package strftime

import "unicode/utf8"

type parser struct {
        format  func(spec, flag byte) error
        literal func(byte) error
}

func (p *parser) parse(fmt string) error {
        const (
                initial = iota
                percent
                flagged
                modified
        )

        var flag, modifier byte
        var err error
        state := initial
        start := 0
        for i, b := range []byte(fmt) {
                switch state {
                default:
                        if b == '%' {
                                state = percent
                                start = i
                                continue
                        }
                        err = p.literal(b)

                case percent:
                        if b == '-' || b == ':' {
                                state = flagged
                                flag = b
                                continue
                        }
                        if b == 'E' || b == 'O' {
                                state = modified
                                modifier = b
                                flag = 0
                                continue
                        }
                        err = p.format(b, 0)
                        state = initial

                case flagged:
                        if b == 'E' || b == 'O' {
                                state = modified
                                modifier = b
                                continue
                        }
                        err = p.format(b, flag)
                        state = initial

                case modified:
                        if okModifier(modifier, b) {
                                err = p.format(b, flag)
                        } else {
                                err = p.literals(fmt[start : i+1])
                        }
                        state = initial
                }

                if err != nil {
                        if err, ok := err.(formatError); ok {
                                err.setDirective(fmt, start, i)
                                return err
                        }
                        return err
                }
        }

        if state != initial {
                return p.literals(fmt[start:])
        }
        return nil
}

func (p *parser) literals(literal string) error {
        for _, b := range []byte(literal) {
                if err := p.literal(b); err != nil {
                        return err
                }
        }
        return nil
}

type literalErr string

func (e literalErr) Error() string {
        return "strftime: unsupported literal: " + string(e)
}

type formatError struct {
        message   string
        directive string
}

func (e formatError) Error() string {
        return "strftime: unsupported directive: " + e.directive + " " + e.message
}

func (e *formatError) setDirective(str string, i, j int) {
        _, n := utf8.DecodeRuneInString(str[j:])
        e.directive = str[i : j+n]
}

package strftime

import "strings"

// https://strftime.org/
func goLayout(spec, flag byte, parsing bool) string {
        switch spec {
        default:
                return ""

        case 'B':
                return "January"
        case 'b', 'h':
                return "Jan"
        case 'm':
                if flag == '-' || parsing {
                        return "1"
                }
                return "01"
        case 'A':
                return "Monday"
        case 'a':
                return "Mon"
        case 'e':
                return "_2"
        case 'd':
                if flag == '-' || parsing {
                        return "2"
                }
                return "02"
        case 'j':
                if flag == '-' {
                        if parsing {
                                return "__2"
                        }
                        return ""
                }
                return "002"
        case 'I':
                if flag == '-' || parsing {
                        return "3"
                }
                return "03"
        case 'H':
                if flag == '-' && !parsing {
                        return ""
                }
                return "15"
        case 'M':
                if flag == '-' || parsing {
                        return "4"
                }
                return "04"
        case 'S':
                if flag == '-' || parsing {
                        return "5"
                }
                return "05"
        case 'y':
                return "06"
        case 'Y':
                return "2006"
        case 'p':
                return "PM"
        case 'P':
                return "pm"
        case 'Z':
                return "MST"
        case 'z':
                if flag == ':' {
                        if parsing {
                                return "Z07:00"
                        }
                        return "-07:00"
                }
                if parsing {
                        return "Z0700"
                }
                return "-0700"

        case '+':
                if parsing {
                        return "Mon Jan _2 15:4:5 MST 2006"
                }
                return "Mon Jan _2 15:04:05 MST 2006"
        case 'c':
                if parsing {
                        return "Mon Jan _2 15:4:5 2006"
                }
                return "Mon Jan _2 15:04:05 2006"
        case 'v':
                return "_2-Jan-2006"
        case 'F':
                if parsing {
                        return "2006-1-2"
                }
                return "2006-01-02"
        case 'D', 'x':
                if parsing {
                        return "1/2/06"
                }
                return "01/02/06"
        case 'r':
                if parsing {
                        return "3:4:5 PM"
                }
                return "03:04:05 PM"
        case 'T', 'X':
                if parsing {
                        return "15:4:5"
                }
                return "15:04:05"
        case 'R':
                if parsing {
                        return "15:4"
                }
                return "15:04"

        case '%':
                return "%"
        case 't':
                return "\t"
        case 'n':
                return "\n"
        }
}

// https://nsdateformatter.com/
func uts35Pattern(spec, flag byte) string {
        switch spec {
        default:
                return ""

        case 'B':
                return "MMMM"
        case 'b', 'h':
                return "MMM"
        case 'm':
                if flag == '-' {
                        return "M"
                }
                return "MM"
        case 'A':
                return "EEEE"
        case 'a':
                return "E"
        case 'd':
                if flag == '-' {
                        return "d"
                }
                return "dd"
        case 'j':
                if flag == '-' {
                        return "D"
                }
                return "DDD"
        case 'I':
                if flag == '-' {
                        return "h"
                }
                return "hh"
        case 'H':
                if flag == '-' {
                        return "H"
                }
                return "HH"
        case 'M':
                if flag == '-' {
                        return "m"
                }
                return "mm"
        case 'S':
                if flag == '-' {
                        return "s"
                }
                return "ss"
        case 'y':
                return "yy"
        case 'Y':
                return "yyyy"
        case 'g':
                return "YY"
        case 'G':
                return "YYYY"
        case 'V':
                if flag == '-' {
                        return "w"
                }
                return "ww"
        case 'p':
                return "a"
        case 'Z':
                return "zzz"
        case 'z':
                if flag == ':' {
                        return "xxx"
                }
                return "xx"
        case 'L':
                return "SSS"
        case 'f':
                return "SSSSSS"
        case 'N':
                return "SSSSSSSSS"

        case '+':
                return "E MMM d HH:mm:ss zzz yyyy"
        case 'c':
                return "E MMM d HH:mm:ss yyyy"
        case 'v':
                return "d-MMM-yyyy"
        case 'F':
                return "yyyy-MM-dd"
        case 'D', 'x':
                return "MM/dd/yy"
        case 'r':
                return "hh:mm:ss a"
        case 'T', 'X':
                return "HH:mm:ss"
        case 'R':
                return "HH:mm"

        case '%':
                return "%"
        case 't':
                return "\t"
        case 'n':
                return "\n"
        }
}

// http://man.he.net/man3/strftime
func okModifier(mod, spec byte) bool {
        if mod == 'E' {
                return strings.Contains("cCxXyY", string(spec))
        }
        if mod == 'O' {
                return strings.Contains("deHImMSuUVwWy", string(spec))
        }
        return false
}

package strftime

import (
        "bytes"
        "strconv"
        "time"
)

// Format returns a textual representation of the time value
// formatted according to the strftime format specification.
func Format(fmt string, t time.Time) string {
        buf := buffer(fmt)
        return string(AppendFormat(buf, fmt, t))
}

// AppendFormat is like Format, but appends the textual representation
// to dst and returns the extended buffer.
func AppendFormat(dst []byte, fmt string, t time.Time) []byte {
        var parser parser

        parser.literal = func(b byte) error {
                dst = append(dst, b)
                return nil
        }

        parser.format = func(spec, flag byte) error {
                switch spec {
                case 'A':
                        dst = append(dst, t.Weekday().String()...)
                        return nil
                case 'a':
                        dst = append(dst, t.Weekday().String()[:3]...)
                        return nil
                case 'B':
                        dst = append(dst, t.Month().String()...)
                        return nil
                case 'b', 'h':
                        dst = append(dst, t.Month().String()[:3]...)
                        return nil
                case 'm':
                        dst = appendInt2(dst, int(t.Month()), flag)
                        return nil
                case 'd':
                        dst = appendInt2(dst, int(t.Day()), flag)
                        return nil
                case 'e':
                        dst = appendInt2(dst, int(t.Day()), ' ')
                        return nil
                case 'I':
                        dst = append12Hour(dst, t, flag)
                        return nil
                case 'l':
                        dst = append12Hour(dst, t, ' ')
                        return nil
                case 'H':
                        dst = appendInt2(dst, t.Hour(), flag)
                        return nil
                case 'k':
                        dst = appendInt2(dst, t.Hour(), ' ')
                        return nil
                case 'M':
                        dst = appendInt2(dst, t.Minute(), flag)
                        return nil
                case 'S':
                        dst = appendInt2(dst, t.Second(), flag)
                        return nil
                case 'L':
                        dst = append(dst, t.Format(".000")[1:]...)
                        return nil
                case 'f':
                        dst = append(dst, t.Format(".000000")[1:]...)
                        return nil
                case 'N':
                        dst = append(dst, t.Format(".000000000")[1:]...)
                        return nil
                case 'y':
                        dst = t.AppendFormat(dst, "06")
                        return nil
                case 'Y':
                        dst = t.AppendFormat(dst, "2006")
                        return nil
                case 'C':
                        dst = t.AppendFormat(dst, "2006")
                        dst = dst[:len(dst)-2]
                        return nil
                case 'U':
                        dst = appendWeekNumber(dst, t, flag, true)
                        return nil
                case 'W':
                        dst = appendWeekNumber(dst, t, flag, false)
                        return nil
                case 'V':
                        _, w := t.ISOWeek()
                        dst = appendInt2(dst, w, flag)
                        return nil
                case 'g':
                        y, _ := t.ISOWeek()
                        dst = year(y).AppendFormat(dst, "06")
                        return nil
                case 'G':
                        y, _ := t.ISOWeek()
                        dst = year(y).AppendFormat(dst, "2006")
                        return nil
                case 's':
                        dst = strconv.AppendInt(dst, t.Unix(), 10)
                        return nil
                case 'Q':
                        dst = strconv.AppendInt(dst, t.UnixMilli(), 10)
                        return nil
                case 'w':
                        w := t.Weekday()
                        dst = appendInt1(dst, int(w))
                        return nil
                case 'u':
                        if w := t.Weekday(); w == 0 {
                                dst = append(dst, '7')
                        } else {
                                dst = appendInt1(dst, int(w))
                        }
                        return nil
                case 'j':
                        if flag == '-' {
                                dst = strconv.AppendInt(dst, int64(t.YearDay()), 10)
                        } else {
                                dst = t.AppendFormat(dst, "002")
                        }
                        return nil
                }

                if layout := goLayout(spec, flag, false); layout != "" {
                        dst = t.AppendFormat(dst, layout)
                        return nil
                }

                dst = append(dst, '%')
                if flag != 0 {
                        dst = append(dst, flag)
                }
                dst = append(dst, spec)
                return nil
        }

        parser.parse(fmt)
        return dst
}

// Parse converts a textual representation of time to the time value it represents
// according to the strptime format specification.
//
// The following specifiers are not supported for parsing:
//
//        %g %k %l %s %u %w %C %G %Q %U %V %W
//
// You must also avoid digits and these letter sequences
// in fmt literals:
//
//        Jan Mon MST PM pm
func Parse(fmt, value string) (time.Time, error) {
        pattern, err := layout(fmt, true)
        if err != nil {
                return time.Time{}, err
        }
        return time.Parse(pattern, value)
}

// Layout converts a strftime format specification
// to a Go time pattern specification.
//
// The following specifiers are not supported by Go patterns:
//
//        %f %g %k %l %s %u %w %C %G %L %N %Q %U %V %W
//
// You must also avoid digits and these letter sequences
// in fmt literals:
//
//        Jan Mon MST PM pm
func Layout(fmt string) (string, error) {
        return layout(fmt, false)
}

func layout(fmt string, parsing bool) (string, error) {
        dst := buffer(fmt)
        var parser parser

        parser.literal = func(b byte) error {
                if '0' <= b && b <= '9' {
                        return literalErr(b)
                }
                dst = append(dst, b)
                if b == 'M' || b == 'T' || b == 'm' || b == 'n' {
                        switch {
                        case bytes.HasSuffix(dst, []byte("Jan")):
                                return literalErr("Jan")
                        case bytes.HasSuffix(dst, []byte("Mon")):
                                return literalErr("Mon")
                        case bytes.HasSuffix(dst, []byte("MST")):
                                return literalErr("MST")
                        case bytes.HasSuffix(dst, []byte("PM")):
                                return literalErr("PM")
                        case bytes.HasSuffix(dst, []byte("pm")):
                                return literalErr("pm")
                        }
                }
                return nil
        }

        parser.format = func(spec, flag byte) error {
                if layout := goLayout(spec, flag, parsing); layout != "" {
                        dst = append(dst, layout...)
                        return nil
                }

                switch spec {
                default:
                        return formatError{}

                case 'L', 'f', 'N':
                        if bytes.HasSuffix(dst, []byte(".")) || bytes.HasSuffix(dst, []byte(",")) {
                                switch spec {
                                default:
                                        dst = append(dst, "000"...)
                                case 'f':
                                        dst = append(dst, "000000"...)
                                case 'N':
                                        dst = append(dst, "000000000"...)
                                }
                                return nil
                        }
                        return formatError{message: "must follow '.' or ','"}
                }
        }

        if err := parser.parse(fmt); err != nil {
                return "", err
        }
        return string(dst), nil
}

// UTS35 converts a strftime format specification
// to a Unicode Technical Standard #35 Date Format Pattern.
//
// The following specifiers are not supported by UTS35:
//
//        %e %k %l %u %w %C %P %U %W
func UTS35(fmt string) (string, error) {
        const quote = '\''
        var quoted bool
        dst := buffer(fmt)

        var parser parser

        parser.literal = func(b byte) error {
                if b == quote {
                        dst = append(dst, quote, quote)
                        return nil
                }
                if !quoted && ('a' <= b && b <= 'z' || 'A' <= b && b <= 'Z') {
                        dst = append(dst, quote)
                        quoted = true
                }
                dst = append(dst, b)
                return nil
        }

        parser.format = func(spec, flag byte) error {
                if quoted {
                        dst = append(dst, quote)
                        quoted = false
                }
                if pattern := uts35Pattern(spec, flag); pattern != "" {
                        dst = append(dst, pattern...)
                        return nil
                }
                return formatError{}
        }

        if err := parser.parse(fmt); err != nil {
                return "", err
        }
        if quoted {
                dst = append(dst, quote)
        }
        return string(dst), nil
}

func buffer(format string) (buf []byte) {
        const bufSize = 64
        max := len(format) + 10
        if max < bufSize {
                var b [bufSize]byte
                buf = b[:0]
        } else {
                buf = make([]byte, 0, max)
        }
        return
}

func year(y int) time.Time {
        return time.Date(y, time.January, 1, 0, 0, 0, 0, time.UTC)
}

func appendWeekNumber(dst []byte, t time.Time, flag byte, sunday bool) []byte {
        offset := int(t.Weekday())
        if sunday {
                offset = 6 - offset
        } else if offset != 0 {
                offset = 7 - offset
        }
        return appendInt2(dst, (t.YearDay()+offset)/7, flag)
}

func append12Hour(dst []byte, t time.Time, flag byte) []byte {
        h := t.Hour()
        if h == 0 {
                h = 12
        } else if h > 12 {
                h -= 12
        }
        return appendInt2(dst, h, flag)
}

func appendInt1(dst []byte, i int) []byte {
        return append(dst, byte('0'+i))
}

func appendInt2(dst []byte, i int, flag byte) []byte {
        if flag == 0 || i >= 10 {
                return append(dst, smallsString[i*2:i*2+2]...)
        }
        if flag == ' ' {
                dst = append(dst, flag)
        }
        return appendInt1(dst, i)
}

const smallsString = "" +
        "00010203040506070809" +
        "10111213141516171819" +
        "20212223242526272829" +
        "30313233343536373839" +
        "40414243444546474849" +
        "50515253545556575859" +
        "60616263646566676869" +
        "70717273747576777879" +
        "80818283848586878889" +
        "90919293949596979899"