Source file src/time/format.go

Documentation: time

     1  // Copyright 2010 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 "errors"
     8  
     9  // These are predefined layouts for use in Time.Format and time.Parse.
    10  // The reference time used in the layouts is the specific time:
    11  //	Mon Jan 2 15:04:05 MST 2006
    12  // which is Unix time 1136239445. Since MST is GMT-0700,
    13  // the reference time can be thought of as
    14  //	01/02 03:04:05PM '06 -0700
    15  // To define your own format, write down what the reference time would look
    16  // like formatted your way; see the values of constants like ANSIC,
    17  // StampMicro or Kitchen for examples. The model is to demonstrate what the
    18  // reference time looks like so that the Format and Parse methods can apply
    19  // the same transformation to a general time value.
    20  //
    21  // Some valid layouts are invalid time values for time.Parse, due to formats
    22  // such as _ for space padding and Z for zone information.
    23  //
    24  // Within the format string, an underscore _ represents a space that may be
    25  // replaced by a digit if the following number (a day) has two digits; for
    26  // compatibility with fixed-width Unix time formats.
    27  //
    28  // A decimal point followed by one or more zeros represents a fractional
    29  // second, printed to the given number of decimal places. A decimal point
    30  // followed by one or more nines represents a fractional second, printed to
    31  // the given number of decimal places, with trailing zeros removed.
    32  // When parsing (only), the input may contain a fractional second
    33  // field immediately after the seconds field, even if the layout does not
    34  // signify its presence. In that case a decimal point followed by a maximal
    35  // series of digits is parsed as a fractional second.
    36  //
    37  // Numeric time zone offsets format as follows:
    38  //	-0700  ±hhmm
    39  //	-07:00 ±hh:mm
    40  //	-07    ±hh
    41  // Replacing the sign in the format with a Z triggers
    42  // the ISO 8601 behavior of printing Z instead of an
    43  // offset for the UTC zone. Thus:
    44  //	Z0700  Z or ±hhmm
    45  //	Z07:00 Z or ±hh:mm
    46  //	Z07    Z or ±hh
    47  //
    48  // The recognized day of week formats are "Mon" and "Monday".
    49  // The recognized month formats are "Jan" and "January".
    50  //
    51  // Text in the format string that is not recognized as part of the reference
    52  // time is echoed verbatim during Format and expected to appear verbatim
    53  // in the input to Parse.
    54  //
    55  // The executable example for Time.Format demonstrates the working
    56  // of the layout string in detail and is a good reference.
    57  //
    58  // Note that the RFC822, RFC850, and RFC1123 formats should be applied
    59  // only to local times. Applying them to UTC times will use "UTC" as the
    60  // time zone abbreviation, while strictly speaking those RFCs require the
    61  // use of "GMT" in that case.
    62  // In general RFC1123Z should be used instead of RFC1123 for servers
    63  // that insist on that format, and RFC3339 should be preferred for new protocols.
    64  // RFC3339, RFC822, RFC822Z, RFC1123, and RFC1123Z are useful for formatting;
    65  // when used with time.Parse they do not accept all the time formats
    66  // permitted by the RFCs.
    67  // The RFC3339Nano format removes trailing zeros from the seconds field
    68  // and thus may not sort correctly once formatted.
    69  const (
    70  	ANSIC       = "Mon Jan _2 15:04:05 2006"
    71  	UnixDate    = "Mon Jan _2 15:04:05 MST 2006"
    72  	RubyDate    = "Mon Jan 02 15:04:05 -0700 2006"
    73  	RFC822      = "02 Jan 06 15:04 MST"
    74  	RFC822Z     = "02 Jan 06 15:04 -0700" // RFC822 with numeric zone
    75  	RFC850      = "Monday, 02-Jan-06 15:04:05 MST"
    76  	RFC1123     = "Mon, 02 Jan 2006 15:04:05 MST"
    77  	RFC1123Z    = "Mon, 02 Jan 2006 15:04:05 -0700" // RFC1123 with numeric zone
    78  	RFC3339     = "2006-01-02T15:04:05Z07:00"
    79  	RFC3339Nano = "2006-01-02T15:04:05.999999999Z07:00"
    80  	Kitchen     = "3:04PM"
    81  	// Handy time stamps.
    82  	Stamp      = "Jan _2 15:04:05"
    83  	StampMilli = "Jan _2 15:04:05.000"
    84  	StampMicro = "Jan _2 15:04:05.000000"
    85  	StampNano  = "Jan _2 15:04:05.000000000"
    86  )
    87  
    88  const (
    89  	_                        = iota
    90  	stdLongMonth             = iota + stdNeedDate  // "January"
    91  	stdMonth                                       // "Jan"
    92  	stdNumMonth                                    // "1"
    93  	stdZeroMonth                                   // "01"
    94  	stdLongWeekDay                                 // "Monday"
    95  	stdWeekDay                                     // "Mon"
    96  	stdDay                                         // "2"
    97  	stdUnderDay                                    // "_2"
    98  	stdZeroDay                                     // "02"
    99  	stdHour                  = iota + stdNeedClock // "15"
   100  	stdHour12                                      // "3"
   101  	stdZeroHour12                                  // "03"
   102  	stdMinute                                      // "4"
   103  	stdZeroMinute                                  // "04"
   104  	stdSecond                                      // "5"
   105  	stdZeroSecond                                  // "05"
   106  	stdLongYear              = iota + stdNeedDate  // "2006"
   107  	stdYear                                        // "06"
   108  	stdPM                    = iota + stdNeedClock // "PM"
   109  	stdpm                                          // "pm"
   110  	stdTZ                    = iota                // "MST"
   111  	stdISO8601TZ                                   // "Z0700"  // prints Z for UTC
   112  	stdISO8601SecondsTZ                            // "Z070000"
   113  	stdISO8601ShortTZ                              // "Z07"
   114  	stdISO8601ColonTZ                              // "Z07:00" // prints Z for UTC
   115  	stdISO8601ColonSecondsTZ                       // "Z07:00:00"
   116  	stdNumTZ                                       // "-0700"  // always numeric
   117  	stdNumSecondsTz                                // "-070000"
   118  	stdNumShortTZ                                  // "-07"    // always numeric
   119  	stdNumColonTZ                                  // "-07:00" // always numeric
   120  	stdNumColonSecondsTZ                           // "-07:00:00"
   121  	stdFracSecond0                                 // ".0", ".00", ... , trailing zeros included
   122  	stdFracSecond9                                 // ".9", ".99", ..., trailing zeros omitted
   123  
   124  	stdNeedDate  = 1 << 8             // need month, day, year
   125  	stdNeedClock = 2 << 8             // need hour, minute, second
   126  	stdArgShift  = 16                 // extra argument in high bits, above low stdArgShift
   127  	stdMask      = 1<<stdArgShift - 1 // mask out argument
   128  )
   129  
   130  // std0x records the std values for "01", "02", ..., "06".
   131  var std0x = [...]int{stdZeroMonth, stdZeroDay, stdZeroHour12, stdZeroMinute, stdZeroSecond, stdYear}
   132  
   133  // startsWithLowerCase reports whether the string has a lower-case letter at the beginning.
   134  // Its purpose is to prevent matching strings like "Month" when looking for "Mon".
   135  func startsWithLowerCase(str string) bool {
   136  	if len(str) == 0 {
   137  		return false
   138  	}
   139  	c := str[0]
   140  	return 'a' <= c && c <= 'z'
   141  }
   142  
   143  // nextStdChunk finds the first occurrence of a std string in
   144  // layout and returns the text before, the std string, and the text after.
   145  func nextStdChunk(layout string) (prefix string, std int, suffix string) {
   146  	for i := 0; i < len(layout); i++ {
   147  		switch c := int(layout[i]); c {
   148  		case 'J': // January, Jan
   149  			if len(layout) >= i+3 && layout[i:i+3] == "Jan" {
   150  				if len(layout) >= i+7 && layout[i:i+7] == "January" {
   151  					return layout[0:i], stdLongMonth, layout[i+7:]
   152  				}
   153  				if !startsWithLowerCase(layout[i+3:]) {
   154  					return layout[0:i], stdMonth, layout[i+3:]
   155  				}
   156  			}
   157  
   158  		case 'M': // Monday, Mon, MST
   159  			if len(layout) >= i+3 {
   160  				if layout[i:i+3] == "Mon" {
   161  					if len(layout) >= i+6 && layout[i:i+6] == "Monday" {
   162  						return layout[0:i], stdLongWeekDay, layout[i+6:]
   163  					}
   164  					if !startsWithLowerCase(layout[i+3:]) {
   165  						return layout[0:i], stdWeekDay, layout[i+3:]
   166  					}
   167  				}
   168  				if layout[i:i+3] == "MST" {
   169  					return layout[0:i], stdTZ, layout[i+3:]
   170  				}
   171  			}
   172  
   173  		case '0': // 01, 02, 03, 04, 05, 06
   174  			if len(layout) >= i+2 && '1' <= layout[i+1] && layout[i+1] <= '6' {
   175  				return layout[0:i], std0x[layout[i+1]-'1'], layout[i+2:]
   176  			}
   177  
   178  		case '1': // 15, 1
   179  			if len(layout) >= i+2 && layout[i+1] == '5' {
   180  				return layout[0:i], stdHour, layout[i+2:]
   181  			}
   182  			return layout[0:i], stdNumMonth, layout[i+1:]
   183  
   184  		case '2': // 2006, 2
   185  			if len(layout) >= i+4 && layout[i:i+4] == "2006" {
   186  				return layout[0:i], stdLongYear, layout[i+4:]
   187  			}
   188  			return layout[0:i], stdDay, layout[i+1:]
   189  
   190  		case '_': // _2, _2006
   191  			if len(layout) >= i+2 && layout[i+1] == '2' {
   192  				//_2006 is really a literal _, followed by stdLongYear
   193  				if len(layout) >= i+5 && layout[i+1:i+5] == "2006" {
   194  					return layout[0 : i+1], stdLongYear, layout[i+5:]
   195  				}
   196  				return layout[0:i], stdUnderDay, layout[i+2:]
   197  			}
   198  
   199  		case '3':
   200  			return layout[0:i], stdHour12, layout[i+1:]
   201  
   202  		case '4':
   203  			return layout[0:i], stdMinute, layout[i+1:]
   204  
   205  		case '5':
   206  			return layout[0:i], stdSecond, layout[i+1:]
   207  
   208  		case 'P': // PM
   209  			if len(layout) >= i+2 && layout[i+1] == 'M' {
   210  				return layout[0:i], stdPM, layout[i+2:]
   211  			}
   212  
   213  		case 'p': // pm
   214  			if len(layout) >= i+2 && layout[i+1] == 'm' {
   215  				return layout[0:i], stdpm, layout[i+2:]
   216  			}
   217  
   218  		case '-': // -070000, -07:00:00, -0700, -07:00, -07
   219  			if len(layout) >= i+7 && layout[i:i+7] == "-070000" {
   220  				return layout[0:i], stdNumSecondsTz, layout[i+7:]
   221  			}
   222  			if len(layout) >= i+9 && layout[i:i+9] == "-07:00:00" {
   223  				return layout[0:i], stdNumColonSecondsTZ, layout[i+9:]
   224  			}
   225  			if len(layout) >= i+5 && layout[i:i+5] == "-0700" {
   226  				return layout[0:i], stdNumTZ, layout[i+5:]
   227  			}
   228  			if len(layout) >= i+6 && layout[i:i+6] == "-07:00" {
   229  				return layout[0:i], stdNumColonTZ, layout[i+6:]
   230  			}
   231  			if len(layout) >= i+3 && layout[i:i+3] == "-07" {
   232  				return layout[0:i], stdNumShortTZ, layout[i+3:]
   233  			}
   234  
   235  		case 'Z': // Z070000, Z07:00:00, Z0700, Z07:00,
   236  			if len(layout) >= i+7 && layout[i:i+7] == "Z070000" {
   237  				return layout[0:i], stdISO8601SecondsTZ, layout[i+7:]
   238  			}
   239  			if len(layout) >= i+9 && layout[i:i+9] == "Z07:00:00" {
   240  				return layout[0:i], stdISO8601ColonSecondsTZ, layout[i+9:]
   241  			}
   242  			if len(layout) >= i+5 && layout[i:i+5] == "Z0700" {
   243  				return layout[0:i], stdISO8601TZ, layout[i+5:]
   244  			}
   245  			if len(layout) >= i+6 && layout[i:i+6] == "Z07:00" {
   246  				return layout[0:i], stdISO8601ColonTZ, layout[i+6:]
   247  			}
   248  			if len(layout) >= i+3 && layout[i:i+3] == "Z07" {
   249  				return layout[0:i], stdISO8601ShortTZ, layout[i+3:]
   250  			}
   251  
   252  		case '.': // .000 or .999 - repeated digits for fractional seconds.
   253  			if i+1 < len(layout) && (layout[i+1] == '0' || layout[i+1] == '9') {
   254  				ch := layout[i+1]
   255  				j := i + 1
   256  				for j < len(layout) && layout[j] == ch {
   257  					j++
   258  				}
   259  				// String of digits must end here - only fractional second is all digits.
   260  				if !isDigit(layout, j) {
   261  					std := stdFracSecond0
   262  					if layout[i+1] == '9' {
   263  						std = stdFracSecond9
   264  					}
   265  					std |= (j - (i + 1)) << stdArgShift
   266  					return layout[0:i], std, layout[j:]
   267  				}
   268  			}
   269  		}
   270  	}
   271  	return layout, 0, ""
   272  }
   273  
   274  var longDayNames = []string{
   275  	"Sunday",
   276  	"Monday",
   277  	"Tuesday",
   278  	"Wednesday",
   279  	"Thursday",
   280  	"Friday",
   281  	"Saturday",
   282  }
   283  
   284  var shortDayNames = []string{
   285  	"Sun",
   286  	"Mon",
   287  	"Tue",
   288  	"Wed",
   289  	"Thu",
   290  	"Fri",
   291  	"Sat",
   292  }
   293  
   294  var shortMonthNames = []string{
   295  	"Jan",
   296  	"Feb",
   297  	"Mar",
   298  	"Apr",
   299  	"May",
   300  	"Jun",
   301  	"Jul",
   302  	"Aug",
   303  	"Sep",
   304  	"Oct",
   305  	"Nov",
   306  	"Dec",
   307  }
   308  
   309  var longMonthNames = []string{
   310  	"January",
   311  	"February",
   312  	"March",
   313  	"April",
   314  	"May",
   315  	"June",
   316  	"July",
   317  	"August",
   318  	"September",
   319  	"October",
   320  	"November",
   321  	"December",
   322  }
   323  
   324  // match reports whether s1 and s2 match ignoring case.
   325  // It is assumed s1 and s2 are the same length.
   326  func match(s1, s2 string) bool {
   327  	for i := 0; i < len(s1); i++ {
   328  		c1 := s1[i]
   329  		c2 := s2[i]
   330  		if c1 != c2 {
   331  			// Switch to lower-case; 'a'-'A' is known to be a single bit.
   332  			c1 |= 'a' - 'A'
   333  			c2 |= 'a' - 'A'
   334  			if c1 != c2 || c1 < 'a' || c1 > 'z' {
   335  				return false
   336  			}
   337  		}
   338  	}
   339  	return true
   340  }
   341  
   342  func lookup(tab []string, val string) (int, string, error) {
   343  	for i, v := range tab {
   344  		if len(val) >= len(v) && match(val[0:len(v)], v) {
   345  			return i, val[len(v):], nil
   346  		}
   347  	}
   348  	return -1, val, errBad
   349  }
   350  
   351  // appendInt appends the decimal form of x to b and returns the result.
   352  // If the decimal form (excluding sign) is shorter than width, the result is padded with leading 0's.
   353  // Duplicates functionality in strconv, but avoids dependency.
   354  func appendInt(b []byte, x int, width int) []byte {
   355  	u := uint(x)
   356  	if x < 0 {
   357  		b = append(b, '-')
   358  		u = uint(-x)
   359  	}
   360  
   361  	// Assemble decimal in reverse order.
   362  	var buf [20]byte
   363  	i := len(buf)
   364  	for u >= 10 {
   365  		i--
   366  		q := u / 10
   367  		buf[i] = byte('0' + u - q*10)
   368  		u = q
   369  	}
   370  	i--
   371  	buf[i] = byte('0' + u)
   372  
   373  	// Add 0-padding.
   374  	for w := len(buf) - i; w < width; w++ {
   375  		b = append(b, '0')
   376  	}
   377  
   378  	return append(b, buf[i:]...)
   379  }
   380  
   381  // Never printed, just needs to be non-nil for return by atoi.
   382  var atoiError = errors.New("time: invalid number")
   383  
   384  // Duplicates functionality in strconv, but avoids dependency.
   385  func atoi(s string) (x int, err error) {
   386  	neg := false
   387  	if s != "" && (s[0] == '-' || s[0] == '+') {
   388  		neg = s[0] == '-'
   389  		s = s[1:]
   390  	}
   391  	q, rem, err := leadingInt(s)
   392  	x = int(q)
   393  	if err != nil || rem != "" {
   394  		return 0, atoiError
   395  	}
   396  	if neg {
   397  		x = -x
   398  	}
   399  	return x, nil
   400  }
   401  
   402  // formatNano appends a fractional second, as nanoseconds, to b
   403  // and returns the result.
   404  func formatNano(b []byte, nanosec uint, n int, trim bool) []byte {
   405  	u := nanosec
   406  	var buf [9]byte
   407  	for start := len(buf); start > 0; {
   408  		start--
   409  		buf[start] = byte(u%10 + '0')
   410  		u /= 10
   411  	}
   412  
   413  	if n > 9 {
   414  		n = 9
   415  	}
   416  	if trim {
   417  		for n > 0 && buf[n-1] == '0' {
   418  			n--
   419  		}
   420  		if n == 0 {
   421  			return b
   422  		}
   423  	}
   424  	b = append(b, '.')
   425  	return append(b, buf[:n]...)
   426  }
   427  
   428  // String returns the time formatted using the format string
   429  //	"2006-01-02 15:04:05.999999999 -0700 MST"
   430  //
   431  // If the time has a monotonic clock reading, the returned string
   432  // includes a final field "m=±<value>", where value is the monotonic
   433  // clock reading formatted as a decimal number of seconds.
   434  //
   435  // The returned string is meant for debugging; for a stable serialized
   436  // representation, use t.MarshalText, t.MarshalBinary, or t.Format
   437  // with an explicit format string.
   438  func (t Time) String() string {
   439  	s := t.Format("2006-01-02 15:04:05.999999999 -0700 MST")
   440  
   441  	// Format monotonic clock reading as m=±ddd.nnnnnnnnn.
   442  	if t.wall&hasMonotonic != 0 {
   443  		m2 := uint64(t.ext)
   444  		sign := byte('+')
   445  		if t.ext < 0 {
   446  			sign = '-'
   447  			m2 = -m2
   448  		}
   449  		m1, m2 := m2/1e9, m2%1e9
   450  		m0, m1 := m1/1e9, m1%1e9
   451  		var buf []byte
   452  		buf = append(buf, " m="...)
   453  		buf = append(buf, sign)
   454  		wid := 0
   455  		if m0 != 0 {
   456  			buf = appendInt(buf, int(m0), 0)
   457  			wid = 9
   458  		}
   459  		buf = appendInt(buf, int(m1), wid)
   460  		buf = append(buf, '.')
   461  		buf = appendInt(buf, int(m2), 9)
   462  		s += string(buf)
   463  	}
   464  	return s
   465  }
   466  
   467  // Format returns a textual representation of the time value formatted
   468  // according to layout, which defines the format by showing how the reference
   469  // time, defined to be
   470  //	Mon Jan 2 15:04:05 -0700 MST 2006
   471  // would be displayed if it were the value; it serves as an example of the
   472  // desired output. The same display rules will then be applied to the time
   473  // value.
   474  //
   475  // A fractional second is represented by adding a period and zeros
   476  // to the end of the seconds section of layout string, as in "15:04:05.000"
   477  // to format a time stamp with millisecond precision.
   478  //
   479  // Predefined layouts ANSIC, UnixDate, RFC3339 and others describe standard
   480  // and convenient representations of the reference time. For more information
   481  // about the formats and the definition of the reference time, see the
   482  // documentation for ANSIC and the other constants defined by this package.
   483  func (t Time) Format(layout string) string {
   484  	const bufSize = 64
   485  	var b []byte
   486  	max := len(layout) + 10
   487  	if max < bufSize {
   488  		var buf [bufSize]byte
   489  		b = buf[:0]
   490  	} else {
   491  		b = make([]byte, 0, max)
   492  	}
   493  	b = t.AppendFormat(b, layout)
   494  	return string(b)
   495  }
   496  
   497  // AppendFormat is like Format but appends the textual
   498  // representation to b and returns the extended buffer.
   499  func (t Time) AppendFormat(b []byte, layout string) []byte {
   500  	var (
   501  		name, offset, abs = t.locabs()
   502  
   503  		year  int = -1
   504  		month Month
   505  		day   int
   506  		hour  int = -1
   507  		min   int
   508  		sec   int
   509  	)
   510  	// Each iteration generates one std value.
   511  	for layout != "" {
   512  		prefix, std, suffix := nextStdChunk(layout)
   513  		if prefix != "" {
   514  			b = append(b, prefix...)
   515  		}
   516  		if std == 0 {
   517  			break
   518  		}
   519  		layout = suffix
   520  
   521  		// Compute year, month, day if needed.
   522  		if year < 0 && std&stdNeedDate != 0 {
   523  			year, month, day, _ = absDate(abs, true)
   524  		}
   525  
   526  		// Compute hour, minute, second if needed.
   527  		if hour < 0 && std&stdNeedClock != 0 {
   528  			hour, min, sec = absClock(abs)
   529  		}
   530  
   531  		switch std & stdMask {
   532  		case stdYear:
   533  			y := year
   534  			if y < 0 {
   535  				y = -y
   536  			}
   537  			b = appendInt(b, y%100, 2)
   538  		case stdLongYear:
   539  			b = appendInt(b, year, 4)
   540  		case stdMonth:
   541  			b = append(b, month.String()[:3]...)
   542  		case stdLongMonth:
   543  			m := month.String()
   544  			b = append(b, m...)
   545  		case stdNumMonth:
   546  			b = appendInt(b, int(month), 0)
   547  		case stdZeroMonth:
   548  			b = appendInt(b, int(month), 2)
   549  		case stdWeekDay:
   550  			b = append(b, absWeekday(abs).String()[:3]...)
   551  		case stdLongWeekDay:
   552  			s := absWeekday(abs).String()
   553  			b = append(b, s...)
   554  		case stdDay:
   555  			b = appendInt(b, day, 0)
   556  		case stdUnderDay:
   557  			if day < 10 {
   558  				b = append(b, ' ')
   559  			}
   560  			b = appendInt(b, day, 0)
   561  		case stdZeroDay:
   562  			b = appendInt(b, day, 2)
   563  		case stdHour:
   564  			b = appendInt(b, hour, 2)
   565  		case stdHour12:
   566  			// Noon is 12PM, midnight is 12AM.
   567  			hr := hour % 12
   568  			if hr == 0 {
   569  				hr = 12
   570  			}
   571  			b = appendInt(b, hr, 0)
   572  		case stdZeroHour12:
   573  			// Noon is 12PM, midnight is 12AM.
   574  			hr := hour % 12
   575  			if hr == 0 {
   576  				hr = 12
   577  			}
   578  			b = appendInt(b, hr, 2)
   579  		case stdMinute:
   580  			b = appendInt(b, min, 0)
   581  		case stdZeroMinute:
   582  			b = appendInt(b, min, 2)
   583  		case stdSecond:
   584  			b = appendInt(b, sec, 0)
   585  		case stdZeroSecond:
   586  			b = appendInt(b, sec, 2)
   587  		case stdPM:
   588  			if hour >= 12 {
   589  				b = append(b, "PM"...)
   590  			} else {
   591  				b = append(b, "AM"...)
   592  			}
   593  		case stdpm:
   594  			if hour >= 12 {
   595  				b = append(b, "pm"...)
   596  			} else {
   597  				b = append(b, "am"...)
   598  			}
   599  		case stdISO8601TZ, stdISO8601ColonTZ, stdISO8601SecondsTZ, stdISO8601ShortTZ, stdISO8601ColonSecondsTZ, stdNumTZ, stdNumColonTZ, stdNumSecondsTz, stdNumShortTZ, stdNumColonSecondsTZ:
   600  			// Ugly special case. We cheat and take the "Z" variants
   601  			// to mean "the time zone as formatted for ISO 8601".
   602  			if offset == 0 && (std == stdISO8601TZ || std == stdISO8601ColonTZ || std == stdISO8601SecondsTZ || std == stdISO8601ShortTZ || std == stdISO8601ColonSecondsTZ) {
   603  				b = append(b, 'Z')
   604  				break
   605  			}
   606  			zone := offset / 60 // convert to minutes
   607  			absoffset := offset
   608  			if zone < 0 {
   609  				b = append(b, '-')
   610  				zone = -zone
   611  				absoffset = -absoffset
   612  			} else {
   613  				b = append(b, '+')
   614  			}
   615  			b = appendInt(b, zone/60, 2)
   616  			if std == stdISO8601ColonTZ || std == stdNumColonTZ || std == stdISO8601ColonSecondsTZ || std == stdNumColonSecondsTZ {
   617  				b = append(b, ':')
   618  			}
   619  			if std != stdNumShortTZ && std != stdISO8601ShortTZ {
   620  				b = appendInt(b, zone%60, 2)
   621  			}
   622  
   623  			// append seconds if appropriate
   624  			if std == stdISO8601SecondsTZ || std == stdNumSecondsTz || std == stdNumColonSecondsTZ || std == stdISO8601ColonSecondsTZ {
   625  				if std == stdNumColonSecondsTZ || std == stdISO8601ColonSecondsTZ {
   626  					b = append(b, ':')
   627  				}
   628  				b = appendInt(b, absoffset%60, 2)
   629  			}
   630  
   631  		case stdTZ:
   632  			if name != "" {
   633  				b = append(b, name...)
   634  				break
   635  			}
   636  			// No time zone known for this time, but we must print one.
   637  			// Use the -0700 format.
   638  			zone := offset / 60 // convert to minutes
   639  			if zone < 0 {
   640  				b = append(b, '-')
   641  				zone = -zone
   642  			} else {
   643  				b = append(b, '+')
   644  			}
   645  			b = appendInt(b, zone/60, 2)
   646  			b = appendInt(b, zone%60, 2)
   647  		case stdFracSecond0, stdFracSecond9:
   648  			b = formatNano(b, uint(t.Nanosecond()), std>>stdArgShift, std&stdMask == stdFracSecond9)
   649  		}
   650  	}
   651  	return b
   652  }
   653  
   654  var errBad = errors.New("bad value for field") // placeholder not passed to user
   655  
   656  // ParseError describes a problem parsing a time string.
   657  type ParseError struct {
   658  	Layout     string
   659  	Value      string
   660  	LayoutElem string
   661  	ValueElem  string
   662  	Message    string
   663  }
   664  
   665  func quote(s string) string {
   666  	return "\"" + s + "\""
   667  }
   668  
   669  // Error returns the string representation of a ParseError.
   670  func (e *ParseError) Error() string {
   671  	if e.Message == "" {
   672  		return "parsing time " +
   673  			quote(e.Value) + " as " +
   674  			quote(e.Layout) + ": cannot parse " +
   675  			quote(e.ValueElem) + " as " +
   676  			quote(e.LayoutElem)
   677  	}
   678  	return "parsing time " +
   679  		quote(e.Value) + e.Message
   680  }
   681  
   682  // isDigit reports whether s[i] is in range and is a decimal digit.
   683  func isDigit(s string, i int) bool {
   684  	if len(s) <= i {
   685  		return false
   686  	}
   687  	c := s[i]
   688  	return '0' <= c && c <= '9'
   689  }
   690  
   691  // getnum parses s[0:1] or s[0:2] (fixed forces the latter)
   692  // as a decimal integer and returns the integer and the
   693  // remainder of the string.
   694  func getnum(s string, fixed bool) (int, string, error) {
   695  	if !isDigit(s, 0) {
   696  		return 0, s, errBad
   697  	}
   698  	if !isDigit(s, 1) {
   699  		if fixed {
   700  			return 0, s, errBad
   701  		}
   702  		return int(s[0] - '0'), s[1:], nil
   703  	}
   704  	return int(s[0]-'0')*10 + int(s[1]-'0'), s[2:], nil
   705  }
   706  
   707  func cutspace(s string) string {
   708  	for len(s) > 0 && s[0] == ' ' {
   709  		s = s[1:]
   710  	}
   711  	return s
   712  }
   713  
   714  // skip removes the given prefix from value,
   715  // treating runs of space characters as equivalent.
   716  func skip(value, prefix string) (string, error) {
   717  	for len(prefix) > 0 {
   718  		if prefix[0] == ' ' {
   719  			if len(value) > 0 && value[0] != ' ' {
   720  				return value, errBad
   721  			}
   722  			prefix = cutspace(prefix)
   723  			value = cutspace(value)
   724  			continue
   725  		}
   726  		if len(value) == 0 || value[0] != prefix[0] {
   727  			return value, errBad
   728  		}
   729  		prefix = prefix[1:]
   730  		value = value[1:]
   731  	}
   732  	return value, nil
   733  }
   734  
   735  // Parse parses a formatted string and returns the time value it represents.
   736  // The layout defines the format by showing how the reference time,
   737  // defined to be
   738  //	Mon Jan 2 15:04:05 -0700 MST 2006
   739  // would be interpreted if it were the value; it serves as an example of
   740  // the input format. The same interpretation will then be made to the
   741  // input string.
   742  //
   743  // Predefined layouts ANSIC, UnixDate, RFC3339 and others describe standard
   744  // and convenient representations of the reference time. For more information
   745  // about the formats and the definition of the reference time, see the
   746  // documentation for ANSIC and the other constants defined by this package.
   747  // Also, the executable example for Time.Format demonstrates the working
   748  // of the layout string in detail and is a good reference.
   749  //
   750  // Elements omitted from the value are assumed to be zero or, when
   751  // zero is impossible, one, so parsing "3:04pm" returns the time
   752  // corresponding to Jan 1, year 0, 15:04:00 UTC (note that because the year is
   753  // 0, this time is before the zero Time).
   754  // Years must be in the range 0000..9999. The day of the week is checked
   755  // for syntax but it is otherwise ignored.
   756  //
   757  // In the absence of a time zone indicator, Parse returns a time in UTC.
   758  //
   759  // When parsing a time with a zone offset like -0700, if the offset corresponds
   760  // to a time zone used by the current location (Local), then Parse uses that
   761  // location and zone in the returned time. Otherwise it records the time as
   762  // being in a fabricated location with time fixed at the given zone offset.
   763  //
   764  // When parsing a time with a zone abbreviation like MST, if the zone abbreviation
   765  // has a defined offset in the current location, then that offset is used.
   766  // The zone abbreviation "UTC" is recognized as UTC regardless of location.
   767  // If the zone abbreviation is unknown, Parse records the time as being
   768  // in a fabricated location with the given zone abbreviation and a zero offset.
   769  // This choice means that such a time can be parsed and reformatted with the
   770  // same layout losslessly, but the exact instant used in the representation will
   771  // differ by the actual zone offset. To avoid such problems, prefer time layouts
   772  // that use a numeric zone offset, or use ParseInLocation.
   773  func Parse(layout, value string) (Time, error) {
   774  	return parse(layout, value, UTC, Local)
   775  }
   776  
   777  // ParseInLocation is like Parse but differs in two important ways.
   778  // First, in the absence of time zone information, Parse interprets a time as UTC;
   779  // ParseInLocation interprets the time as in the given location.
   780  // Second, when given a zone offset or abbreviation, Parse tries to match it
   781  // against the Local location; ParseInLocation uses the given location.
   782  func ParseInLocation(layout, value string, loc *Location) (Time, error) {
   783  	return parse(layout, value, loc, loc)
   784  }
   785  
   786  func parse(layout, value string, defaultLocation, local *Location) (Time, error) {
   787  	alayout, avalue := layout, value
   788  	rangeErrString := "" // set if a value is out of range
   789  	amSet := false       // do we need to subtract 12 from the hour for midnight?
   790  	pmSet := false       // do we need to add 12 to the hour?
   791  
   792  	// Time being constructed.
   793  	var (
   794  		year       int
   795  		month      int = 1 // January
   796  		day        int = 1
   797  		hour       int
   798  		min        int
   799  		sec        int
   800  		nsec       int
   801  		z          *Location
   802  		zoneOffset int = -1
   803  		zoneName   string
   804  	)
   805  
   806  	// Each iteration processes one std value.
   807  	for {
   808  		var err error
   809  		prefix, std, suffix := nextStdChunk(layout)
   810  		stdstr := layout[len(prefix) : len(layout)-len(suffix)]
   811  		value, err = skip(value, prefix)
   812  		if err != nil {
   813  			return Time{}, &ParseError{alayout, avalue, prefix, value, ""}
   814  		}
   815  		if std == 0 {
   816  			if len(value) != 0 {
   817  				return Time{}, &ParseError{alayout, avalue, "", value, ": extra text: " + value}
   818  			}
   819  			break
   820  		}
   821  		layout = suffix
   822  		var p string
   823  		switch std & stdMask {
   824  		case stdYear:
   825  			if len(value) < 2 {
   826  				err = errBad
   827  				break
   828  			}
   829  			p, value = value[0:2], value[2:]
   830  			year, err = atoi(p)
   831  			if year >= 69 { // Unix time starts Dec 31 1969 in some time zones
   832  				year += 1900
   833  			} else {
   834  				year += 2000
   835  			}
   836  		case stdLongYear:
   837  			if len(value) < 4 || !isDigit(value, 0) {
   838  				err = errBad
   839  				break
   840  			}
   841  			p, value = value[0:4], value[4:]
   842  			year, err = atoi(p)
   843  		case stdMonth:
   844  			month, value, err = lookup(shortMonthNames, value)
   845  			month++
   846  		case stdLongMonth:
   847  			month, value, err = lookup(longMonthNames, value)
   848  			month++
   849  		case stdNumMonth, stdZeroMonth:
   850  			month, value, err = getnum(value, std == stdZeroMonth)
   851  			if month <= 0 || 12 < month {
   852  				rangeErrString = "month"
   853  			}
   854  		case stdWeekDay:
   855  			// Ignore weekday except for error checking.
   856  			_, value, err = lookup(shortDayNames, value)
   857  		case stdLongWeekDay:
   858  			_, value, err = lookup(longDayNames, value)
   859  		case stdDay, stdUnderDay, stdZeroDay:
   860  			if std == stdUnderDay && len(value) > 0 && value[0] == ' ' {
   861  				value = value[1:]
   862  			}
   863  			day, value, err = getnum(value, std == stdZeroDay)
   864  			if day < 0 {
   865  				// Note that we allow any one- or two-digit day here.
   866  				rangeErrString = "day"
   867  			}
   868  		case stdHour:
   869  			hour, value, err = getnum(value, false)
   870  			if hour < 0 || 24 <= hour {
   871  				rangeErrString = "hour"
   872  			}
   873  		case stdHour12, stdZeroHour12:
   874  			hour, value, err = getnum(value, std == stdZeroHour12)
   875  			if hour < 0 || 12 < hour {
   876  				rangeErrString = "hour"
   877  			}
   878  		case stdMinute, stdZeroMinute:
   879  			min, value, err = getnum(value, std == stdZeroMinute)
   880  			if min < 0 || 60 <= min {
   881  				rangeErrString = "minute"
   882  			}
   883  		case stdSecond, stdZeroSecond:
   884  			sec, value, err = getnum(value, std == stdZeroSecond)
   885  			if sec < 0 || 60 <= sec {
   886  				rangeErrString = "second"
   887  				break
   888  			}
   889  			// Special case: do we have a fractional second but no
   890  			// fractional second in the format?
   891  			if len(value) >= 2 && value[0] == '.' && isDigit(value, 1) {
   892  				_, std, _ = nextStdChunk(layout)
   893  				std &= stdMask
   894  				if std == stdFracSecond0 || std == stdFracSecond9 {
   895  					// Fractional second in the layout; proceed normally
   896  					break
   897  				}
   898  				// No fractional second in the layout but we have one in the input.
   899  				n := 2
   900  				for ; n < len(value) && isDigit(value, n); n++ {
   901  				}
   902  				nsec, rangeErrString, err = parseNanoseconds(value, n)
   903  				value = value[n:]
   904  			}
   905  		case stdPM:
   906  			if len(value) < 2 {
   907  				err = errBad
   908  				break
   909  			}
   910  			p, value = value[0:2], value[2:]
   911  			switch p {
   912  			case "PM":
   913  				pmSet = true
   914  			case "AM":
   915  				amSet = true
   916  			default:
   917  				err = errBad
   918  			}
   919  		case stdpm:
   920  			if len(value) < 2 {
   921  				err = errBad
   922  				break
   923  			}
   924  			p, value = value[0:2], value[2:]
   925  			switch p {
   926  			case "pm":
   927  				pmSet = true
   928  			case "am":
   929  				amSet = true
   930  			default:
   931  				err = errBad
   932  			}
   933  		case stdISO8601TZ, stdISO8601ColonTZ, stdISO8601SecondsTZ, stdISO8601ShortTZ, stdISO8601ColonSecondsTZ, stdNumTZ, stdNumShortTZ, stdNumColonTZ, stdNumSecondsTz, stdNumColonSecondsTZ:
   934  			if (std == stdISO8601TZ || std == stdISO8601ShortTZ || std == stdISO8601ColonTZ) && len(value) >= 1 && value[0] == 'Z' {
   935  				value = value[1:]
   936  				z = UTC
   937  				break
   938  			}
   939  			var sign, hour, min, seconds string
   940  			if std == stdISO8601ColonTZ || std == stdNumColonTZ {
   941  				if len(value) < 6 {
   942  					err = errBad
   943  					break
   944  				}
   945  				if value[3] != ':' {
   946  					err = errBad
   947  					break
   948  				}
   949  				sign, hour, min, seconds, value = value[0:1], value[1:3], value[4:6], "00", value[6:]
   950  			} else if std == stdNumShortTZ || std == stdISO8601ShortTZ {
   951  				if len(value) < 3 {
   952  					err = errBad
   953  					break
   954  				}
   955  				sign, hour, min, seconds, value = value[0:1], value[1:3], "00", "00", value[3:]
   956  			} else if std == stdISO8601ColonSecondsTZ || std == stdNumColonSecondsTZ {
   957  				if len(value) < 9 {
   958  					err = errBad
   959  					break
   960  				}
   961  				if value[3] != ':' || value[6] != ':' {
   962  					err = errBad
   963  					break
   964  				}
   965  				sign, hour, min, seconds, value = value[0:1], value[1:3], value[4:6], value[7:9], value[9:]
   966  			} else if std == stdISO8601SecondsTZ || std == stdNumSecondsTz {
   967  				if len(value) < 7 {
   968  					err = errBad
   969  					break
   970  				}
   971  				sign, hour, min, seconds, value = value[0:1], value[1:3], value[3:5], value[5:7], value[7:]
   972  			} else {
   973  				if len(value) < 5 {
   974  					err = errBad
   975  					break
   976  				}
   977  				sign, hour, min, seconds, value = value[0:1], value[1:3], value[3:5], "00", value[5:]
   978  			}
   979  			var hr, mm, ss int
   980  			hr, err = atoi(hour)
   981  			if err == nil {
   982  				mm, err = atoi(min)
   983  			}
   984  			if err == nil {
   985  				ss, err = atoi(seconds)
   986  			}
   987  			zoneOffset = (hr*60+mm)*60 + ss // offset is in seconds
   988  			switch sign[0] {
   989  			case '+':
   990  			case '-':
   991  				zoneOffset = -zoneOffset
   992  			default:
   993  				err = errBad
   994  			}
   995  		case stdTZ:
   996  			// Does it look like a time zone?
   997  			if len(value) >= 3 && value[0:3] == "UTC" {
   998  				z = UTC
   999  				value = value[3:]
  1000  				break
  1001  			}
  1002  			n, ok := parseTimeZone(value)
  1003  			if !ok {
  1004  				err = errBad
  1005  				break
  1006  			}
  1007  			zoneName, value = value[:n], value[n:]
  1008  
  1009  		case stdFracSecond0:
  1010  			// stdFracSecond0 requires the exact number of digits as specified in
  1011  			// the layout.
  1012  			ndigit := 1 + (std >> stdArgShift)
  1013  			if len(value) < ndigit {
  1014  				err = errBad
  1015  				break
  1016  			}
  1017  			nsec, rangeErrString, err = parseNanoseconds(value, ndigit)
  1018  			value = value[ndigit:]
  1019  
  1020  		case stdFracSecond9:
  1021  			if len(value) < 2 || value[0] != '.' || value[1] < '0' || '9' < value[1] {
  1022  				// Fractional second omitted.
  1023  				break
  1024  			}
  1025  			// Take any number of digits, even more than asked for,
  1026  			// because it is what the stdSecond case would do.
  1027  			i := 0
  1028  			for i < 9 && i+1 < len(value) && '0' <= value[i+1] && value[i+1] <= '9' {
  1029  				i++
  1030  			}
  1031  			nsec, rangeErrString, err = parseNanoseconds(value, 1+i)
  1032  			value = value[1+i:]
  1033  		}
  1034  		if rangeErrString != "" {
  1035  			return Time{}, &ParseError{alayout, avalue, stdstr, value, ": " + rangeErrString + " out of range"}
  1036  		}
  1037  		if err != nil {
  1038  			return Time{}, &ParseError{alayout, avalue, stdstr, value, ""}
  1039  		}
  1040  	}
  1041  	if pmSet && hour < 12 {
  1042  		hour += 12
  1043  	} else if amSet && hour == 12 {
  1044  		hour = 0
  1045  	}
  1046  
  1047  	// Validate the day of the month.
  1048  	if day < 1 || day > daysIn(Month(month), year) {
  1049  		return Time{}, &ParseError{alayout, avalue, "", value, ": day out of range"}
  1050  	}
  1051  
  1052  	if z != nil {
  1053  		return Date(year, Month(month), day, hour, min, sec, nsec, z), nil
  1054  	}
  1055  
  1056  	if zoneOffset != -1 {
  1057  		t := Date(year, Month(month), day, hour, min, sec, nsec, UTC)
  1058  		t.addSec(-int64(zoneOffset))
  1059  
  1060  		// Look for local zone with the given offset.
  1061  		// If that zone was in effect at the given time, use it.
  1062  		name, offset, _, _ := local.lookup(t.unixSec())
  1063  		if offset == zoneOffset && (zoneName == "" || name == zoneName) {
  1064  			t.setLoc(local)
  1065  			return t, nil
  1066  		}
  1067  
  1068  		// Otherwise create fake zone to record offset.
  1069  		t.setLoc(FixedZone(zoneName, zoneOffset))
  1070  		return t, nil
  1071  	}
  1072  
  1073  	if zoneName != "" {
  1074  		t := Date(year, Month(month), day, hour, min, sec, nsec, UTC)
  1075  		// Look for local zone with the given offset.
  1076  		// If that zone was in effect at the given time, use it.
  1077  		offset, ok := local.lookupName(zoneName, t.unixSec())
  1078  		if ok {
  1079  			t.addSec(-int64(offset))
  1080  			t.setLoc(local)
  1081  			return t, nil
  1082  		}
  1083  
  1084  		// Otherwise, create fake zone with unknown offset.
  1085  		if len(zoneName) > 3 && zoneName[:3] == "GMT" {
  1086  			offset, _ = atoi(zoneName[3:]) // Guaranteed OK by parseGMT.
  1087  			offset *= 3600
  1088  		}
  1089  		t.setLoc(FixedZone(zoneName, offset))
  1090  		return t, nil
  1091  	}
  1092  
  1093  	// Otherwise, fall back to default.
  1094  	return Date(year, Month(month), day, hour, min, sec, nsec, defaultLocation), nil
  1095  }
  1096  
  1097  // parseTimeZone parses a time zone string and returns its length. Time zones
  1098  // are human-generated and unpredictable. We can't do precise error checking.
  1099  // On the other hand, for a correct parse there must be a time zone at the
  1100  // beginning of the string, so it's almost always true that there's one
  1101  // there. We look at the beginning of the string for a run of upper-case letters.
  1102  // If there are more than 5, it's an error.
  1103  // If there are 4 or 5 and the last is a T, it's a time zone.
  1104  // If there are 3, it's a time zone.
  1105  // Otherwise, other than special cases, it's not a time zone.
  1106  // GMT is special because it can have an hour offset.
  1107  func parseTimeZone(value string) (length int, ok bool) {
  1108  	if len(value) < 3 {
  1109  		return 0, false
  1110  	}
  1111  	// Special case 1: ChST and MeST are the only zones with a lower-case letter.
  1112  	if len(value) >= 4 && (value[:4] == "ChST" || value[:4] == "MeST") {
  1113  		return 4, true
  1114  	}
  1115  	// Special case 2: GMT may have an hour offset; treat it specially.
  1116  	if value[:3] == "GMT" {
  1117  		length = parseGMT(value)
  1118  		return length, true
  1119  	}
  1120  	// Special Case 3: Some time zones are not named, but have +/-00 format
  1121  	if value[0] == '+' || value[0] == '-' {
  1122  		length = parseSignedOffset(value)
  1123  		ok := length > 0 // parseSignedOffset returns 0 in case of bad input
  1124  		return length, ok
  1125  	}
  1126  	// How many upper-case letters are there? Need at least three, at most five.
  1127  	var nUpper int
  1128  	for nUpper = 0; nUpper < 6; nUpper++ {
  1129  		if nUpper >= len(value) {
  1130  			break
  1131  		}
  1132  		if c := value[nUpper]; c < 'A' || 'Z' < c {
  1133  			break
  1134  		}
  1135  	}
  1136  	switch nUpper {
  1137  	case 0, 1, 2, 6:
  1138  		return 0, false
  1139  	case 5: // Must end in T to match.
  1140  		if value[4] == 'T' {
  1141  			return 5, true
  1142  		}
  1143  	case 4:
  1144  		// Must end in T, except one special case.
  1145  		if value[3] == 'T' || value[:4] == "WITA" {
  1146  			return 4, true
  1147  		}
  1148  	case 3:
  1149  		return 3, true
  1150  	}
  1151  	return 0, false
  1152  }
  1153  
  1154  // parseGMT parses a GMT time zone. The input string is known to start "GMT".
  1155  // The function checks whether that is followed by a sign and a number in the
  1156  // range -23 through +23 excluding zero.
  1157  func parseGMT(value string) int {
  1158  	value = value[3:]
  1159  	if len(value) == 0 {
  1160  		return 3
  1161  	}
  1162  
  1163  	return 3 + parseSignedOffset(value)
  1164  }
  1165  
  1166  // parseSignedOffset parses a signed timezone offset (e.g. "+03" or "-04").
  1167  // The function checks for a signed number in the range -23 through +23 excluding zero.
  1168  // Returns length of the found offset string or 0 otherwise
  1169  func parseSignedOffset(value string) int {
  1170  	sign := value[0]
  1171  	if sign != '-' && sign != '+' {
  1172  		return 0
  1173  	}
  1174  	x, rem, err := leadingInt(value[1:])
  1175  
  1176  	// fail if nothing consumed by leadingInt
  1177  	if err != nil || value[1:] == rem {
  1178  		return 0
  1179  	}
  1180  	if sign == '-' {
  1181  		x = -x
  1182  	}
  1183  	if x < -23 || 23 < x {
  1184  		return 0
  1185  	}
  1186  	return len(value) - len(rem)
  1187  }
  1188  
  1189  func parseNanoseconds(value string, nbytes int) (ns int, rangeErrString string, err error) {
  1190  	if value[0] != '.' {
  1191  		err = errBad
  1192  		return
  1193  	}
  1194  	if ns, err = atoi(value[1:nbytes]); err != nil {
  1195  		return
  1196  	}
  1197  	if ns < 0 || 1e9 <= ns {
  1198  		rangeErrString = "fractional second"
  1199  		return
  1200  	}
  1201  	// We need nanoseconds, which means scaling by the number
  1202  	// of missing digits in the format, maximum length 10. If it's
  1203  	// longer than 10, we won't scale.
  1204  	scaleDigits := 10 - nbytes
  1205  	for i := 0; i < scaleDigits; i++ {
  1206  		ns *= 10
  1207  	}
  1208  	return
  1209  }
  1210  
  1211  var errLeadingInt = errors.New("time: bad [0-9]*") // never printed
  1212  
  1213  // leadingInt consumes the leading [0-9]* from s.
  1214  func leadingInt(s string) (x int64, rem string, err error) {
  1215  	i := 0
  1216  	for ; i < len(s); i++ {
  1217  		c := s[i]
  1218  		if c < '0' || c > '9' {
  1219  			break
  1220  		}
  1221  		if x > (1<<63-1)/10 {
  1222  			// overflow
  1223  			return 0, "", errLeadingInt
  1224  		}
  1225  		x = x*10 + int64(c) - '0'
  1226  		if x < 0 {
  1227  			// overflow
  1228  			return 0, "", errLeadingInt
  1229  		}
  1230  	}
  1231  	return x, s[i:], nil
  1232  }
  1233  
  1234  // leadingFraction consumes the leading [0-9]* from s.
  1235  // It is used only for fractions, so does not return an error on overflow,
  1236  // it just stops accumulating precision.
  1237  func leadingFraction(s string) (x int64, scale float64, rem string) {
  1238  	i := 0
  1239  	scale = 1
  1240  	overflow := false
  1241  	for ; i < len(s); i++ {
  1242  		c := s[i]
  1243  		if c < '0' || c > '9' {
  1244  			break
  1245  		}
  1246  		if overflow {
  1247  			continue
  1248  		}
  1249  		if x > (1<<63-1)/10 {
  1250  			// It's possible for overflow to give a positive number, so take care.
  1251  			overflow = true
  1252  			continue
  1253  		}
  1254  		y := x*10 + int64(c) - '0'
  1255  		if y < 0 {
  1256  			overflow = true
  1257  			continue
  1258  		}
  1259  		x = y
  1260  		scale *= 10
  1261  	}
  1262  	return x, scale, s[i:]
  1263  }
  1264  
  1265  var unitMap = map[string]int64{
  1266  	"ns": int64(Nanosecond),
  1267  	"us": int64(Microsecond),
  1268  	"µs": int64(Microsecond), // U+00B5 = micro symbol
  1269  	"μs": int64(Microsecond), // U+03BC = Greek letter mu
  1270  	"ms": int64(Millisecond),
  1271  	"s":  int64(Second),
  1272  	"m":  int64(Minute),
  1273  	"h":  int64(Hour),
  1274  }
  1275  
  1276  // ParseDuration parses a duration string.
  1277  // A duration string is a possibly signed sequence of
  1278  // decimal numbers, each with optional fraction and a unit suffix,
  1279  // such as "300ms", "-1.5h" or "2h45m".
  1280  // Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".
  1281  func ParseDuration(s string) (Duration, error) {
  1282  	// [-+]?([0-9]*(\.[0-9]*)?[a-z]+)+
  1283  	orig := s
  1284  	var d int64
  1285  	neg := false
  1286  
  1287  	// Consume [-+]?
  1288  	if s != "" {
  1289  		c := s[0]
  1290  		if c == '-' || c == '+' {
  1291  			neg = c == '-'
  1292  			s = s[1:]
  1293  		}
  1294  	}
  1295  	// Special case: if all that is left is "0", this is zero.
  1296  	if s == "0" {
  1297  		return 0, nil
  1298  	}
  1299  	if s == "" {
  1300  		return 0, errors.New("time: invalid duration " + orig)
  1301  	}
  1302  	for s != "" {
  1303  		var (
  1304  			v, f  int64       // integers before, after decimal point
  1305  			scale float64 = 1 // value = v + f/scale
  1306  		)
  1307  
  1308  		var err error
  1309  
  1310  		// The next character must be [0-9.]
  1311  		if !(s[0] == '.' || '0' <= s[0] && s[0] <= '9') {
  1312  			return 0, errors.New("time: invalid duration " + orig)
  1313  		}
  1314  		// Consume [0-9]*
  1315  		pl := len(s)
  1316  		v, s, err = leadingInt(s)
  1317  		if err != nil {
  1318  			return 0, errors.New("time: invalid duration " + orig)
  1319  		}
  1320  		pre := pl != len(s) // whether we consumed anything before a period
  1321  
  1322  		// Consume (\.[0-9]*)?
  1323  		post := false
  1324  		if s != "" && s[0] == '.' {
  1325  			s = s[1:]
  1326  			pl := len(s)
  1327  			f, scale, s = leadingFraction(s)
  1328  			post = pl != len(s)
  1329  		}
  1330  		if !pre && !post {
  1331  			// no digits (e.g. ".s" or "-.s")
  1332  			return 0, errors.New("time: invalid duration " + orig)
  1333  		}
  1334  
  1335  		// Consume unit.
  1336  		i := 0
  1337  		for ; i < len(s); i++ {
  1338  			c := s[i]
  1339  			if c == '.' || '0' <= c && c <= '9' {
  1340  				break
  1341  			}
  1342  		}
  1343  		if i == 0 {
  1344  			return 0, errors.New("time: missing unit in duration " + orig)
  1345  		}
  1346  		u := s[:i]
  1347  		s = s[i:]
  1348  		unit, ok := unitMap[u]
  1349  		if !ok {
  1350  			return 0, errors.New("time: unknown unit " + u + " in duration " + orig)
  1351  		}
  1352  		if v > (1<<63-1)/unit {
  1353  			// overflow
  1354  			return 0, errors.New("time: invalid duration " + orig)
  1355  		}
  1356  		v *= unit
  1357  		if f > 0 {
  1358  			// float64 is needed to be nanosecond accurate for fractions of hours.
  1359  			// v >= 0 && (f*unit/scale) <= 3.6e+12 (ns/h, h is the largest unit)
  1360  			v += int64(float64(f) * (float64(unit) / scale))
  1361  			if v < 0 {
  1362  				// overflow
  1363  				return 0, errors.New("time: invalid duration " + orig)
  1364  			}
  1365  		}
  1366  		d += v
  1367  		if d < 0 {
  1368  			// overflow
  1369  			return 0, errors.New("time: invalid duration " + orig)
  1370  		}
  1371  	}
  1372  
  1373  	if neg {
  1374  		d = -d
  1375  	}
  1376  	return Duration(d), nil
  1377  }
  1378  

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