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// Copyright 2010 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 "errors"

const (
        numeric = iota
        alphabetic
        separator
        plus
        minus
)

// These are predefined layouts for use in Time.Format.
// The standard time used in the layouts is:
//      Mon Jan 2 15:04:05 MST 2006  (MST is GMT-0700)
// which is Unix time 1136243045.
// (Think of it as 01/02 03:04:05PM '06 -0700.)
// To define your own format, write down what the standard
// time would look like formatted your way.
//
// Within the format string, an underscore _ represents a space that may be
// replaced by a digit if the following number (a day) has two digits; for
// compatibility with fixed-width Unix time formats.
//
// A decimal point followed by one or more zeros represents a fractional
// second. When parsing (only), the input may contain a fractional second
// field immediately after the seconds field, even if the layout does not
// signify its presence. In that case a decimal point followed by a maximal
// series of digits is parsed as a fractional second.
//
// Numeric time zone offsets format as follows:
//      -0700  ±hhmm
//      -07:00 ±hh:mm
// Replacing the sign in the format with a Z triggers
// the ISO 8601 behavior of printing Z instead of an
// offset for the UTC zone.  Thus:
//      Z0700  Z or ±hhmm
//      Z07:00 Z or ±hh:mm
const (
        ANSIC    = "Mon Jan _2 15:04:05 2006"
        UnixDate = "Mon Jan _2 15:04:05 MST 2006"
        RubyDate = "Mon Jan 02 15:04:05 -0700 2006"
        RFC822   = "02 Jan 06 1504 MST"
        RFC822Z  = "02 Jan 06 1504 -0700" // RFC822 with numeric zone
        RFC850   = "Monday, 02-Jan-06 15:04:05 MST"
        RFC1123  = "Mon, 02 Jan 2006 15:04:05 MST"
        RFC1123Z = "Mon, 02 Jan 2006 15:04:05 -0700" // RFC1123 with numeric zone
        RFC3339  = "2006-01-02T15:04:05Z07:00"
        Kitchen  = "3:04PM"
        // Handy time stamps.
        Stamp      = "Jan _2 15:04:05"
        StampMilli = "Jan _2 15:04:05.000"
        StampMicro = "Jan _2 15:04:05.000000"
        StampNano  = "Jan _2 15:04:05.000000000"
)

const (
        stdLongMonth      = "January"
        stdMonth          = "Jan"
        stdNumMonth       = "1"
        stdZeroMonth      = "01"
        stdLongWeekDay    = "Monday"
        stdWeekDay        = "Mon"
        stdDay            = "2"
        stdUnderDay       = "_2"
        stdZeroDay        = "02"
        stdHour           = "15"
        stdHour12         = "3"
        stdZeroHour12     = "03"
        stdMinute         = "4"
        stdZeroMinute     = "04"
        stdSecond         = "5"
        stdZeroSecond     = "05"
        stdLongYear       = "2006"
        stdYear           = "06"
        stdPM             = "PM"
        stdpm             = "pm"
        stdTZ             = "MST"
        stdISO8601TZ      = "Z0700"  // prints Z for UTC
        stdISO8601ColonTZ = "Z07:00" // prints Z for UTC
        stdNumTZ          = "-0700"  // always numeric
        stdNumShortTZ     = "-07"    // always numeric
        stdNumColonTZ     = "-07:00" // always numeric
)

// nextStdChunk finds the first occurrence of a std string in
// layout and returns the text before, the std string, and the text after.
func nextStdChunk(layout string) (prefix, std, suffix string) {
        for i := 0; i < len(layout); i++ {
                switch layout[i] {
                case 'J': // January, Jan
                        if len(layout) >= i+7 && layout[i:i+7] == stdLongMonth {
                                return layout[0:i], stdLongMonth, layout[i+7:]
                        }
                        if len(layout) >= i+3 && layout[i:i+3] == stdMonth {
                                return layout[0:i], stdMonth, layout[i+3:]
                        }

                case 'M': // Monday, Mon, MST
                        if len(layout) >= i+6 && layout[i:i+6] == stdLongWeekDay {
                                return layout[0:i], stdLongWeekDay, layout[i+6:]
                        }
                        if len(layout) >= i+3 {
                                if layout[i:i+3] == stdWeekDay {
                                        return layout[0:i], stdWeekDay, layout[i+3:]
                                }
                                if layout[i:i+3] == stdTZ {
                                        return layout[0:i], stdTZ, layout[i+3:]
                                }
                        }

                case '0': // 01, 02, 03, 04, 05, 06
                        if len(layout) >= i+2 && '1' <= layout[i+1] && layout[i+1] <= '6' {
                                return layout[0:i], layout[i : i+2], layout[i+2:]
                        }

                case '1': // 15, 1
                        if len(layout) >= i+2 && layout[i+1] == '5' {
                                return layout[0:i], stdHour, layout[i+2:]
                        }
                        return layout[0:i], stdNumMonth, layout[i+1:]

                case '2': // 2006, 2
                        if len(layout) >= i+4 && layout[i:i+4] == stdLongYear {
                                return layout[0:i], stdLongYear, layout[i+4:]
                        }
                        return layout[0:i], stdDay, layout[i+1:]

                case '_': // _2
                        if len(layout) >= i+2 && layout[i+1] == '2' {
                                return layout[0:i], stdUnderDay, layout[i+2:]
                        }

                case '3', '4', '5': // 3, 4, 5
                        return layout[0:i], layout[i : i+1], layout[i+1:]

                case 'P': // PM
                        if len(layout) >= i+2 && layout[i+1] == 'M' {
                                return layout[0:i], layout[i : i+2], layout[i+2:]
                        }

                case 'p': // pm
                        if len(layout) >= i+2 && layout[i+1] == 'm' {
                                return layout[0:i], layout[i : i+2], layout[i+2:]
                        }

                case '-': // -0700, -07:00, -07
                        if len(layout) >= i+5 && layout[i:i+5] == stdNumTZ {
                                return layout[0:i], layout[i : i+5], layout[i+5:]
                        }
                        if len(layout) >= i+6 && layout[i:i+6] == stdNumColonTZ {
                                return layout[0:i], layout[i : i+6], layout[i+6:]
                        }
                        if len(layout) >= i+3 && layout[i:i+3] == stdNumShortTZ {
                                return layout[0:i], layout[i : i+3], layout[i+3:]
                        }
                case 'Z': // Z0700, Z07:00
                        if len(layout) >= i+5 && layout[i:i+5] == stdISO8601TZ {
                                return layout[0:i], layout[i : i+5], layout[i+5:]
                        }
                        if len(layout) >= i+6 && layout[i:i+6] == stdISO8601ColonTZ {
                                return layout[0:i], layout[i : i+6], layout[i+6:]
                        }
                case '.': // .000 - multiple digits of zeros (only) for fractional seconds.
                        numZeros := 0
                        var j int
                        for j = i + 1; j < len(layout) && layout[j] == '0'; j++ {
                                numZeros++
                        }
                        // String of digits must end here - only fractional second is all zeros.
                        if numZeros > 0 && !isDigit(layout, j) {
                                return layout[0:i], layout[i : i+1+numZeros], layout[i+1+numZeros:]
                        }
                }
        }
        return layout, "", ""
}

var longDayNames = []string{
        "Sunday",
        "Monday",
        "Tuesday",
        "Wednesday",
        "Thursday",
        "Friday",
        "Saturday",
}

var shortDayNames = []string{
        "Sun",
        "Mon",
        "Tue",
        "Wed",
        "Thu",
        "Fri",
        "Sat",
}

var shortMonthNames = []string{
        "---",
        "Jan",
        "Feb",
        "Mar",
        "Apr",
        "May",
        "Jun",
        "Jul",
        "Aug",
        "Sep",
        "Oct",
        "Nov",
        "Dec",
}

var longMonthNames = []string{
        "---",
        "January",
        "February",
        "March",
        "April",
        "May",
        "June",
        "July",
        "August",
        "September",
        "October",
        "November",
        "December",
}

// match returns true if s1 and s2 match ignoring case.
// It is assumed s1 and s2 are the same length.
func match(s1, s2 string) bool {
        for i := 0; i < len(s1); i++ {
                c1 := s1[i]
                c2 := s2[i]
                if c1 != c2 {
                        // Switch to lower-case; 'a'-'A' is known to be a single bit.
                        c1 |= 'a' - 'A'
                        c2 |= 'a' - 'A'
                        if c1 != c2 || c1 < 'a' || c1 > 'z' {
                                return false
                        }
                }
        }
        return true
}

func lookup(tab []string, val string) (int, string, error) {
        for i, v := range tab {
                if len(val) >= len(v) && match(val[0:len(v)], v) {
                        return i, val[len(v):], nil
                }
        }
        return -1, val, errBad
}

// Duplicates functionality in strconv, but avoids dependency.
func itoa(x int) string {
        var buf [32]byte
        n := len(buf)
        if x == 0 {
                return "0"
        }
        u := uint(x)
        if x < 0 {
                u = -u
        }
        for u > 0 {
                n--
                buf[n] = byte(u%10 + '0')
                u /= 10
        }
        if x < 0 {
                n--
                buf[n] = '-'
        }
        return string(buf[n:])
}

// Never printed, just needs to be non-nil for return by atoi.
var atoiError = errors.New("time: invalid number")

// Duplicates functionality in strconv, but avoids dependency.
func atoi(s string) (x int, err error) {
        neg := false
        if s != "" && s[0] == '-' {
                neg = true
                s = s[1:]
        }
        x, rem, err := leadingInt(s)
        if err != nil || rem != "" {
                return 0, atoiError
        }
        if neg {
                x = -x
        }
        return x, nil
}

func pad(i int, padding string) string {
        s := itoa(i)
        if i < 10 {
                s = padding + s
        }
        return s
}

func zeroPad(i int) string { return pad(i, "0") }

// formatNano formats a fractional second, as nanoseconds.
func formatNano(nanosec, n int) string {
        // User might give us bad data. Make sure it's positive and in range.
        // They'll get nonsense output but it will have the right format.
        s := itoa(int(uint(nanosec) % 1e9))
        // Zero pad left without fmt.
        if len(s) < 9 {
                s = "000000000"[:9-len(s)] + s
        }
        if n > 9 {
                n = 9
        }
        return "." + s[:n]
}

// String returns the time formatted using the format string
//      "Mon Jan _2 15:04:05 -0700 MST 2006"
func (t Time) String() string {
        return t.Format("Mon Jan _2 15:04:05 -0700 MST 2006")
}

type buffer []byte

func (b *buffer) WriteString(s string) {
        *b = append(*b, s...)
}

func (b *buffer) String() string {
        return string([]byte(*b))
}

// Format returns a textual representation of the time value formatted
// according to layout.  The layout defines the format by showing the
// representation of a standard time, which is then used to describe
// the time to be formatted.  Predefined layouts ANSIC, UnixDate,
// RFC3339 and others describe standard representations. For more
// information about the formats, see the documentation for ANSIC.
func (t Time) Format(layout string) string {
        var (
                year  int = -1
                month Month
                day   int
                hour  int = -1
                min   int
                sec   int
                b     buffer
        )
        // Each iteration generates one std value.
        for {
                prefix, std, suffix := nextStdChunk(layout)
                b.WriteString(prefix)
                if std == "" {
                        break
                }

                // Compute year, month, day if needed.
                if year < 0 {
                        // Jan 01 02 2006
                        if a, z := std[0], std[len(std)-1]; a == 'J' || a == 'j' || z == '1' || z == '2' || z == '6' {
                                year, month, day = t.Date()
                        }
                }

                // Compute hour, minute, second if needed.
                if hour < 0 {
                        // 03 04 05 15 pm
                        if z := std[len(std)-1]; z == '3' || z == '4' || z == '5' || z == 'm' || z == 'M' {
                                hour, min, sec = t.Clock()
                        }
                }

                var p string
                switch std {
                case stdYear:
                        p = zeroPad(year % 100)
                case stdLongYear:
                        p = itoa(year)
                case stdMonth:
                        p = month.String()[:3]
                case stdLongMonth:
                        p = month.String()
                case stdNumMonth:
                        p = itoa(int(month))
                case stdZeroMonth:
                        p = zeroPad(int(month))
                case stdWeekDay:
                        p = t.Weekday().String()[:3]
                case stdLongWeekDay:
                        p = t.Weekday().String()
                case stdDay:
                        p = itoa(day)
                case stdUnderDay:
                        p = pad(day, " ")
                case stdZeroDay:
                        p = zeroPad(day)
                case stdHour:
                        p = zeroPad(hour)
                case stdHour12:
                        // Noon is 12PM, midnight is 12AM.
                        hr := hour % 12
                        if hr == 0 {
                                hr = 12
                        }
                        p = itoa(hr)
                case stdZeroHour12:
                        // Noon is 12PM, midnight is 12AM.
                        hr := hour % 12
                        if hr == 0 {
                                hr = 12
                        }
                        p = zeroPad(hr)
                case stdMinute:
                        p = itoa(min)
                case stdZeroMinute:
                        p = zeroPad(min)
                case stdSecond:
                        p = itoa(sec)
                case stdZeroSecond:
                        p = zeroPad(sec)
                case stdPM:
                        if hour >= 12 {
                                p = "PM"
                        } else {
                                p = "AM"
                        }
                case stdpm:
                        if hour >= 12 {
                                p = "pm"
                        } else {
                                p = "am"
                        }
                case stdISO8601TZ, stdISO8601ColonTZ, stdNumTZ, stdNumColonTZ:
                        // Ugly special case.  We cheat and take the "Z" variants
                        // to mean "the time zone as formatted for ISO 8601".
                        _, offset := t.Zone()
                        if offset == 0 && std[0] == 'Z' {
                                p = "Z"
                                break
                        }
                        zone := offset / 60 // convert to minutes
                        if zone < 0 {
                                p = "-"
                                zone = -zone
                        } else {
                                p = "+"
                        }
                        p += zeroPad(zone / 60)
                        if std == stdISO8601ColonTZ || std == stdNumColonTZ {
                                p += ":"
                        }
                        p += zeroPad(zone % 60)
                case stdTZ:
                        name, offset := t.Zone()
                        if name != "" {
                                p = name
                        } else {
                                // No time zone known for this time, but we must print one.
                                // Use the -0700 format.
                                zone := offset / 60 // convert to minutes
                                if zone < 0 {
                                        p = "-"
                                        zone = -zone
                                } else {
                                        p = "+"
                                }
                                p += zeroPad(zone / 60)
                                p += zeroPad(zone % 60)
                        }
                default:
                        if len(std) >= 2 && std[0:2] == ".0" {
                                p = formatNano(t.Nanosecond(), len(std)-1)
                        }
                }
                b.WriteString(p)
                layout = suffix
        }
        return b.String()
}

var errBad = errors.New("bad value for field") // placeholder not passed to user

// ParseError describes a problem parsing a time string.
type ParseError struct {
        Layout     string
        Value      string
        LayoutElem string
        ValueElem  string
        Message    string
}

func quote(s string) string {
        return "\"" + s + "\""
}

// Error returns the string representation of a ParseError.
func (e *ParseError) Error() string {
        if e.Message == "" {
                return "parsing time " +
                        quote(e.Value) + " as " +
                        quote(e.Layout) + ": cannot parse " +
                        quote(e.ValueElem) + " as " +
                        quote(e.LayoutElem)
        }
        return "parsing time " +
                quote(e.Value) + e.Message
}

// isDigit returns true if s[i] is a decimal digit, false if not or
// if s[i] is out of range.
func isDigit(s string, i int) bool {
        if len(s) <= i {
                return false
        }
        c := s[i]
        return '0' <= c && c <= '9'
}

// getnum parses s[0:1] or s[0:2] (fixed forces the latter)
// as a decimal integer and returns the integer and the
// remainder of the string.
func getnum(s string, fixed bool) (int, string, error) {
        if !isDigit(s, 0) {
                return 0, s, errBad
        }
        if !isDigit(s, 1) {
                if fixed {
                        return 0, s, errBad
                }
                return int(s[0] - '0'), s[1:], nil
        }
        return int(s[0]-'0')*10 + int(s[1]-'0'), s[2:], nil
}

func cutspace(s string) string {
        for len(s) > 0 && s[0] == ' ' {
                s = s[1:]
        }
        return s
}

// skip removes the given prefix from value,
// treating runs of space characters as equivalent.
func skip(value, prefix string) (string, error) {
        for len(prefix) > 0 {
                if prefix[0] == ' ' {
                        if len(value) > 0 && value[0] != ' ' {
                                return "", errBad
                        }
                        prefix = cutspace(prefix)
                        value = cutspace(value)
                        continue
                }
                if len(value) == 0 || value[0] != prefix[0] {
                        return "", errBad
                }
                prefix = prefix[1:]
                value = value[1:]
        }
        return value, nil
}

// Parse parses a formatted string and returns the time value it represents.
// The layout defines the format by showing the representation of a standard
// time, which is then used to describe the string to be parsed.  Predefined
// layouts ANSIC, UnixDate, RFC3339 and others describe standard
// representations.For more information about the formats, see the
// documentation for ANSIC.
//
// Elements omitted from the value are assumed to be zero, or when
// zero is impossible, one, so parsing "3:04pm" returns the time
// corresponding to Jan 1, year 0, 15:04:00 UTC.
// Years must be in the range 0000..9999. The day of the week is checked
// for syntax but it is otherwise ignored.
func Parse(layout, value string) (Time, error) {
        alayout, avalue := layout, value
        rangeErrString := "" // set if a value is out of range
        amSet := false       // do we need to subtract 12 from the hour for midnight?
        pmSet := false       // do we need to add 12 to the hour?

        // Time being constructed.
        var (
                year       int
                month      int = 1 // January
                day        int = 1
                hour       int
                min        int
                sec        int
                nsec       int
                z          *Location
                zoneOffset int = -1
                zoneName   string
        )

        // Each iteration processes one std value.
        for {
                var err error
                prefix, std, suffix := nextStdChunk(layout)
                value, err = skip(value, prefix)
                if err != nil {
                        return Time{}, &ParseError{alayout, avalue, prefix, value, ""}
                }
                if len(std) == 0 {
                        if len(value) != 0 {
                                return Time{}, &ParseError{alayout, avalue, "", value, ": extra text: " + value}
                        }
                        break
                }
                layout = suffix
                var p string
                switch std {
                case stdYear:
                        if len(value) < 2 {
                                err = errBad
                                break
                        }
                        p, value = value[0:2], value[2:]
                        year, err = atoi(p)
                        if year >= 69 { // Unix time starts Dec 31 1969 in some time zones
                                year += 1900
                        } else {
                                year += 2000
                        }
                case stdLongYear:
                        if len(value) < 4 || !isDigit(value, 0) {
                                err = errBad
                                break
                        }
                        p, value = value[0:4], value[4:]
                        year, err = atoi(p)
                case stdMonth:
                        month, value, err = lookup(shortMonthNames, value)
                case stdLongMonth:
                        month, value, err = lookup(longMonthNames, value)
                case stdNumMonth, stdZeroMonth:
                        month, value, err = getnum(value, std == stdZeroMonth)
                        if month <= 0 || 12 < month {
                                rangeErrString = "month"
                        }
                case stdWeekDay:
                        // Ignore weekday except for error checking.
                        _, value, err = lookup(shortDayNames, value)
                case stdLongWeekDay:
                        _, value, err = lookup(longDayNames, value)
                case stdDay, stdUnderDay, stdZeroDay:
                        if std == stdUnderDay && len(value) > 0 && value[0] == ' ' {
                                value = value[1:]
                        }
                        day, value, err = getnum(value, std == stdZeroDay)
                        if day < 0 || 31 < day {
                                rangeErrString = "day"
                        }
                case stdHour:
                        hour, value, err = getnum(value, false)
                        if hour < 0 || 24 <= hour {
                                rangeErrString = "hour"
                        }
                case stdHour12, stdZeroHour12:
                        hour, value, err = getnum(value, std == stdZeroHour12)
                        if hour < 0 || 12 < hour {
                                rangeErrString = "hour"
                        }
                case stdMinute, stdZeroMinute:
                        min, value, err = getnum(value, std == stdZeroMinute)
                        if min < 0 || 60 <= min {
                                rangeErrString = "minute"
                        }
                case stdSecond, stdZeroSecond:
                        sec, value, err = getnum(value, std == stdZeroSecond)
                        if sec < 0 || 60 <= sec {
                                rangeErrString = "second"
                        }
                        // Special case: do we have a fractional second but no
                        // fractional second in the format?
                        if len(value) > 2 && value[0] == '.' && isDigit(value, 1) {
                                _, std, _ := nextStdChunk(layout)
                                if len(std) > 0 && std[0] == '.' && isDigit(std, 1) {
                                        // Fractional second in the layout; proceed normally
                                        break
                                }
                                // No fractional second in the layout but we have one in the input.
                                n := 2
                                for ; n < len(value) && isDigit(value, n); n++ {
                                }
                                nsec, rangeErrString, err = parseNanoseconds(value, n)
                                value = value[n:]
                        }
                case stdPM:
                        if len(value) < 2 {
                                err = errBad
                                break
                        }
                        p, value = value[0:2], value[2:]
                        switch p {
                        case "PM":
                                pmSet = true
                        case "AM":
                                amSet = true
                        default:
                                err = errBad
                        }
                case stdpm:
                        if len(value) < 2 {
                                err = errBad
                                break
                        }
                        p, value = value[0:2], value[2:]
                        switch p {
                        case "pm":
                                pmSet = true
                        case "am":
                                amSet = true
                        default:
                                err = errBad
                        }
                case stdISO8601TZ, stdISO8601ColonTZ, stdNumTZ, stdNumShortTZ, stdNumColonTZ:
                        if std[0] == 'Z' && len(value) >= 1 && value[0] == 'Z' {
                                value = value[1:]
                                z = UTC
                                break
                        }
                        var sign, hour, min string
                        if std == stdISO8601ColonTZ || std == stdNumColonTZ {
                                if len(value) < 6 {
                                        err = errBad
                                        break
                                }
                                if value[3] != ':' {
                                        err = errBad
                                        break
                                }
                                sign, hour, min, value = value[0:1], value[1:3], value[4:6], value[6:]
                        } else if std == stdNumShortTZ {
                                if len(value) < 3 {
                                        err = errBad
                                        break
                                }
                                sign, hour, min, value = value[0:1], value[1:3], "00", value[3:]
                        } else {
                                if len(value) < 5 {
                                        err = errBad
                                        break
                                }
                                sign, hour, min, value = value[0:1], value[1:3], value[3:5], value[5:]
                        }
                        var hr, mm int
                        hr, err = atoi(hour)
                        if err == nil {
                                mm, err = atoi(min)
                        }
                        zoneOffset = (hr*60 + mm) * 60 // offset is in seconds
                        switch sign[0] {
                        case '+':
                        case '-':
                                zoneOffset = -zoneOffset
                        default:
                                err = errBad
                        }
                case stdTZ:
                        // Does it look like a time zone?
                        if len(value) >= 3 && value[0:3] == "UTC" {
                                z = UTC
                                value = value[3:]
                                break
                        }

                        if len(value) >= 3 && value[2] == 'T' {
                                p, value = value[0:3], value[3:]
                        } else if len(value) >= 4 && value[3] == 'T' {
                                p, value = value[0:4], value[4:]
                        } else {
                                err = errBad
                                break
                        }
                        for i := 0; i < len(p); i++ {
                                if p[i] < 'A' || 'Z' < p[i] {
                                        err = errBad
                                }
                        }
                        if err != nil {
                                break
                        }
                        // It's a valid format.
                        zoneName = p
                default:
                        if len(value) < len(std) {
                                err = errBad
                                break
                        }
                        if len(std) >= 2 && std[0:2] == ".0" {
                                nsec, rangeErrString, err = parseNanoseconds(value, len(std))
                                value = value[len(std):]
                        }
                }
                if rangeErrString != "" {
                        return Time{}, &ParseError{alayout, avalue, std, value, ": " + rangeErrString + " out of range"}
                }
                if err != nil {
                        return Time{}, &ParseError{alayout, avalue, std, value, ""}
                }
        }
        if pmSet && hour < 12 {
                hour += 12
        } else if amSet && hour == 12 {
                hour = 0
        }

        // TODO: be more aggressive checking day?
        if z != nil {
                return Date(year, Month(month), day, hour, min, sec, nsec, z), nil
        }

        t := Date(year, Month(month), day, hour, min, sec, nsec, UTC)
        if zoneOffset != -1 {
                t.sec -= int64(zoneOffset)

                // Look for local zone with the given offset.
                // If that zone was in effect at the given time, use it.
                name, offset, _, _, _ := Local.lookup(t.sec + internalToUnix)
                if offset == zoneOffset && (zoneName == "" || name == zoneName) {
                        t.loc = Local
                        return t, nil
                }

                // Otherwise create fake zone to record offset.
                t.loc = FixedZone(zoneName, zoneOffset)
                return t, nil
        }

        if zoneName != "" {
                // Look for local zone with the given offset.
                // If that zone was in effect at the given time, use it.
                offset, _, ok := Local.lookupName(zoneName)
                if ok {
                        name, off, _, _, _ := Local.lookup(t.sec + internalToUnix - int64(offset))
                        if name == zoneName && off == offset {
                                t.sec -= int64(offset)
                                t.loc = Local
                                return t, nil
                        }
                }

                // Otherwise, create fake zone with unknown offset.
                t.loc = FixedZone(zoneName, 0)
                return t, nil
        }

        // Otherwise, fall back to UTC.
        return t, nil
}

func parseNanoseconds(value string, nbytes int) (ns int, rangeErrString string, err error) {
        if value[0] != '.' {
                err = errBad
                return
        }
        ns, err = atoi(value[1:nbytes])
        if err != nil {
                return
        }
        if ns < 0 || 1e9 <= ns {
                rangeErrString = "fractional second"
                return
        }
        // We need nanoseconds, which means scaling by the number
        // of missing digits in the format, maximum length 10. If it's
        // longer than 10, we won't scale.
        scaleDigits := 10 - nbytes
        for i := 0; i < scaleDigits; i++ {
                ns *= 10
        }
        return
}

var errLeadingInt = errors.New("time: bad [0-9]*") // never printed

// leadingInt consumes the leading [0-9]* from s.
func leadingInt(s string) (x int, rem string, err error) {
        i := 0
        for ; i < len(s); i++ {
                c := s[i]
                if c < '0' || c > '9' {
                        break
                }
                if x >= (1<<31-10)/10 {
                        // overflow
                        return 0, "", errLeadingInt
                }
                x = x*10 + int(c) - '0'
        }
        return x, s[i:], nil
}

var unitMap = map[string]float64{
        "ns": float64(Nanosecond),
        "us": float64(Microsecond),
        "µs": float64(Microsecond), // U+00B5 = micro symbol
        "μs": float64(Microsecond), // U+03BC = Greek letter mu
        "ms": float64(Millisecond),
        "s":  float64(Second),
        "m":  float64(Minute),
        "h":  float64(Hour),
}

// ParseDuration parses a duration string.
// A duration string is a possibly signed sequence of
// decimal numbers, each with optional fraction and a unit suffix,
// such as "300ms", "-1.5h" or "2h45m".
// Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".
func ParseDuration(s string) (Duration, error) {
        // [-+]?([0-9]*(\.[0-9]*)?[a-z]+)+
        orig := s
        f := float64(0)
        neg := false

        // Consume [-+]?
        if s != "" {
                c := s[0]
                if c == '-' || c == '+' {
                        neg = c == '-'
                        s = s[1:]
                }
        }
        // Special case: if all that is left is "0", this is zero.
        if s == "0" {
                return 0, nil
        }
        if s == "" {
                return 0, errors.New("time: invalid duration " + orig)
        }
        for s != "" {
                g := float64(0) // this element of the sequence

                var x int
                var err error

                // The next character must be [0-9.]
                if !(s[0] == '.' || ('0' <= s[0] && s[0] <= '9')) {
                        return 0, errors.New("time: invalid duration " + orig)
                }
                // Consume [0-9]*
                pl := len(s)
                x, s, err = leadingInt(s)
                if err != nil {
                        return 0, errors.New("time: invalid duration " + orig)
                }
                g = float64(x)
                pre := pl != len(s) // whether we consumed anything before a period

                // Consume (\.[0-9]*)?
                post := false
                if s != "" && s[0] == '.' {
                        s = s[1:]
                        pl := len(s)
                        x, s, err = leadingInt(s)
                        if err != nil {
                                return 0, errors.New("time: invalid duration " + orig)
                        }
                        scale := 1
                        for n := pl - len(s); n > 0; n-- {
                                scale *= 10
                        }
                        g += float64(x) / float64(scale)
                        post = pl != len(s)
                }
                if !pre && !post {
                        // no digits (e.g. ".s" or "-.s")
                        return 0, errors.New("time: invalid duration " + orig)
                }

                // Consume unit.
                i := 0
                for ; i < len(s); i++ {
                        c := s[i]
                        if c == '.' || ('0' <= c && c <= '9') {
                                break
                        }
                }
                if i == 0 {
                        return 0, errors.New("time: missing unit in duration " + orig)
                }
                u := s[:i]
                s = s[i:]
                unit, ok := unitMap[u]
                if !ok {
                        return 0, errors.New("time: unknown unit " + u + " in duration " + orig)
                }

                f += g * unit
        }

        if neg {
                f = -f
        }
        return Duration(f), nil
}