utils.go

package kal

import (
	"errors"
	"fmt"
	"time"
)

// Checks if the given time is at the given month and day
func atMD(date time.Time, month, day int) bool {
	return (date.Month() == time.Month(month)) && (date.Day() == day)
}

// Checks if the two given times are at the same months and days
func atDate(t, when time.Time) bool {
	return (t.Month() == when.Month()) && (t.Day() == when.Day())
}

// Return the count of a given weekday from day t, +- a few days
func numberOfWeekdaysInPeriod(date time.Time, days int, whichWeekday time.Weekday) int {
	specialWeekdayCounter := 0
	when := date
	if days < 0 {
		for i := days; i <= 0; i++ {
			when = date.AddDate(0, 0, i)
			if when.Weekday() == whichWeekday {
				specialWeekdayCounter++
			}
		}
	} else {
		for i := 0; i <= days; i++ {
			when = date.AddDate(0, 0, i)
			if when.Weekday() == whichWeekday {
				specialWeekdayCounter++
			}
		}
	}
	return specialWeekdayCounter
}

// Return the number of sundays from day t, +- a few days
func sundaysInPeriod(date time.Time, days int) int {
	return numberOfWeekdaysInPeriod(date, days, time.Sunday)
}

// Find a preceding sunday, same year
func searchBackwardsForSunday(date time.Time) (time.Time, error) {
	return searchBackwardsForDaySameYear(date, time.Sunday)
}

// Find the last weekday given a month/year
func lastDayOfMonth(date time.Time, weekday time.Weekday) time.Time {

	var found time.Time

	// Start with the first day in the given month
	current := time.Date(date.Year(), date.Month(), 1, 0, 0, 0, 0, time.UTC)

	// Stay within the same month
	for current.Month() == date.Month() {
		// Check if it's the given weekday
		if current.Weekday() == weekday {
			// Found one
			found = current
		}

		// Go the next day
		current = current.AddDate(0, 0, 1)
	}

	// Return the last found day
	return found
}

// Find a later weekday, same month
func searchForwardsForDaySameMonth(date time.Time, weekday time.Weekday) (time.Time, error) {
	// Start with the day after the given date
	current := date.AddDate(0, 0, 1)

	// Stay within the same month
	for current.Month() == date.Month() {
		// Check if it's the given weekday
		if current.Weekday() == weekday {
			// Found one
			return current, nil
		}

		// Go the next day
		current = current.AddDate(0, 0, 1)
	}

	return date, errors.New("Could not find a later " + weekday.String() + " the same month!")
}

// Find a preceding weekday, same year
func searchBackwardsForDaySameYear(date time.Time, weekday time.Weekday) (time.Time, error) {
	// Start with the day before the given date
	current := date.AddDate(0, 0, -1)

	// Stay within the same year
	for current.Year() == date.Year() {
		// Check if it's the given weekday
		if current.Weekday() == weekday {
			// Found one
			return current, nil
		}

		// Go the previous day
		current = current.AddDate(0, 0, -1)
	}

	return date, errors.New("Could not find an earlier " + weekday.String() + " the same year!")
}

// Check if the given date is at the Nth weekday (for istance Sunday) of a given month
func atNthWeekdayOfMonth(date time.Time, n int, weekday time.Weekday, month time.Month) bool {
	if date.Month() != month {
		return false
	}
	nthDay, err := nthWeekdayOfMonth(date, n, weekday)
	if err != nil {
		return false
	}
	if atDate(date, nthDay) {
		return true
	}
	return false
}

// Check if the given date is at the last type of weekday (like monday)
func atLastWeekday(date time.Time, weekday time.Weekday, month time.Month) bool {
	return (date.Month() == month) && atDate(date, lastDayOfMonth(date, weekday))
}

// Find the Nth type of weekday of a given year and month
func nthWeekdayOfMonth(date time.Time, n int, whichWeekday time.Weekday) (time.Time, error) {

	specialWeekdayCounter := 0

	// Start at the first day in the given month
	current := time.Date(date.Year(), date.Month(), 1, 0, 0, 0, 0, time.UTC)

	// As long as we are in the same month
	for current.Month() == date.Month() {

		// Which weekday is it?
		if current.Weekday() == whichWeekday {
			specialWeekdayCounter++
		}

		// Is it the Nth occurrence?
		if specialWeekdayCounter == n {
			return current, nil
		}

		// If it's the given weekday, advance almost one week forward
		if current.Weekday() == whichWeekday {
			current = current.AddDate(0, 0, 7)
			continue
		}

		// Advance to the next day
		current = current.AddDate(0, 0, 1)
	}

	return date, fmt.Errorf("could not find the %dth %s in %s", n, whichWeekday, date.Month())
}

// Find the Nth sunday of a given year and month
func nthSundayOfMonth(date time.Time, n int) (time.Time, error) {
	return nthWeekdayOfMonth(date, n, time.Sunday)
}