 0
 */

export default function getOverlappingDaysInIntervals(
  dirtyIntervalLeft: Interval,
  dirtyIntervalRight: Interval
): number {
  requiredArgs(2, arguments)

  const intervalLeft = dirtyIntervalLeft || {}
  const intervalRight = dirtyIntervalRight || {}
  const leftStartTime = toDate(intervalLeft.start).getTime()
  const leftEndTime = toDate(intervalLeft.end).getTime()
  const rightStartTime = toDate(intervalRight.start).getTime()
  const rightEndTime = toDate(intervalRight.end).getTime()

  // Throw an exception if start date is after end date or if any date is `Invalid Date`
  if (!(leftStartTime <= leftEndTime && rightStartTime <= rightEndTime)) {
    throw new RangeError('Invalid interval')
  }

  const isOverlapping =
    leftStartTime < rightEndTime && rightStartTime < leftEndTime

  if (!isOverlapping) {
    return 0
  }

  const overlapStartDate =
    rightStartTime < leftStartTime ? leftStartTime : rightStartTime

  const overlapEndDate = rightEndTime > leftEndTime ? leftEndTime : rightEndTime

  const differenceInMs = overlapEndDate - overlapStartDate

  return Math.ceil(differenceInMs / MILLISECONDS_IN_DAY)
}