format.go

package chrono

import (
	"fmt"
	"strconv"
	"strings"
)

// These are predefined layouts used for the parsing and formatting of dates, times and date-times.
// Additional layouts can be composed using the specifiers detailed below:
//
//   - %Y: The ISO 8601 year as a decimal number, padded to 4 digits with leading 0s.
//   - %EY: The year in the era as a decimal number, padded to 4 digits with leading 0s.
//   - %y: The ISO 8601 year without a century as a decimal number, padded to 2 digits with a leading 0, in the range 00 to 99. See note (1).
//   - %Ey: The year in the era without a century as a decimal number, padded to 2 digits with a leading 0, in the range 00 to 99. See notes (1) and (9).
//   - %C: The century as a decimal number, padded to 2 digits with a leading 0, e.g. 19 for 1980. See note (9).
//   - %EC: The name of the era, either "CE" (for Common Era) "BCE" (for Before the Common Era).
//   - %j: The day of the year as a decimal number, padded to 3 digits with leading 0s, in the range 001 to 366. See note (2).
//   - %m: The month as a decimal number, padded to 2 digits with a leading 0, in the range 01 to 12.
//   - %B: The full month name, e.g. January, February, etc.
//   - %b: The abbreviated month name, e.g. Jan, Feb, etc.
//   - %d: The day of the month as a decimal number, padded to 2 digits with a leading 0, in the range 01 to 31.
//
// Days of week:
//
//   - %u: The day of the week as a decimal number, e.g. 1 for Monday, 2 for Tuesday, etc. See note (3).
//   - %A: The full name of the day of the week, e.g. Monday, Tuesday, etc. See note (3).
//   - %a: The abbreviated name of the day of the week, e.g. Mon, Tue, etc. See note (3).
//
// Week numbers:
//
//   - %G: The ISO 8601 week-based year, padded to 4 digits with leading 0s. This may differ by ±1 to the actual calendar year. See note (2).
//   - %V: The ISO week number, padded to 2 digits with a leading 0, in the range 01 to 53. See note (2).
//
// Times of day:
//
//   - %P: Either "am" or "pm", where noon is "pm" and midnight is "am".
//   - %p: Either "AM" or "PM", where noon is "PM" and midnight is "AM".
//   - %I: The hour of the day using the 12-hour clock as a decimal number, padded to 2 digits with a leading 0, in the range 01 to 12. See note (4).
//
// Time components:
//
//   - %H: The hour of the day using the 24-hour clock as a decimal number, padded to 2 digits with a leading 0, in the range 00 to 23. See note (5).
//   - %M: The minute as a decimal number, padded to 2 digits with a leading 0, in the range 00 to 59.
//   - %S: The second as a decimal number, padded to 2 digits with a leading 0, in the range 00 to 59.
//
// Millisecond precisions:
//
//   - %f: Equivalent to %6f.
//   - %3f: The millisecond offset within the represented second, rounded either up or down and padded to 3 digits with leading 0s.
//   - %6f: The microsecond offset within the represented second, rounded either up or down and padded to 6 digits with leading 0s.
//   - %9f: The nanosecond offset within the represented second, padded to 9 digits with leading 0s.
//
// Time offsets:
//
//   - %z: The UTC offset in the format ±HHMM, preceded always by the sign ('+' or '-'), and padded to 4 digits with leading zeros. See notes (6), (7), and (8).
//   - %Ez: Equivalent to %z, except that an offset of +0000 is formatted at 'Z', and other offsets as ±HH:MM. See notes (6) and (7).
//
// When formatting using specifiers that represent padded decimals, leading 0s can be omitted using the '-' character after the '%'.
// For example, '%m' may produce the string '04' (for March), but '%-m' produces '4'.
// However, when parsing using these specifiers, it is not required that the input string contains any leading zeros.
//
// When parsing using specifier that represent textual values (month names, etc.), the input text is treated case insensitively.
//
// Depending on the context in which the layout is used, only a subset of specifiers may be supported by a particular function.
// For example, %H is not supported when parsing or formatting a date.
//
// When parsing, if multiple instances of the same specifier, or multiple instances of a specifier that represent the same value,
// are encountered, only the instance will be considered. See note (2).
//
// If a specifier is encountered which is not recognized (defined in the list above), or not supported by a particular function,
// the function will panic with a message that includes the unrecognized sequence.
//
// Any other text is enchoed verbatim when formatting, and is expected to appear verbatim in the parsed text.
// In order to print the '%' character verbatim (which normally signifies a specifier), the sequence '%%' can be used.
//
// For familiarity, the examples below use the time package's reference time of "2nd Jan 2006 15:04:05 -0700" (Unix time 1136239445).
// But note that this reference format is not relevant at all to the functioning of this package.
//
// Notes:
//
//  1. When 2-digit years are parsed (%y or %Ey), they are converted according to the POSIX and ISO C standards:
//     values 69–99 are mapped to 1969–1999, and values 0–68 are mapped to 2000–2068.
//  2. When a date is parsed in combination with a day of year (%j), and/or an ISO week-based date (%G and/or %V),
//     an error will be returned if the represented dates to not match.
//  3. When a date is parsed in combination with a day of the week (%a, %A and/or %u),
//     an error will be returned if it does not match the day represented by the parsed date.
//     The day of the week is otherwise ignored - it does not have any effect on the result.
//  4. When a time represented in the 12-hour clock format (%I) is parsed, and no time of day (%P or %p) is present,
//     the time of day is assumed to be before noon, i.e. am or AM.
//  5. When a time is parsed that contains the time of day (%P or %p), any hour (%H) that is present
//     must be valid on the 12-hour clock.
//  6. When UTC offsets are parsed (%z or %Ez) into a type which do not include a time offset element,
//     the offset present in the string is ignored.
//     When UTC offsets are formatted from a type which does not include a time offset element,
//     the offset will not be present in the returned string.
//  7. When UTC offsets are parsed (%z or %Ez), the shorted form of ±HH is accepted.
//     However, when formatted, only the full forms are returned (either ±HHMM or ±HH:MM).
//  8. When %z is used for parsing a UTC offset, 'Z' can be used to represent an offset of +0000.
//  9. When parsing partial years (%Ey and %C) in combination with a full year (%Y or %EY),
//     an error will be returned if the represented years to not match.
const (
	// ISO 8601.
	ISO8601                          = ISO8601DateTimeExtended
	ISO8601DateSimple                = "%Y%m%d"                                  // 20060102
	ISO8601DateExtended              = "%Y-%m-%d"                                // 2006-01-02
	ISO8601DateTruncated             = "%Y-%m"                                   // 2006-01
	ISO8601TimeSimple                = "T%H%M%S%z"                               // T030405-0700
	ISO8601TimeExtended              = "T%H:%M:%S%Ez"                            // T03:04:05-07:00
	ISO8601TimeMillisSimple          = "T%H%M%S.%3f%z"                           // T030405.000-0700
	ISO8601TimeMillisExtended        = "T%H:%M:%S.%3f%Ez"                        // T03:04:05.000-07:00
	ISO8601TimeTruncatedMinsSimple   = "T%H%M"                                   // T0304
	ISO8601TimeTruncatedMinsExtended = "T%H:%M"                                  // T03:04
	ISO8601TimeTruncatedHours        = "T%H"                                     // T03
	ISO8601DateTimeSimple            = ISO8601DateSimple + ISO8601TimeSimple     // 20060102T030405-0700
	ISO8601DateTimeExtended          = ISO8601DateExtended + ISO8601TimeExtended // 2006-01-02T03:04:05-07:00
	ISO8601WeekSimple                = "%GW%V"                                   // 2006W01
	ISO8601WeekExtended              = "%G-W%V"                                  // 2006-W01
	ISO8601WeekDaySimple             = "%GW%V%u"                                 // 2006W011
	ISO8601WeekDayExtended           = "%G-W%V-%u"                               // 2006-W01-1
	ISO8601OrdinalDateSimple         = "%Y%j"                                    // 2006002
	ISO8601OrdinalDateExtended       = "%Y-%j"                                   // 2006-002
	// Layouts defined by the time package.
	ANSIC   = "%a %b %d %H:%M:%S %Y" // Mon Jan 02 15:04:05 2006
	Kitchen = "%I:%M%p"              // 3:04PM
)

func formatDateTimeOffset(layout string, date *int32, time *int64, offset *int64) (string, error) {
	var (
		year  int
		month int
		day   int
		hour  int
		min   int
		sec   int
	)

	var err error
	if date != nil {
		v := int64(*date)
		if year, month, day, err = fromDate(v); err != nil {
			return "", err
		}
	}

	if time != nil {
		v := int64(*time)
		hour, min, sec, _ = fromTime(v)
	}

	var buf, out []rune
NextChar:
	for _, c := range layout {
		buf = append(buf, c)

		if len(buf) >= 2 && buf[0] == '%' {
			if c == '-' || c == 'E' || (c >= '0' && c <= '9') {
				continue NextChar
			}

			nopad, localed, precision, main, err := parseSpecifier(buf)
			if err != nil {
				return "", err
			}

			decimal := func(v int, len int) string {
				if nopad {
					return strconv.Itoa(v)
				}
				return fmt.Sprintf("%0*d", len, v)
			}

			switch {
			case date != nil && main == 'a': // %a
				out = append(out, []rune(shortWeekdayName(getWeekday(*date)))...)
			case date != nil && main == 'A': // %A
				out = append(out, []rune(longWeekdayName(getWeekday(*date)))...)
			case date != nil && main == 'b': // %b
				out = append(out, []rune(shortMonthName(month))...)
			case date != nil && main == 'B': // %B
				out = append(out, []rune(longMonthName(month))...)
			case date != nil && main == 'C':
				if localed { // %EC
					if _, isBCE := convertISOToGregorianYear(year); isBCE {
						out = append(out, []rune("BCE")...)
					} else {
						out = append(out, []rune("CE")...)
					}
				} else { // %C
					out = append(out, []rune(fmt.Sprintf("%02d", year/100))...)
				}
			case date != nil && main == 'd': // %d
				out = append(out, []rune(decimal(day, 2))...)
			case time != nil && main == 'f': // %f
				if precision == 0 {
					precision = 6
				}

				nanos := timeNanoseconds(*time)
				switch precision {
				case 3: // %3f
					out = append(out, []rune(decimal(divideAndRoundInt(nanos, 1000000), 3))...)
				case 6: // %6f
					out = append(out, []rune(decimal(divideAndRoundInt(nanos, 1000), 6))...)
				case 9: // %9f
					out = append(out, []rune(decimal(nanos, 9))...)
				default:
					panic(fmt.Sprintf("unsupported specifier '%df'", precision))
				}
			case date != nil && main == 'G': // %G
				v := int64(*date)
				y, _, err := getISOWeek(v)
				if err != nil {
					panic(err.Error())
				}
				out = append(out, []rune(decimal(y, 4))...)
			case time != nil && main == 'H': // %H
				out = append(out, []rune(decimal(hour, 2))...)
			case time != nil && main == 'I': // %I
				h, _ := convert24To12HourClock(hour)
				out = append(out, []rune(decimal(h, 2))...)
			case date != nil && main == 'j': // %j
				v := int64(*date)
				d, err := getYearDay(v)
				if err != nil {
					panic(err.Error())
				}
				out = append(out, []rune(decimal(d, 3))...)
			case date != nil && main == 'm': // %m
				out = append(out, []rune(decimal(int(month), 2))...)
			case time != nil && main == 'M': // %M
				out = append(out, []rune(decimal(min, 2))...)
			case time != nil && main == 'p': // %p
				if _, isAfternoon := convert24To12HourClock(hour); !isAfternoon {
					out = append(out, []rune("AM")...)
				} else {
					out = append(out, []rune("PM")...)
				}
			case time != nil && main == 'P': // %P
				if _, isAfternoon := convert24To12HourClock(hour); !isAfternoon {
					out = append(out, []rune("am")...)
				} else {
					out = append(out, []rune("pm")...)
				}
			case time != nil && main == 'S': // %S
				out = append(out, []rune(decimal(sec, 2))...)
			case date != nil && main == 'u': // %u
				out = append(out, []rune(strconv.Itoa(getWeekday(*date)))...)
			case date != nil && main == 'V': // %V
				v := int64(*date)
				_, w, err := getISOWeek(v)
				if err != nil {
					panic(err.Error())
				}
				out = append(out, []rune(decimal(w, 2))...)
			case date != nil && main == 'y': // %y
				y := year
				if localed { // %Ey
					y, _ = convertISOToGregorianYear(y)
				}
				out = append(out, []rune(decimal(y%100, 2))...)
			case date != nil && main == 'Y': // %Y
				y := year
				if localed { // %EY
					y, _ = convertISOToGregorianYear(y)
				}
				out = append(out, []rune(decimal(y, 4))...)
			case time != nil && main == 'z':
				// Formatting %z from a type that contains no offset (e.g. LocalTime, LocalDateTime)
				// is valid, although it will not be printed.
				if offset == nil {
					break
				}

				if localed { // %Ez
					out = append(out, []rune(offsetString(*offset, ":"))...)
				} else { // %z
					out = append(out, []rune(offsetString(*offset, ""))...)
				}
			case main == '%': // %%
				out = append(out, '%')
			default:
				return "", fmt.Errorf("unsupported sequence %q", string(buf))
			}

			buf = nil
		} else if len(buf) == 1 && buf[0] != '%' {
			out = append(out, buf...)
			buf = nil
		}
	}

	return string(out), nil
}

var overrideCentury *int

func getCentury(year int) int {
	switch {
	case overrideCentury != nil:
		return *overrideCentury
	case year >= 69 && year <= 99:
		return 1900
	default:
		return 2000
	}
}

const (
	extraTextErrMsg   = "parsing time \"%s\": extra text: \"%s\""
	endOfStringErrMsg = "parsing time \"%s\": end of string"
)

// parseDateAndTime parses the supplied value according to the specified layout.
// date, time and offset must be provided in order for those components to be parsed.
// If not provided, and the specifiers that pertain to those components are
// encountered in the supplied layout, then an error is returned.
// If non-zero, date, time, and offset and taken as starting points, where the individual values
// that they represent are replaced only if present in the supplied layout.
func parseDateAndTime(layout, value string, date, time, offset *int64) error {
	var (
		haveDate          bool
		haveGregorianYear bool
		isBCE             bool
		year              int
		yearCentury       *int
		shortYear         *int
		yearType          int // -1 = short/century, 0 = none, 1 = full year

		month int
		day   int

		dayOfWeek int

		dayOfYear int

		haveISODate bool
		isoYear     int
		isoWeek     int

		have12HourClock bool
		isAfternoon     bool
		hour            int
		min             int
		sec             int
		nsec            int
	)

	var err error
	if date != nil {
		if year, month, day, err = fromDate(*date); err != nil {
			return err
		}

		if isoYear, isoWeek, err = getISOWeek(*date); err != nil {
			return err
		}
	}

	if time != nil {
		hour, min, sec, nsec = fromTime(*time)
		_, isAfternoon = convert24To12HourClock(hour)
	}

	var pos int
	var buf []rune
	for i := 0; i <= len(layout); i++ {
		verifyText := func() error {
			if len(buf) == 0 {
				return nil
			}

			verify := func() error {
				if !strings.HasPrefix(value[pos:], string(buf)) {
					return fmt.Errorf("parsing time \"%s\" as \"%s\": cannot parse \"%s\" as \"%s\"", value, layout, value[pos:], string(buf))
				}
				return nil
			}

			if buf[len(buf)-1] == '%' {
				if err := verify(); err != nil {
					return err
				}
				buf = []rune{'%'}
			} else {
				if err := verify(); err != nil {
					return err
				}
			}

			pos += len(buf)
			buf = nil
			return nil
		}

		processSpecifier := func() error {
			integer := func(maxLen int) (int, error) {
				var neg bool

				str := value[pos:]
				if len(str) >= 1 {
					switch str[0] {
					case '-':
						neg = true
						fallthrough
					case '+':
						str = str[1:]
						pos++
					}
				}

				if l := len(str); l == 0 {
					return 0, fmt.Errorf(endOfStringErrMsg, value)
				} else if l < maxLen {
					maxLen = l
				}
				str = str[:maxLen]

				var i int
				for _, char := range str {
					if (char < '0' || char > '9') && char != '.' && char != ',' {
						break
					}
					i++
				}
				pos += i

				if i == 0 {
					return 0, fmt.Errorf(extraTextErrMsg, value, str)
				}

				out, err := strconv.Atoi(str[:i])
				if err != nil {
					return 0, fmt.Errorf(extraTextErrMsg, value, str)
				}

				if neg {
					return out * -1, nil
				}
				return out, nil
			}

			hasMore := func() bool {
				return len(value[pos:]) > 0
			}

			casedAlpha := func(char rune) (rune, bool) {
				str := value[pos:]
				if len(str) != 0 {
					r := rune(str[0])
					if r == char {
						pos++
						return r, true
					}
				}
				return ' ', false
			}

			alphas := func(maxLen int) (lower, original string) {
				str := value[pos:]

				if l := len(str); l < maxLen {
					maxLen = l
				}
				str = value[pos : pos+maxLen]

				_lower := make([]rune, maxLen)
				_original := make([]rune, maxLen)

				var i int
				for _, char := range str {
					if char >= 'a' && char <= 'z' {
						_lower[i] = char
						_original[i] = char
						i++
					} else if char >= 'A' && char <= 'Z' {
						_lower[i] = char + 32
						_original[i] = char
						i++
					} else {
						break
					}
				}
				pos += i

				return string(_lower[:i]), string(_original[:i])
			}

			_, localed, precision, main, err := parseSpecifier(buf)
			if err != nil {
				return err
			}

			switch {
			case date != nil && main == 'a': // %a
				lower, original := alphas(3)
				var ok bool
				if dayOfWeek, ok = shortDayNameLookup[lower]; !ok {
					return fmt.Errorf("unrecognized short day name %q", original)
				}
			case date != nil && main == 'A': // %A
				lower, original := alphas(9)
				var ok bool
				if dayOfWeek, ok = longDayNameLookup[lower]; !ok {
					return fmt.Errorf("unrecognized day name %q", original)
				}
			case date != nil && main == 'b': // %b
				lower, original := alphas(3)
				var ok bool
				if month, ok = shortMonthNameLookup[lower]; !ok {
					return fmt.Errorf("unrecognized short month name %q", original)
				}
			case date != nil && main == 'B': // %B
				lower, original := alphas(9)
				var ok bool
				if month, ok = longMonthNameLookup[lower]; !ok {
					return fmt.Errorf("unrecognized month name %q", original)
				}
			case date != nil && main == 'C':
				if localed { // %EC
					haveGregorianYear = true
					lower, original := alphas(3)
					switch lower {
					case "ce":
					case "bce":
						isBCE = true
					default:
						return fmt.Errorf("unrecognized era %q", original)
					}
				} else { // %C
					var v int
					if v, err = integer(2); err != nil {
						return err
					}
					yearCentury = &v
					yearType = -1
				}
			case date != nil && main == 'd': // %d
				haveDate = true
				if day, err = integer(2); err != nil {
					return err
				}
			case time != nil && main == 'f': // %f
				if precision == 0 {
					precision = 6
				}

				switch precision {
				case 3: // %3f
					millis, err := integer(3)
					if err != nil {
						return err
					}
					nsec = millis * 1000000
				case 6: // %6f
					micros, err := integer(6)
					if err != nil {
						return err
					}
					nsec = micros * 1000
				case 9: // %9f
					if nsec, err = integer(9); err != nil {
						return err
					}
				default:
				}
			case date != nil && main == 'G': // %G
				haveISODate = true
				if isoYear, err = integer(4); err != nil {
					return err
				}
			case time != nil && main == 'H': // %H
				if hour, err = integer(2); err != nil {
					return err
				}
			case time != nil && main == 'I': // %I
				have12HourClock = true
				if hour, err = integer(2); err != nil {
					return err
				}
			case date != nil && main == 'j': // %j
				if dayOfYear, err = integer(3); err != nil {
					return err
				}
			case date != nil && main == 'm': // %m
				if month, err = integer(2); err != nil {
					return err
				}
			case time != nil && main == 'M': // %M
				if min, err = integer(2); err != nil {
					return err
				}
			case time != nil && main == 'p': // %p
				lower, original := alphas(2)
				switch strings.ToUpper(lower) {
				case "AM":
				case "PM":
					isAfternoon = true
				default:
					return fmt.Errorf("failed to parse time of day %q", original)
				}
			case time != nil && main == 'P': // %P
				lower, original := alphas(2)
				switch lower {
				case "am":
				case "pm":
					isAfternoon = true
				default:
					return fmt.Errorf("failed to parse time of day %q", original)
				}
			case time != nil && main == 'S': // %S
				if sec, err = integer(2); err != nil {
					return err
				}
			case date != nil && main == 'u': // %u
				if dayOfWeek, err = integer(1); err != nil {
					return err
				}
			case date != nil && main == 'V': // %V
				haveISODate = true
				if isoWeek, err = integer(2); err != nil {
					return err
				}
			case date != nil && main == 'y': // %y
				if localed { // %Ey
					haveGregorianYear = true
				}

				var v int
				if v, err = integer(2); err != nil {
					return err
				}
				shortYear = &v
				yearType = -1
			case date != nil && main == 'Y': // %Y
				if localed { // %EY
					haveGregorianYear = true
				}

				if year, err = integer(4); err != nil {
					return err
				}
				yearType = 1
			case time != nil && main == 'z': // %z
				// If at end of input and no offset is requested, break.
				// But continue to parse in the case where offset is not requested, but may be present.
				if offset == nil && !hasMore() {
					break
				}

				var v int64
				var h, m int
				var err error

				// Catch the 'Z' case, which is valid for both %z and %Ez.
				// Continue instead of breaking because offset may need updating.
				if _, ok := casedAlpha('Z'); ok {
					goto CalculateOffset
				}

				if h, err = integer(2); err != nil {
					return err
				}

				if !hasMore() {
					goto CalculateOffset
				}

				if localed { // %Ez
					if actual, ok := casedAlpha(':'); !ok {
						return fmt.Errorf(extraTextErrMsg, value, string(actual))
					}
				}

				if m, err = integer(2); err != nil {
					return err
				}

			CalculateOffset:
				if h >= 0 {
					v = int64(h)*oneHour + int64(m)*oneMinute
				} else {
					v = int64(h)*oneHour - int64(m)*oneMinute
				}

				// Parsing %z into a type that contains no offset (e.g. LocalTime, LocalDateTime)
				// is valid, although the value itself is ignored. But it needed to be consumed above, just now discarded.
				if offset != nil {
					*offset = v
				}
			case main == '%': // %%
			default:
				return fmt.Errorf("unsupported sequence %q", string(buf))
			}

			buf = nil
			return nil
		}

		// Some short-hands.
		var (
			valid             = i < len(layout)
			isSpecifier       = len(buf) >= 2 && buf[0] == '%'
			specifierComplete = isSpecifier && (buf[len(buf)-1] != '-' && buf[len(buf)-1] != 'E' && (buf[len(buf)-1] < '0' || buf[len(buf)-1] > '9'))
			isText            = len(buf) >= 1 && buf[0] != '%'
		)

		if valid {
			c := layout[i]
			if len(buf) == 0 {
				goto AppendToBuffer
			} else if isSpecifier {
				if !specifierComplete {
					goto AppendToBuffer
				}

				if err := processSpecifier(); err != nil {
					return err
				}
				goto AppendToBuffer
			} else if isText && c == '%' {
				if err := verifyText(); err != nil {
					return err
				}
				goto AppendToBuffer
			}

		AppendToBuffer:
			buf = append(buf, rune(c))
		} else if isSpecifier {
			if err := processSpecifier(); err != nil {
				return err
			}
		} else if isText {
			if err := verifyText(); err != nil {
				return err
			}
		}
	}

	if pos < len(value) {
		return fmt.Errorf(extraTextErrMsg, value, value[pos:])
	}

	if date != nil {
		// Check century according to note (9).
		if yearCentury != nil {
			if yearType == 1 && year/100 != *yearCentury {
				return fmt.Errorf("year century %d does not agree with year %d", *yearCentury, year)
			} else if yearType != 1 {
				year = *yearCentury * 100
			}
		}

		// Check 2-digit year according to note (9).
		if shortYear != nil {
			_year := getCentury(*shortYear) + *shortYear
			if yearCentury != nil {
				_year = *yearCentury*100 + *shortYear
			}

			if yearType == 1 && year-(year/100*100) != *shortYear {
				return fmt.Errorf("short year %d (%d) does not agree with year %d", *shortYear, _year, year)
			} else if yearType != 1 {
				year = _year
			}
		}

		if haveGregorianYear {
			if year, err = convertGregorianToISOYear(year, isBCE); err != nil {
				return err
			}
		}

		if !isDateValid(year, month, day) {
			return fmt.Errorf("invalid date %q", simpleDateStr(year, month, day))
		}

		_date, err := makeDate(year, month, day)
		if err != nil {
			return err
		}

		*date = _date

		// Check day of year according to note (2).
		if dayOfYear != 0 {
			doyDate, err := ofDayOfYear(year, dayOfYear)
			if err != nil {
				return err
			}

			if haveDate && (doyDate != _date) {
				return fmt.Errorf("day-of-year date %q does not agree with date %q",
					LocalDate(doyDate).String(),
					simpleDateStr(year, month, day),
				)
			}

			*date = doyDate
		}

		// Check ISO week-year according to note (2).
		if haveISODate {
			weekday := dayOfWeek
			if dayOfWeek == 0 {
				weekday = int(Monday)
			}

			isoDate, err := ofISOWeek(isoYear, isoWeek, weekday)
			if err != nil {
				return fmt.Errorf("invalid ISO week-year date %q", getISODateSimpleStr(isoYear, isoWeek, day))
			}

			if haveDate && (isoDate != _date) {
				return fmt.Errorf("ISO week-year date %q does not agree with date %q",
					getISODateSimpleStr(isoYear, isoWeek, day),
					simpleDateStr(year, month, day),
				)
			}

			*date = isoDate
		}

		// Check day of week according to note (3).
		haveDate = haveDate || dayOfYear != 0
		if dayOfWeek != 0 && haveDate {
			if actual := getWeekday(int32(*date)); dayOfWeek != actual {
				return fmt.Errorf("day of week %q does not agree with actual day of week %q",
					longWeekdayName(dayOfWeek),
					longWeekdayName(actual),
				)
			}
		}
	}

	if time != nil {
		// Check validity of hour on 12-hour clock according to note (5).
		if have12HourClock {
			if hour < 1 || hour > 12 {
				return fmt.Errorf("hour %d is not valid on the 12-hour clock", hour)
			}
			hour = convert12To24HourClock(hour, isAfternoon)
		}

		v, err := makeTime(hour, min, sec, nsec)
		if err != nil {
			return err
		}
		*time = v
	}

	return nil
}

func parseSpecifier(buf []rune) (nopad, localed bool, precision uint, main rune, err error) {
	if len(buf) == 3 {
		switch {
		case buf[1] == '-':
			nopad = true
		case buf[1] == 'E':
			localed = true
		case buf[1] >= '0' && buf[1] <= '9':
			precision = uint(buf[1] - 48)
		default:
			return false, false, 0, 0, fmt.Errorf("unsupported modifier '%c'", buf[1])
		}
	} else if len(buf) == 4 {
		switch buf[1] {
		case '-':
			nopad = true
		default:
			return false, false, 0, 0, fmt.Errorf("unsupported modifier '%c'", buf[1])
		}

		switch buf[2] {
		case 'E':
			localed = true
		default:
			return false, false, 0, 0, fmt.Errorf("unsupported modifier '%c'", buf[1])
		}
	}
	return nopad, localed, precision, buf[len(buf)-1], nil
}

func convert12To24HourClock(hour12 int, isAfternoon bool) (hour24 int) {
	if isAfternoon && hour12 == 12 {
		return 12
	} else if isAfternoon {
		return hour12 + 12
	} else if hour12 == 12 {
		return 0
	}
	return hour12
}

func convert24To12HourClock(hour24 int) (hour12 int, isAfternoon bool) {
	if hour24 == 0 {
		return 12, false
	} else if hour24 == 12 {
		return 12, true
	} else if hour24 < 12 {
		return hour24, false
	}
	return hour24 % 12, true
}

func convertGregorianToISOYear(gregorianYear int, isBCE bool) (isoYear int, err error) {
	if gregorianYear == 0 {
		return 0, fmt.Errorf("invalid Gregorian year %04d", gregorianYear)
	}

	if isBCE {
		return (gregorianYear * -1) + 1, nil
	}
	return gregorianYear, nil
}

func convertISOToGregorianYear(isoYear int) (gregorianYear int, isBCE bool) {
	if isoYear <= 0 {
		return (isoYear * -1) + 1, true
	}
	return isoYear, false
}

func shortWeekdayName(d int) string {
	if d < int(Monday) || d > int(Sunday) {
		return fmt.Sprintf("%%!Weekday(%d)", d)
	}
	return shortDayNames[d-1]
}

var longDayNameLookup = map[string]int{
	"monday":    int(Monday),
	"tuesday":   int(Tuesday),
	"wednesday": int(Wednesday),
	"thursday":  int(Thursday),
	"friday":    int(Friday),
	"saturday":  int(Saturday),
	"sunday":    int(Sunday),
}

var shortDayNames = [7]string{
	Monday - 1:    "Mon",
	Tuesday - 1:   "Tue",
	Wednesday - 1: "Wed",
	Thursday - 1:  "Thu",
	Friday - 1:    "Fri",
	Saturday - 1:  "Sat",
	Sunday - 1:    "Sun",
}

var shortDayNameLookup = map[string]int{
	"mon": int(Monday),
	"tue": int(Tuesday),
	"wed": int(Wednesday),
	"thu": int(Thursday),
	"fri": int(Friday),
	"sat": int(Saturday),
	"sun": int(Sunday),
}

func (m Month) short() string {
	return shortMonthName(int(m))
}

func shortMonthName(m int) string {
	if m < int(January) || m > int(December) {
		return fmt.Sprintf("%%!Month(%d)", m)
	}
	return shortMonthNames[m-1]
}

var longMonthNameLookup = map[string]int{
	"january":   int(January),
	"february":  int(February),
	"march":     int(March),
	"april":     int(April),
	"may":       int(May),
	"june":      int(June),
	"july":      int(July),
	"august":    int(August),
	"september": int(September),
	"october":   int(October),
	"november":  int(November),
	"december":  int(December),
}

var shortMonthNames = [12]string{
	January - 1:   "Jan",
	February - 1:  "Feb",
	March - 1:     "Mar",
	April - 1:     "Apr",
	May - 1:       "May",
	June - 1:      "Jun",
	July - 1:      "Jul",
	August - 1:    "Aug",
	September - 1: "Sep",
	October - 1:   "Oct",
	November - 1:  "Nov",
	December - 1:  "Dec",
}

var shortMonthNameLookup = map[string]int{
	"jan": int(January),
	"feb": int(February),
	"mar": int(March),
	"apr": int(April),
	"may": int(May),
	"jun": int(June),
	"jul": int(July),
	"aug": int(August),
	"sep": int(September),
	"oct": int(October),
	"nov": int(November),
	"dec": int(December),
}