// Package daterange resolves recurring calendar windows and normalized dates.
package daterange

import "time"

// CalendarWindow resolves a recurring month/day range around now. A range whose
// start month is after its end month crosses the year boundary.
func CalendarWindow(now time.Time, monthFrom int, dayFrom *int, monthTo int, dayTo *int) (time.Time, time.Time) {
	location := now.Location()
	startYear := now.Year()
	if monthFrom > monthTo && int(now.Month()) <= monthTo {
		startYear--
	}
	endYear := startYear
	if monthFrom > monthTo {
		endYear++
	}
	startDay := 1
	if dayFrom != nil {
		startDay = clampDay(startYear, time.Month(monthFrom), *dayFrom)
	}
	endDay := daysInMonth(endYear, time.Month(monthTo))
	if dayTo != nil {
		endDay = clampDay(endYear, time.Month(monthTo), *dayTo)
	}
	return time.Date(startYear, time.Month(monthFrom), startDay, 0, 0, 0, 0, location), time.Date(endYear, time.Month(monthTo), endDay, 23, 59, 59, 0, location)
}

// Date returns value at midnight in its original location.
func Date(value time.Time) time.Time {
	return time.Date(value.Year(), value.Month(), value.Day(), 0, 0, 0, 0, value.Location())
}

func daysInMonth(year int, month time.Month) int {
	return time.Date(year, month+1, 0, 0, 0, 0, 0, time.UTC).Day()
}

func clampDay(year int, month time.Month, day int) int {
	if maximum := daysInMonth(year, month); day > maximum {
		return maximum
	}
	if day < 1 {
		return 1
	}
	return day
}