format.go

// 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.
//
// Additions and modifications under the MIT License.

package dateformatter

import (
	"errors"
	"fmt"
	"time"

	"github.com/altipla-consulting/dateformatter/symbols"
)

const (
	_                        = iota
	stdLongMonth             = iota + stdNeedDate  // "January"
	stdMonth                                       // "Jan"
	stdNumMonth                                    // "1"
	stdZeroMonth                                   // "01"
	stdLongWeekDay                                 // "Monday"
	stdWeekDay                                     // "Mon"
	stdDay                                         // "2"
	stdUnderDay                                    // "_2"
	stdZeroDay                                     // "02"
	stdHour                  = iota + stdNeedClock // "15"
	stdHour12                                      // "3"
	stdZeroHour12                                  // "03"
	stdMinute                                      // "4"
	stdZeroMinute                                  // "04"
	stdSecond                                      // "5"
	stdZeroSecond                                  // "05"
	stdLongYear              = iota + stdNeedDate  // "2006"
	stdYear                                        // "06"
	stdPM                    = iota + stdNeedClock // "PM"
	stdpm                                          // "pm"
	stdTZ                    = iota                // "MST"
	stdISO8601TZ                                   // "Z0700"  // prints Z for UTC
	stdISO8601SecondsTZ                            // "Z070000"
	stdISO8601ColonTZ                              // "Z07:00" // prints Z for UTC
	stdISO8601ColonSecondsTZ                       // "Z07:00:00"
	stdNumTZ                                       // "-0700"  // always numeric
	stdNumSecondsTz                                // "-070000"
	stdNumShortTZ                                  // "-07"    // always numeric
	stdNumColonTZ                                  // "-07:00" // always numeric
	stdNumColonSecondsTZ                           // "-07:00:00"
	stdFracSecond0                                 // ".0", ".00", ... , trailing zeros included
	stdFracSecond9                                 // ".9", ".99", ..., trailing zeros omitted

	stdNeedDate  = 1 << 8             // need month, day, year
	stdNeedClock = 2 << 8             // need hour, minute, second
	stdArgShift  = 16                 // extra argument in high bits, above low stdArgShift
	stdMask      = 1<<stdArgShift - 1 // mask out argument
)

// std0x records the std values for "01", "02", ..., "06".
var std0x = [...]int{stdZeroMonth, stdZeroDay, stdZeroHour12, stdZeroMinute, stdZeroSecond, stdYear}

// startsWithLowerCase reports whether the string has a lower-case letter at the beginning.
// Its purpose is to prevent matching strings like "Month" when looking for "Mon".
func startsWithLowerCase(str string) bool {
	if len(str) == 0 {
		return false
	}
	c := str[0]
	return 'a' <= c && c <= 'z'
}

// 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 string, std int, suffix string) {
	for i := 0; i < len(layout); i++ {
		switch c := int(layout[i]); c {
		case 'J': // January, Jan
			if len(layout) >= i+3 && layout[i:i+3] == "Jan" {
				if len(layout) >= i+7 && layout[i:i+7] == "January" {
					return layout[0:i], stdLongMonth, layout[i+7:]
				}
				if !startsWithLowerCase(layout[i+3:]) {
					return layout[0:i], stdMonth, layout[i+3:]
				}
			}

		case 'M': // Monday, Mon, MST
			if len(layout) >= i+3 {
				if layout[i:i+3] == "Mon" {
					if len(layout) >= i+6 && layout[i:i+6] == "Monday" {
						return layout[0:i], stdLongWeekDay, layout[i+6:]
					}
					if !startsWithLowerCase(layout[i+3:]) {
						return layout[0:i], stdWeekDay, layout[i+3:]
					}
				}
				if layout[i:i+3] == "MST" {
					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], std0x[layout[i+1]-'1'], 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] == "2006" {
				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':
			return layout[0:i], stdHour12, layout[i+1:]

		case '4':
			return layout[0:i], stdMinute, layout[i+1:]

		case '5':
			return layout[0:i], stdSecond, layout[i+1:]

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

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

// match reports whether 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
}

// appendInt appends the decimal form of x to b and returns the result.
// If the decimal form (excluding sign) is shorter than width, the result is padded with leading 0's.
// Duplicates functionality in strconv, but avoids dependency.
func appendInt(b []byte, x int, width int) []byte {
	format := fmt.Sprintf("%%0%dd", width)
	return append(b, []byte(fmt.Sprintf(format, x))...)
}

// Format returns a textual representation of the time value formatted
// according to layout, which defines the format by showing how the reference
// time, defined to be
//  Mon Jan 2 15:04:05 2006
// would be displayed if it were the value; it serves as an example of the
// desired output. The same display rules will then be applied to the time
// value.
//
// Predefined layouts ANSIC, UnixDate, RFC3339 and others describe standard
// and convenient representations of the reference time. For more information
// about the formats and the definition of the reference time, see the
// documentation for ANSIC and the other constants defined by this package.
func Format(t time.Time, locale, layout string) string {
	var b []byte

	var (
		year  int = -1
		month time.Month
		day   int
		hour  int = -1
		min   int
		sec   int
	)
	// Each iteration generates one std value.
	for layout != "" {
		prefix, std, suffix := nextStdChunk(layout)
		if prefix != "" {
			b = append(b, prefix...)
		}
		if std == 0 {
			break
		}
		layout = suffix

		// Compute year, month, day if needed.
		if year < 0 && std&stdNeedDate != 0 {
			year, month, day = t.Date()
		}

		// Compute hour, minute, second if needed.
		if hour < 0 && std&stdNeedClock != 0 {
			hour, min, sec = t.Clock()
		}

		switch std & stdMask {
		case stdYear:
			y := year
			if y < 0 {
				y = -y
			}
			b = appendInt(b, y%100, 2)
		case stdLongYear:
			b = appendInt(b, year, 4)
		case stdMonth:
			m := symbols.ShortMonthNames[locale][month]
			b = append(b, m...)
		case stdLongMonth:
			m := symbols.LongMonthNames[locale][month]
			b = append(b, m...)
		case stdNumMonth:
			b = appendInt(b, int(month), 0)
		case stdZeroMonth:
			b = appendInt(b, int(month), 2)
		case stdWeekDay:
			s := symbols.ShortWeekdays[locale][t.Weekday()]
			b = append(b, s...)
		case stdLongWeekDay:
			s := symbols.LongWeekdays[locale][t.Weekday()]
			b = append(b, s...)
		case stdDay:
			b = appendInt(b, day, 0)
		case stdUnderDay:
			if day < 10 {
				b = append(b, ' ')
			}
			b = appendInt(b, day, 0)
		case stdZeroDay:
			b = appendInt(b, day, 2)
		case stdHour:
			b = appendInt(b, hour, 2)
		case stdHour12:
			// Noon is 12PM, midnight is 12AM.
			hr := hour % 12
			if hr == 0 {
				hr = 12
			}
			b = appendInt(b, hr, 0)
		case stdZeroHour12:
			// Noon is 12PM, midnight is 12AM.
			hr := hour % 12
			if hr == 0 {
				hr = 12
			}
			b = appendInt(b, hr, 2)
		case stdMinute:
			b = appendInt(b, min, 0)
		case stdZeroMinute:
			b = appendInt(b, min, 2)
		case stdSecond:
			b = appendInt(b, sec, 2)
		case stdZeroSecond:
			b = appendInt(b, sec, 2)
		case stdPM:
			if hour >= 12 {
				b = append(b, "PM"...)
			} else {
				b = append(b, "AM"...)
			}
		case stdpm:
			if hour >= 12 {
				b = append(b, "pm"...)
			} else {
				b = append(b, "am"...)
			}
		}
	}

	return string(b)
}

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

// isDigit reports whether s[i] is in range and is a decimal digit.
func isDigit(s string, i int) bool {
	if len(s) <= i {
		return false
	}
	c := s[i]
	return '0' <= c && c <= '9'
}