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// Copyright 2009 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 (
"syscall"
"sync"
"os"
)
// BUG(brainman): The Windows implementation assumes that
// this year's rules for daylight savings time apply to all previous
// and future years as well.
// TODO(brainman): use GetDynamicTimeZoneInformation, whenever posible (Vista and up),
// to improve on situation described in the bug above.
type zone struct {
name string
offset int
year int64
month, day, dayofweek int
hour, minute, second int
abssec int64
prev *zone
}
// Populate zone struct with Windows supplied information. Returns true, if data is valid.
func (z *zone) populate(bias, biasdelta int32, d *syscall.Systemtime, name []uint16) (dateisgood bool) {
z.name = syscall.UTF16ToString(name)
z.offset = int(bias)
z.year = int64(d.Year)
z.month = int(d.Month)
z.day = int(d.Day)
z.dayofweek = int(d.DayOfWeek)
z.hour = int(d.Hour)
z.minute = int(d.Minute)
z.second = int(d.Second)
dateisgood = d.Month != 0
if dateisgood {
z.offset += int(biasdelta)
}
z.offset = -z.offset * 60
return
}
// Pre-calculte cutoff time in seconds since the Unix epoch, if data is supplied in "absolute" format.
func (z *zone) preCalculateAbsSec() {
if z.year != 0 {
z.abssec = (&Time{z.year, int(z.month), int(z.day), int(z.hour), int(z.minute), int(z.second), 0, 0, ""}).Seconds()
// Time given is in "local" time. Adjust it for "utc".
z.abssec -= int64(z.prev.offset)
}
}
// Convert zone cutoff time to sec in number of seconds since the Unix epoch, given particualar year.
func (z *zone) cutoffSeconds(year int64) int64 {
// Windows specifies daylight savings information in "day in month" format:
// z.month is month number (1-12)
// z.dayofweek is appropriate weekday (Sunday=0 to Saturday=6)
// z.day is week within the month (1 to 5, where 5 is last week of the month)
// z.hour, z.minute and z.second are absolute time
t := &Time{year, int(z.month), 1, int(z.hour), int(z.minute), int(z.second), 0, 0, ""}
t = SecondsToUTC(t.Seconds())
i := int(z.dayofweek) - t.Weekday
if i < 0 {
i += 7
}
t.Day += i
if week := int(z.day) - 1; week < 4 {
t.Day += week * 7
} else {
// "Last" instance of the day.
t.Day += 4 * 7
if t.Day > months(year)[t.Month] {
t.Day -= 7
}
}
// Result is in "local" time. Adjust it for "utc".
return t.Seconds() - int64(z.prev.offset)
}
// Is t before the cutoff for switching to z?
func (z *zone) isBeforeCutoff(t *Time) bool {
var coff int64
if z.year == 0 {
// "day in month" format used
coff = z.cutoffSeconds(t.Year)
} else {
// "absolute" format used
coff = z.abssec
}
return t.Seconds() < coff
}
type zoneinfo struct {
disabled bool // daylight saving time is not used localy
offsetIfDisabled int
januaryIsStd bool // is january 1 standard time?
std, dst zone
}
// Pick zone (std or dst) t time belongs to.
func (zi *zoneinfo) pickZone(t *Time) *zone {
z := &zi.std
if tz.januaryIsStd {
if !zi.dst.isBeforeCutoff(t) && zi.std.isBeforeCutoff(t) {
// after switch to daylight time and before the switch back to standard
z = &zi.dst
}
} else {
if zi.std.isBeforeCutoff(t) || !zi.dst.isBeforeCutoff(t) {
// before switch to standard time or after the switch back to daylight
z = &zi.dst
}
}
return z
}
var tz zoneinfo
var initError os.Error
var onceSetupZone sync.Once
func setupZone() {
var i syscall.Timezoneinformation
if _, e := syscall.GetTimeZoneInformation(&i); e != 0 {
initError = os.NewSyscallError("GetTimeZoneInformation", e)
return
}
if !tz.std.populate(i.Bias, i.StandardBias, &i.StandardDate, i.StandardName[0:]) {
tz.disabled = true
tz.offsetIfDisabled = tz.std.offset
return
}
tz.std.prev = &tz.dst
tz.dst.populate(i.Bias, i.DaylightBias, &i.DaylightDate, i.DaylightName[0:])
tz.dst.prev = &tz.std
tz.std.preCalculateAbsSec()
tz.dst.preCalculateAbsSec()
// Is january 1 standard time this year?
t := UTC()
tz.januaryIsStd = tz.dst.cutoffSeconds(t.Year) < tz.std.cutoffSeconds(t.Year)
}
// Look up the correct time zone (daylight savings or not) for the given unix time, in the current location.
func lookupTimezone(sec int64) (zone string, offset int) {
onceSetupZone.Do(setupZone)
if initError != nil {
return "", 0
}
if tz.disabled {
return "", tz.offsetIfDisabled
}
t := SecondsToUTC(sec)
z := &tz.std
if tz.std.year == 0 {
// "day in month" format used
z = tz.pickZone(t)
} else {
// "absolute" format used
if tz.std.year == t.Year {
// we have rule for the year in question
z = tz.pickZone(t)
} else {
// we do not have any information for that year,
// will assume standard offset all year around
}
}
return z.name, z.offset
}
// lookupByName returns the time offset for the
// time zone with the given abbreviation. It only considers
// time zones that apply to the current system.
func lookupByName(name string) (off int, found bool) {
onceSetupZone.Do(setupZone)
if initError != nil {
return 0, false
}
if tz.disabled {
return tz.offsetIfDisabled, false
}
switch name {
case tz.std.name:
return tz.std.offset, true
case tz.dst.name:
return tz.dst.offset, true
}
return 0, false
}
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