package calendar

import (
	"fmt"
	"sync"
	"time"

	"github.com/golang/glog"
)

// UpcomingEvent is a calendar event that will happen in the near future, or is
// currently happening (relative to same arbitrary timestamp of 'now',
// depending on the way the UpcomingEvent is crated).
//
// It is a best-effort parse of an ICS/iCal event into some event that can be
// interpreted as a 'community event', to be displayed publicly on a site.
type UpcomingEvent struct {
	// UID is the unique ICS/iCal ID of this event.
	UID string
	// Summary is the 'title' of the event, usually a short one-liner.
	Summary string
	// Start and End of the events, potentially whole-day dates. See EventTime
	// for more information.
	// If Start is WholeDay then so is End, and vice-versa.
	Start *EventTime
	// End of the event, exclusive of the time range (ie. if a timestamp it
	// defines the timestamp at which the next event can start; if it's whole
	// day it defines the first day on which the event does not take place).
	End *EventTime
	// Tentative is whether this event is marked as 'Tentative' in the source
	// calendar.
	Tentative bool
}

// WholeDay returns true if this is a whole-day (or multi-day) event.
func (u *UpcomingEvent) WholeDay() bool {
	return u.Start.WholeDay
}

var (
	// onceComplainWarsawGone gates throwing a very verbose message about being
	// unable to localize UpcomingEvents into Warsaw local time by WarsawDate.
	onceComplainWarsawGone sync.Once
)

// WarsawDate prints a human-readable timestamp that makes sense within the
// context of this event taking place in Warsaw, or at least in the same
// timezone as Warsaw.
// It will return a time in one of the following formats:
//
//   YEAR/MONTH/DAY
//   (For one-day events)
//
//   YEAR/MONTH/DAY - DAY
//   (For multi-day events within the same month)
//
//   YEAR/MONTH/DAY - YEAR/MONTH/DAY
//   (For multi-day events spanning more than one month)
//
//   YEAR/MONTH/DAY HH:MM - HH:MM
//   (For timestamped events within the same day)
//
//   YEAR/MONTH/DAY HH:MM - YEAR/MONTH/DAY HH:MM
//   (For timestamped events spanning more than one day)
//
func (u *UpcomingEvent) WarsawDate() string {
	YM := "2006/01"
	D := "02"
	YMD := "2006/01/02"
	HM := "15:04"
	YMDHM := "2006/01/02 15:04"

	if u.WholeDay() {
		start := u.Start.Time
		// ICS whole-day dates are [start, end), ie. 'end' is exclusive.
		end := u.End.Time.AddDate(0, 0, -1)
		if start == end {
			// Event is one-day.
			return start.Format(YMD)
		}
		if start.Year() == end.Year() && start.Month() == end.Month() {
			// Event starts and ends on the same month, print shortened form.
			return fmt.Sprintf("%s/%s - %s", start.Format(YM), start.Format(D), end.Format(D))
		}
		// Event spans multiple months, print full form.
		return fmt.Sprintf("%s - %s", start.Format(YMD), end.Format(YMD))
	}

	warsaw, err := time.LoadLocation("Europe/Warsaw")
	if err != nil {
		onceComplainWarsawGone.Do(func() {
			glog.Errorf("Could not load Europe/Warsaw timezone, did the city cease to exist? LoadLoaction: %v", err)
		})
		// Even in the face of a cataclysm, degrade gracefully and assume the
		// users are local to this service's timezone.
		warsaw = time.Local
	}

	start := u.Start.Time.In(warsaw)
	end := u.End.Time.In(warsaw)
	if start.Year() == end.Year() && start.Month() == end.Month() && start.Day() == end.Day() {
		// Event starts and ends on same day, print shortened form.
		return fmt.Sprintf("%s %s - %s", start.Format(YMD), start.Format(HM), end.Format(HM))
	}
	// Event spans multiple days, print full form.
	return fmt.Sprintf("%s - %s", start.Format(YMDHM), end.Format(YMDHM))
}

func (u *UpcomingEvent) String() string {
	return fmt.Sprintf("%s (%s)", u.Summary, u.WarsawDate())
}

func (e *UpcomingEvent) Elapsed(t time.Time) bool {
	// Event hasn't started yet?
	if e.Start.Time.After(t) {
		return false
	}
	// Event has started, but hasn't ended?
	if e.End.Time.After(t) {
		return false
	}
	return true
}