llgo/internal/lib/time/zoneinfo.go

// 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")
}

// tzset takes a timezone string like the one found in the TZ environment
// variable, the time of the last time zone transition expressed as seconds
// since January 1, 1970 00:00:00 UTC, and a time expressed the same way.
// We call this a tzset string since in C the function tzset reads TZ.
// The return values are as for lookup, plus ok which reports whether the
// parse succeeded.
func tzset(s string, lastTxSec, sec int64) (name string, offset int, start, end int64, isDST, ok bool) {
	var (
		stdName, dstName     string
		stdOffset, dstOffset int
	)

	stdName, s, ok = tzsetName(s)
	if ok {
		stdOffset, s, ok = tzsetOffset(s)
	}
	if !ok {
		return "", 0, 0, 0, false, false
	}

	// The numbers in the tzset string are added to local time to get UTC,
	// but our offsets are added to UTC to get local time,
	// so we negate the number we see here.
	stdOffset = -stdOffset

	if len(s) == 0 || s[0] == ',' {
		// No daylight savings time.
		return stdName, stdOffset, lastTxSec, omega, false, true
	}

	dstName, s, ok = tzsetName(s)
	if ok {
		if len(s) == 0 || s[0] == ',' {
			dstOffset = stdOffset + secondsPerHour
		} else {
			dstOffset, s, ok = tzsetOffset(s)
			dstOffset = -dstOffset // as with stdOffset, above
		}
	}
	if !ok {
		return "", 0, 0, 0, false, false
	}

	if len(s) == 0 {
		// Default DST rules per tzcode.
		s = ",M3.2.0,M11.1.0"
	}
	// The TZ definition does not mention ';' here but tzcode accepts it.
	if s[0] != ',' && s[0] != ';' {
		return "", 0, 0, 0, false, false
	}
	s = s[1:]

	var startRule, endRule rule
	startRule, s, ok = tzsetRule(s)
	if !ok || len(s) == 0 || s[0] != ',' {
		return "", 0, 0, 0, false, false
	}
	s = s[1:]
	endRule, s, ok = tzsetRule(s)
	if !ok || len(s) > 0 {
		return "", 0, 0, 0, false, false
	}

	year, _, _, yday := absDate(uint64(sec+unixToInternal+internalToAbsolute), false)

	ysec := int64(yday*secondsPerDay) + sec%secondsPerDay

	// Compute start of year in seconds since Unix epoch.
	d := daysSinceEpoch(year)
	abs := int64(d * secondsPerDay)
	abs += absoluteToInternal + internalToUnix

	startSec := int64(tzruleTime(year, startRule, stdOffset))
	endSec := int64(tzruleTime(year, endRule, dstOffset))
	dstIsDST, stdIsDST := true, false
	// Note: this is a flipping of "DST" and "STD" while retaining the labels
	// This happens in southern hemispheres. The labelling here thus is a little
	// inconsistent with the goal.
	if endSec < startSec {
		startSec, endSec = endSec, startSec
		stdName, dstName = dstName, stdName
		stdOffset, dstOffset = dstOffset, stdOffset
		stdIsDST, dstIsDST = dstIsDST, stdIsDST
	}

	// The start and end values that we return are accurate
	// close to a daylight savings transition, but are otherwise
	// just the start and end of the year. That suffices for
	// the only caller that cares, which is Date.
	if ysec < startSec {
		return stdName, stdOffset, abs, startSec + abs, stdIsDST, true
	} else if ysec >= endSec {
		return stdName, stdOffset, endSec + abs, abs + 365*secondsPerDay, stdIsDST, true
	} else {
		return dstName, dstOffset, startSec + abs, endSec + abs, dstIsDST, true
	}
}

// tzsetName returns the timezone name at the start of the tzset string s,
// and the remainder of s, and reports whether the parsing is OK.
func tzsetName(s string) (string, string, bool) {
	if len(s) == 0 {
		return "", "", false
	}
	if s[0] != '<' {
		for i, r := range s {
			switch r {
			case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', ',', '-', '+':
				if i < 3 {
					return "", "", false
				}
				return s[:i], s[i:], true
			}
		}
		if len(s) < 3 {
			return "", "", false
		}
		return s, "", true
	} else {
		for i, r := range s {
			if r == '>' {
				return s[1:i], s[i+1:], true
			}
		}
		return "", "", false
	}
}

// tzsetOffset returns the timezone offset at the start of the tzset string s,
// and the remainder of s, and reports whether the parsing is OK.
// The timezone offset is returned as a number of seconds.
func tzsetOffset(s string) (offset int, rest string, ok bool) {
	if len(s) == 0 {
		return 0, "", false
	}
	neg := false
	if s[0] == '+' {
		s = s[1:]
	} else if s[0] == '-' {
		s = s[1:]
		neg = true
	}

	// The tzdata code permits values up to 24 * 7 here,
	// although POSIX does not.
	var hours int
	hours, s, ok = tzsetNum(s, 0, 24*7)
	if !ok {
		return 0, "", false
	}
	off := hours * secondsPerHour
	if len(s) == 0 || s[0] != ':' {
		if neg {
			off = -off
		}
		return off, s, true
	}

	var mins int
	mins, s, ok = tzsetNum(s[1:], 0, 59)
	if !ok {
		return 0, "", false
	}
	off += mins * secondsPerMinute
	if len(s) == 0 || s[0] != ':' {
		if neg {
			off = -off
		}
		return off, s, true
	}

	var secs int
	secs, s, ok = tzsetNum(s[1:], 0, 59)
	if !ok {
		return 0, "", false
	}
	off += secs

	if neg {
		off = -off
	}
	return off, s, true
}

// ruleKind is the kinds of rules that can be seen in a tzset string.
type ruleKind int

const (
	ruleJulian ruleKind = iota
	ruleDOY
	ruleMonthWeekDay
)

// rule is a rule read from a tzset string.
type rule struct {
	kind ruleKind
	day  int
	week int
	mon  int
	time int // transition time
}

// tzsetRule parses a rule from a tzset string.
// It returns the rule, and the remainder of the string, and reports success.
func tzsetRule(s string) (rule, string, bool) {
	var r rule
	if len(s) == 0 {
		return rule{}, "", false
	}
	ok := false
	if s[0] == 'J' {
		var jday int
		jday, s, ok = tzsetNum(s[1:], 1, 365)
		if !ok {
			return rule{}, "", false
		}
		r.kind = ruleJulian
		r.day = jday
	} else if s[0] == 'M' {
		var mon int
		mon, s, ok = tzsetNum(s[1:], 1, 12)
		if !ok || len(s) == 0 || s[0] != '.' {
			return rule{}, "", false

		}
		var week int
		week, s, ok = tzsetNum(s[1:], 1, 5)
		if !ok || len(s) == 0 || s[0] != '.' {
			return rule{}, "", false
		}
		var day int
		day, s, ok = tzsetNum(s[1:], 0, 6)
		if !ok {
			return rule{}, "", false
		}
		r.kind = ruleMonthWeekDay
		r.day = day
		r.week = week
		r.mon = mon
	} else {
		var day int
		day, s, ok = tzsetNum(s, 0, 365)
		if !ok {
			return rule{}, "", false
		}
		r.kind = ruleDOY
		r.day = day
	}

	if len(s) == 0 || s[0] != '/' {
		r.time = 2 * secondsPerHour // 2am is the default
		return r, s, true
	}

	offset, s, ok := tzsetOffset(s[1:])
	if !ok {
		return rule{}, "", false
	}
	r.time = offset

	return r, s, true
}

// tzsetNum parses a number from a tzset string.
// It returns the number, and the remainder of the string, and reports success.
// The number must be between min and max.
func tzsetNum(s string, min, max int) (num int, rest string, ok bool) {
	if len(s) == 0 {
		return 0, "", false
	}
	num = 0
	for i, r := range s {
		if r < '0' || r > '9' {
			if i == 0 || num < min {
				return 0, "", false
			}
			return num, s[i:], true
		}
		num *= 10
		num += int(r) - '0'
		if num > max {
			return 0, "", false
		}
	}
	if num < min {
		return 0, "", false
	}
	return num, "", true
}

// tzruleTime takes a year, a rule, and a timezone offset,
// and returns the number of seconds since the start of the year
// that the rule takes effect.
func tzruleTime(year int, r rule, off int) int {
	var s int
	switch r.kind {
	case ruleJulian:
		s = (r.day - 1) * secondsPerDay
		if isLeap(year) && r.day >= 60 {
			s += secondsPerDay
		}
	case ruleDOY:
		s = r.day * secondsPerDay
	case ruleMonthWeekDay:
		// Zeller's Congruence.
		m1 := (r.mon+9)%12 + 1
		yy0 := year
		if r.mon <= 2 {
			yy0--
		}
		yy1 := yy0 / 100
		yy2 := yy0 % 100
		dow := ((26*m1-2)/10 + 1 + yy2 + yy2/4 + yy1/4 - 2*yy1) % 7
		if dow < 0 {
			dow += 7
		}
		// Now dow is the day-of-week of the first day of r.mon.
		// Get the day-of-month of the first "dow" day.
		d := r.day - dow
		if d < 0 {
			d += 7
		}
		for i := 1; i < r.week; i++ {
			if d+7 >= daysIn(Month(r.mon), year) {
				break
			}
			d += 7
		}
		d += int(daysBefore[r.mon-1])
		if isLeap(year) && r.mon > 2 {
			d++
		}
		s = d * secondsPerDay
	}

	return s + r.time - off
}