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Source file src/time/zoneinfo.go

Documentation: time

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package time
     6  
     7  import (
     8  	"errors"
     9  	"sync"
    10  	"syscall"
    11  )
    12  
    13  //go:generate env ZONEINFO=$GOROOT/lib/time/zoneinfo.zip go run genzabbrs.go -output zoneinfo_abbrs_windows.go
    14  
    15  // A Location maps time instants to the zone in use at that time.
    16  // Typically, the Location represents the collection of time offsets
    17  // in use in a geographical area, such as CEST and CET for central Europe.
    18  type Location struct {
    19  	name string
    20  	zone []zone
    21  	tx   []zoneTrans
    22  
    23  	// Most lookups will be for the current time.
    24  	// To avoid the binary search through tx, keep a
    25  	// static one-element cache that gives the correct
    26  	// zone for the time when the Location was created.
    27  	// if cacheStart <= t < cacheEnd,
    28  	// lookup can return cacheZone.
    29  	// The units for cacheStart and cacheEnd are seconds
    30  	// since January 1, 1970 UTC, to match the argument
    31  	// to lookup.
    32  	cacheStart int64
    33  	cacheEnd   int64
    34  	cacheZone  *zone
    35  }
    36  
    37  // A zone represents a single time zone such as CEST or CET.
    38  type zone struct {
    39  	name   string // abbreviated name, "CET"
    40  	offset int    // seconds east of UTC
    41  	isDST  bool   // is this zone Daylight Savings Time?
    42  }
    43  
    44  // A zoneTrans represents a single time zone transition.
    45  type zoneTrans struct {
    46  	when         int64 // transition time, in seconds since 1970 GMT
    47  	index        uint8 // the index of the zone that goes into effect at that time
    48  	isstd, isutc bool  // ignored - no idea what these mean
    49  }
    50  
    51  // alpha and omega are the beginning and end of time for zone
    52  // transitions.
    53  const (
    54  	alpha = -1 << 63  // math.MinInt64
    55  	omega = 1<<63 - 1 // math.MaxInt64
    56  )
    57  
    58  // UTC represents Universal Coordinated Time (UTC).
    59  var UTC *Location = &utcLoc
    60  
    61  // utcLoc is separate so that get can refer to &utcLoc
    62  // and ensure that it never returns a nil *Location,
    63  // even if a badly behaved client has changed UTC.
    64  var utcLoc = Location{name: "UTC"}
    65  
    66  // Local represents the system's local time zone.
    67  var Local *Location = &localLoc
    68  
    69  // localLoc is separate so that initLocal can initialize
    70  // it even if a client has changed Local.
    71  var localLoc Location
    72  var localOnce sync.Once
    73  
    74  func (l *Location) get() *Location {
    75  	if l == nil {
    76  		return &utcLoc
    77  	}
    78  	if l == &localLoc {
    79  		localOnce.Do(initLocal)
    80  	}
    81  	return l
    82  }
    83  
    84  // String returns a descriptive name for the time zone information,
    85  // corresponding to the name argument to LoadLocation or FixedZone.
    86  func (l *Location) String() string {
    87  	return l.get().name
    88  }
    89  
    90  // FixedZone returns a Location that always uses
    91  // the given zone name and offset (seconds east of UTC).
    92  func FixedZone(name string, offset int) *Location {
    93  	l := &Location{
    94  		name:       name,
    95  		zone:       []zone{{name, offset, false}},
    96  		tx:         []zoneTrans{{alpha, 0, false, false}},
    97  		cacheStart: alpha,
    98  		cacheEnd:   omega,
    99  	}
   100  	l.cacheZone = &l.zone[0]
   101  	return l
   102  }
   103  
   104  // lookup returns information about the time zone in use at an
   105  // instant in time expressed as seconds since January 1, 1970 00:00:00 UTC.
   106  //
   107  // The returned information gives the name of the zone (such as "CET"),
   108  // the start and end times bracketing sec when that zone is in effect,
   109  // the offset in seconds east of UTC (such as -5*60*60), and whether
   110  // the daylight savings is being observed at that time.
   111  func (l *Location) lookup(sec int64) (name string, offset int, start, end int64) {
   112  	l = l.get()
   113  
   114  	if len(l.zone) == 0 {
   115  		name = "UTC"
   116  		offset = 0
   117  		start = alpha
   118  		end = omega
   119  		return
   120  	}
   121  
   122  	if zone := l.cacheZone; zone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
   123  		name = zone.name
   124  		offset = zone.offset
   125  		start = l.cacheStart
   126  		end = l.cacheEnd
   127  		return
   128  	}
   129  
   130  	if len(l.tx) == 0 || sec < l.tx[0].when {
   131  		zone := &l.zone[l.lookupFirstZone()]
   132  		name = zone.name
   133  		offset = zone.offset
   134  		start = alpha
   135  		if len(l.tx) > 0 {
   136  			end = l.tx[0].when
   137  		} else {
   138  			end = omega
   139  		}
   140  		return
   141  	}
   142  
   143  	// Binary search for entry with largest time <= sec.
   144  	// Not using sort.Search to avoid dependencies.
   145  	tx := l.tx
   146  	end = omega
   147  	lo := 0
   148  	hi := len(tx)
   149  	for hi-lo > 1 {
   150  		m := lo + (hi-lo)/2
   151  		lim := tx[m].when
   152  		if sec < lim {
   153  			end = lim
   154  			hi = m
   155  		} else {
   156  			lo = m
   157  		}
   158  	}
   159  	zone := &l.zone[tx[lo].index]
   160  	name = zone.name
   161  	offset = zone.offset
   162  	start = tx[lo].when
   163  	// end = maintained during the search
   164  	return
   165  }
   166  
   167  // lookupFirstZone returns the index of the time zone to use for times
   168  // before the first transition time, or when there are no transition
   169  // times.
   170  //
   171  // The reference implementation in localtime.c from
   172  // https://www.iana.org/time-zones/repository/releases/tzcode2013g.tar.gz
   173  // implements the following algorithm for these cases:
   174  // 1) If the first zone is unused by the transitions, use it.
   175  // 2) Otherwise, if there are transition times, and the first
   176  //    transition is to a zone in daylight time, find the first
   177  //    non-daylight-time zone before and closest to the first transition
   178  //    zone.
   179  // 3) Otherwise, use the first zone that is not daylight time, if
   180  //    there is one.
   181  // 4) Otherwise, use the first zone.
   182  func (l *Location) lookupFirstZone() int {
   183  	// Case 1.
   184  	if !l.firstZoneUsed() {
   185  		return 0
   186  	}
   187  
   188  	// Case 2.
   189  	if len(l.tx) > 0 && l.zone[l.tx[0].index].isDST {
   190  		for zi := int(l.tx[0].index) - 1; zi >= 0; zi-- {
   191  			if !l.zone[zi].isDST {
   192  				return zi
   193  			}
   194  		}
   195  	}
   196  
   197  	// Case 3.
   198  	for zi := range l.zone {
   199  		if !l.zone[zi].isDST {
   200  			return zi
   201  		}
   202  	}
   203  
   204  	// Case 4.
   205  	return 0
   206  }
   207  
   208  // firstZoneUsed returns whether the first zone is used by some
   209  // transition.
   210  func (l *Location) firstZoneUsed() bool {
   211  	for _, tx := range l.tx {
   212  		if tx.index == 0 {
   213  			return true
   214  		}
   215  	}
   216  	return false
   217  }
   218  
   219  // lookupName returns information about the time zone with
   220  // the given name (such as "EST") at the given pseudo-Unix time
   221  // (what the given time of day would be in UTC).
   222  func (l *Location) lookupName(name string, unix int64) (offset int, ok bool) {
   223  	l = l.get()
   224  
   225  	// First try for a zone with the right name that was actually
   226  	// in effect at the given time. (In Sydney, Australia, both standard
   227  	// and daylight-savings time are abbreviated "EST". Using the
   228  	// offset helps us pick the right one for the given time.
   229  	// It's not perfect: during the backward transition we might pick
   230  	// either one.)
   231  	for i := range l.zone {
   232  		zone := &l.zone[i]
   233  		if zone.name == name {
   234  			nam, offset, _, _ := l.lookup(unix - int64(zone.offset))
   235  			if nam == zone.name {
   236  				return offset, true
   237  			}
   238  		}
   239  	}
   240  
   241  	// Otherwise fall back to an ordinary name match.
   242  	for i := range l.zone {
   243  		zone := &l.zone[i]
   244  		if zone.name == name {
   245  			return zone.offset, true
   246  		}
   247  	}
   248  
   249  	// Otherwise, give up.
   250  	return
   251  }
   252  
   253  // NOTE(rsc): Eventually we will need to accept the POSIX TZ environment
   254  // syntax too, but I don't feel like implementing it today.
   255  
   256  var errLocation = errors.New("time: invalid location name")
   257  
   258  var zoneinfo *string
   259  var zoneinfoOnce sync.Once
   260  
   261  // LoadLocation returns the Location with the given name.
   262  //
   263  // If the name is "" or "UTC", LoadLocation returns UTC.
   264  // If the name is "Local", LoadLocation returns Local.
   265  //
   266  // Otherwise, the name is taken to be a location name corresponding to a file
   267  // in the IANA Time Zone database, such as "America/New_York".
   268  //
   269  // The time zone database needed by LoadLocation may not be
   270  // present on all systems, especially non-Unix systems.
   271  // LoadLocation looks in the directory or uncompressed zip file
   272  // named by the ZONEINFO environment variable, if any, then looks in
   273  // known installation locations on Unix systems,
   274  // and finally looks in $GOROOT/lib/time/zoneinfo.zip.
   275  func LoadLocation(name string) (*Location, error) {
   276  	if name == "" || name == "UTC" {
   277  		return UTC, nil
   278  	}
   279  	if name == "Local" {
   280  		return Local, nil
   281  	}
   282  	if containsDotDot(name) || name[0] == '/' || name[0] == '\\' {
   283  		// No valid IANA Time Zone name contains a single dot,
   284  		// much less dot dot. Likewise, none begin with a slash.
   285  		return nil, errLocation
   286  	}
   287  	zoneinfoOnce.Do(func() {
   288  		env, _ := syscall.Getenv("ZONEINFO")
   289  		zoneinfo = &env
   290  	})
   291  	if *zoneinfo != "" {
   292  		if zoneData, err := loadTzinfoFromDirOrZip(*zoneinfo, name); err == nil {
   293  			if z, err := LoadLocationFromTZData(name, zoneData); err == nil {
   294  				return z, nil
   295  			}
   296  		}
   297  	}
   298  	return loadLocation(name, zoneSources)
   299  }
   300  
   301  // containsDotDot reports whether s contains "..".
   302  func containsDotDot(s string) bool {
   303  	if len(s) < 2 {
   304  		return false
   305  	}
   306  	for i := 0; i < len(s)-1; i++ {
   307  		if s[i] == '.' && s[i+1] == '.' {
   308  			return true
   309  		}
   310  	}
   311  	return false
   312  }
   313  

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