// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package time

import "sync"

// A Location maps time instants to the zone in use at that time.
// Typically, the Location represents the collection of time offsets
// in use in a geographical area. For many Locations the time offset varies
// depending on whether daylight savings time is in use at the time instant.
type Location struct {
	name string
	zone []zone
	tx   []zoneTrans

	// The tzdata information can be followed by a string that describes
	// how to handle DST transitions not recorded in zoneTrans.
	// The format is the TZ environment variable without a colon; see
	// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html.
	// Example string, for America/Los_Angeles: PST8PDT,M3.2.0,M11.1.0
	extend string

	// Most lookups will be for the current time.
	// To avoid the binary search through tx, keep a
	// static one-element cache that gives the correct
	// zone for the time when the Location was created.
	// if cacheStart <= t < cacheEnd,
	// lookup can return cacheZone.
	// The units for cacheStart and cacheEnd are seconds
	// since January 1, 1970 UTC, to match the argument
	// to lookup.
	cacheStart int64
	cacheEnd   int64
	cacheZone  *zone
}

// A zone represents a single time zone such as CET.
type zone struct {
	name   string // abbreviated name, "CET"
	offset int    // seconds east of UTC
	isDST  bool   // is this zone Daylight Savings Time?
}

// A zoneTrans represents a single time zone transition.
type zoneTrans struct {
	when         int64 // transition time, in seconds since 1970 GMT
	index        uint8 // the index of the zone that goes into effect at that time
	isstd, isutc bool  // ignored - no idea what these mean
}

// alpha and omega are the beginning and end of time for zone
// transitions.
const (
	alpha = -1 << 63  // math.MinInt64
	omega = 1<<63 - 1 // math.MaxInt64
)

// UTC represents Universal Coordinated Time (UTC).
var UTC *Location = &utcLoc

// utcLoc is separate so that get can refer to &utcLoc
// and ensure that it never returns a nil *Location,
// even if a badly behaved client has changed UTC.
var utcLoc = Location{name: "UTC"}

// Local represents the system's local time zone.
// On Unix systems, Local consults the TZ environment
// variable to find the time zone to use. No TZ means
// use the system default /etc/localtime.
// TZ="" means use UTC.
// TZ="foo" means use file foo in the system timezone directory.
var Local *Location = &localLoc

// localLoc is separate so that initLocal can initialize
// it even if a client has changed Local.
var localLoc Location
var localOnce sync.Once

func (l *Location) get() *Location {
	if l == nil {
		return &utcLoc
	}
	if l == &localLoc {
		localOnce.Do(initLocal)
	}
	return l
}

// String returns a descriptive name for the time zone information,
// corresponding to the name argument to LoadLocation or FixedZone.
func (l *Location) String() string {
	return l.get().name
}

// lookup returns information about the time zone in use at an
// instant in time expressed as seconds since January 1, 1970 00:00:00 UTC.
//
// The returned information gives the name of the zone (such as "CET"),
// the start and end times bracketing sec when that zone is in effect,
// the offset in seconds east of UTC (such as -5*60*60), and whether
// the daylight savings is being observed at that time.
func (l *Location) lookup(sec int64) (name string, offset int, start, end int64, isDST bool) {
	l = l.get()

	if len(l.zone) == 0 {
		name = "UTC"
		offset = 0
		start = alpha
		end = omega
		isDST = false
		return
	}

	if zone := l.cacheZone; zone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
		name = zone.name
		offset = zone.offset
		start = l.cacheStart
		end = l.cacheEnd
		isDST = zone.isDST
		return
	}

	/*
		if len(l.tx) == 0 || sec < l.tx[0].when {
			zone := &l.zone[l.lookupFirstZone()]
			name = zone.name
			offset = zone.offset
			start = alpha
			if len(l.tx) > 0 {
				end = l.tx[0].when
			} else {
				end = omega
			}
			isDST = zone.isDST
			return
		}

		// Binary search for entry with largest time <= sec.
		// Not using sort.Search to avoid dependencies.
		tx := l.tx
		end = omega
		lo := 0
		hi := len(tx)
		for hi-lo > 1 {
			m := lo + (hi-lo)/2
			lim := tx[m].when
			if sec < lim {
				end = lim
				hi = m
			} else {
				lo = m
			}
		}
		zone := &l.zone[tx[lo].index]
		name = zone.name
		offset = zone.offset
		start = tx[lo].when
		// end = maintained during the search
		isDST = zone.isDST

		// If we're at the end of the known zone transitions,
		// try the extend string.
		if lo == len(tx)-1 && l.extend != "" {
			if ename, eoffset, estart, eend, eisDST, ok := tzset(l.extend, start, sec); ok {
				return ename, eoffset, estart, eend, eisDST
			}
		}

		return
	*/
	panic("todo: Location.lookup")
}