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// 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"
"syscall"
)
// 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, such as CEST and CET for central Europe.
type Location struct {
name string
zone []zone
tx []zoneTrans
// 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 CEST or 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
}
// 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.
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 argument to LoadLocation.
func (l *Location) String() string {
return l.get().name
}
// FixedZone returns a Location that always uses
// the given zone name and offset (seconds east of UTC).
func FixedZone(name string, offset int) *Location {
l := &Location{
name: name,
zone: []zone{{name, offset, false}},
tx: []zoneTrans{{-1 << 63, 0, false, false}},
cacheStart: -1 << 63,
cacheEnd: 1<<63 - 1,
}
l.cacheZone = &l.zone[0]
return l
}
// 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, isDST bool, start, end int64) {
l = l.get()
if len(l.tx) == 0 {
name = "UTC"
offset = 0
isDST = false
start = -1 << 63
end = 1<<63 - 1
return
}
if zone := l.cacheZone; zone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
name = zone.name
offset = zone.offset
isDST = zone.isDST
start = l.cacheStart
end = l.cacheEnd
return
}
// Binary search for entry with largest time <= sec.
// Not using sort.Search to avoid dependencies.
tx := l.tx
end = 1<<63 - 1
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
isDST = zone.isDST
start = tx[lo].when
// end = maintained during the search
return
}
// lookupName returns information about the time zone with
// the given name (such as "EST") at the given pseudo-Unix time
// (what the given time of day would be in UTC).
func (l *Location) lookupName(name string, unix int64) (offset int, isDST bool, ok bool) {
l = l.get()
// First try for a zone with the right name that was actually
// in effect at the given time. (In Sydney, Australia, both standard
// and daylight-savings time are abbreviated "EST". Using the
// offset helps us pick the right one for the given time.
// It's not perfect: during the backward transition we might pick
// either one.)
for i := range l.zone {
zone := &l.zone[i]
if zone.name == name {
nam, offset, isDST, _, _ := l.lookup(unix - int64(zone.offset))
if nam == zone.name {
return offset, isDST, true
}
}
}
// Otherwise fall back to an ordinary name match.
for i := range l.zone {
zone := &l.zone[i]
if zone.name == name {
return zone.offset, zone.isDST, true
}
}
// Otherwise, give up.
return
}
// NOTE(rsc): Eventually we will need to accept the POSIX TZ environment
// syntax too, but I don't feel like implementing it today.
var zoneinfo, _ = syscall.Getenv("ZONEINFO")
// LoadLocation returns the Location with the given name.
//
// If the name is "" or "UTC", LoadLocation returns UTC.
// If the name is "Local", LoadLocation returns Local.
//
// Otherwise, the name is taken to be a location name corresponding to a file
// in the IANA Time Zone database, such as "America/New_York".
//
// The time zone database needed by LoadLocation may not be
// present on all systems, especially non-Unix systems.
// LoadLocation looks in the directory or uncompressed zip file
// named by the ZONEINFO environment variable, if any, then looks in
// known installation locations on Unix systems,
// and finally looks in $GOROOT/lib/time/zoneinfo.zip.
func LoadLocation(name string) (*Location, error) {
if name == "" || name == "UTC" {
return UTC, nil
}
if name == "Local" {
return Local, nil
}
if zoneinfo != "" {
if z, err := loadZoneFile(zoneinfo, name); err == nil {
z.name = name
return z, nil
}
}
return loadLocation(name)
}
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