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// Copyright 2009 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 strconv

import (
        "bytes"
        "strings"
        "unicode"
        "unicode/utf8"
)

const lowerhex = "0123456789abcdef"

func quoteWith(s string, quote byte, ASCIIonly bool) string {
        var buf bytes.Buffer
        buf.WriteByte(quote)
        for width := 0; len(s) > 0; s = s[width:] {
                r := rune(s[0])
                width = 1
                if r >= utf8.RuneSelf {
                        r, width = utf8.DecodeRuneInString(s)
                }
                if width == 1 && r == utf8.RuneError {
                        buf.WriteString(`\x`)
                        buf.WriteByte(lowerhex[s[0]>>4])
                        buf.WriteByte(lowerhex[s[0]&0xF])
                        continue
                }
                if r == rune(quote) || r == '\\' { // always backslashed
                        buf.WriteByte('\\')
                        buf.WriteByte(byte(r))
                        continue
                }
                if ASCIIonly {
                        if r <= unicode.MaxASCII && unicode.IsPrint(r) {
                                buf.WriteRune(r)
                                continue
                        }
                } else if unicode.IsPrint(r) {
                        buf.WriteRune(r)
                        continue
                }
                switch r {
                case '\a':
                        buf.WriteString(`\a`)
                case '\b':
                        buf.WriteString(`\b`)
                case '\f':
                        buf.WriteString(`\f`)
                case '\n':
                        buf.WriteString(`\n`)
                case '\r':
                        buf.WriteString(`\r`)
                case '\t':
                        buf.WriteString(`\t`)
                case '\v':
                        buf.WriteString(`\v`)
                default:
                        switch {
                        case r < ' ':
                                buf.WriteString(`\x`)
                                buf.WriteByte(lowerhex[s[0]>>4])
                                buf.WriteByte(lowerhex[s[0]&0xF])
                        case r > unicode.MaxRune:
                                r = 0xFFFD
                                fallthrough
                        case r < 0x10000:
                                buf.WriteString(`\u`)
                                for s := 12; s >= 0; s -= 4 {
                                        buf.WriteByte(lowerhex[r>>uint(s)&0xF])
                                }
                        default:
                                buf.WriteString(`\U`)
                                for s := 28; s >= 0; s -= 4 {
                                        buf.WriteByte(lowerhex[r>>uint(s)&0xF])
                                }
                        }
                }
        }
        buf.WriteByte(quote)
        return buf.String()

}

// Quote returns a double-quoted Go string literal representing s.  The
// returned string uses Go escape sequences (\t, \n, \xFF, \u0100) for
// control characters and non-printable characters as defined by
// unicode.IsPrint.
func Quote(s string) string {
        return quoteWith(s, '"', false)
}

// AppendQuote appends a double-quoted Go string literal representing s,
// as generated by Quote, to dst and returns the extended buffer.
func AppendQuote(dst []byte, s string) []byte {
        return append(dst, Quote(s)...)
}

// QuoteToASCII returns a double-quoted Go string literal representing s.
// The returned string uses Go escape sequences (\t, \n, \xFF, \u0100) for
// non-ASCII characters and non-printable characters as defined by
// unicode.IsPrint.
func QuoteToASCII(s string) string {
        return quoteWith(s, '"', true)
}

// AppendQuoteToASCII appends a double-quoted Go string literal representing s,
// as generated by QuoteToASCII, to dst and returns the extended buffer.
func AppendQuoteToASCII(dst []byte, s string) []byte {
        return append(dst, QuoteToASCII(s)...)
}

// QuoteRune returns a single-quoted Go character literal representing the
// rune.  The returned string uses Go escape sequences (\t, \n, \xFF, \u0100)
// for control characters and non-printable characters as defined by
// unicode.IsPrint.
func QuoteRune(r rune) string {
        // TODO: avoid the allocation here.
        return quoteWith(string(r), '\'', false)
}

// AppendQuoteRune appends a single-quoted Go character literal representing the rune,
// as generated by QuoteRune, to dst and returns the extended buffer.
func AppendQuoteRune(dst []byte, r rune) []byte {
        return append(dst, QuoteRune(r)...)
}

// QuoteRuneToASCII returns a single-quoted Go character literal representing
// the rune.  The returned string uses Go escape sequences (\t, \n, \xFF,
// \u0100) for non-ASCII characters and non-printable characters as defined
// by unicode.IsPrint.
func QuoteRuneToASCII(r rune) string {
        // TODO: avoid the allocation here.
        return quoteWith(string(r), '\'', true)
}

// AppendQuoteRune appends a single-quoted Go character literal representing the rune,
// as generated by QuoteRuneToASCII, to dst and returns the extended buffer.
func AppendQuoteRuneToASCII(dst []byte, r rune) []byte {
        return append(dst, QuoteRuneToASCII(r)...)
}

// CanBackquote returns whether the string s would be
// a valid Go string literal if enclosed in backquotes.
func CanBackquote(s string) bool {
        for i := 0; i < len(s); i++ {
                if (s[i] < ' ' && s[i] != '\t') || s[i] == '`' {
                        return false
                }
        }
        return true
}

func unhex(b byte) (v rune, ok bool) {
        c := rune(b)
        switch {
        case '0' <= c && c <= '9':
                return c - '0', true
        case 'a' <= c && c <= 'f':
                return c - 'a' + 10, true
        case 'A' <= c && c <= 'F':
                return c - 'A' + 10, true
        }
        return
}

// UnquoteChar decodes the first character or byte in the escaped string
// or character literal represented by the string s.
// It returns four values:
//
//      1) value, the decoded Unicode code point or byte value;
//      2) multibyte, a boolean indicating whether the decoded character requires a multibyte UTF-8 representation;
//      3) tail, the remainder of the string after the character; and
//      4) an error that will be nil if the character is syntactically valid.
//
// The second argument, quote, specifies the type of literal being parsed
// and therefore which escaped quote character is permitted.
// If set to a single quote, it permits the sequence \' and disallows unescaped '.
// If set to a double quote, it permits \" and disallows unescaped ".
// If set to zero, it does not permit either escape and allows both quote characters to appear unescaped.
func UnquoteChar(s string, quote byte) (value rune, multibyte bool, tail string, err error) {
        // easy cases
        switch c := s[0]; {
        case c == quote && (quote == '\'' || quote == '"'):
                err = ErrSyntax
                return
        case c >= utf8.RuneSelf:
                r, size := utf8.DecodeRuneInString(s)
                return r, true, s[size:], nil
        case c != '\\':
                return rune(s[0]), false, s[1:], nil
        }

        // hard case: c is backslash
        if len(s) <= 1 {
                err = ErrSyntax
                return
        }
        c := s[1]
        s = s[2:]

        switch c {
        case 'a':
                value = '\a'
        case 'b':
                value = '\b'
        case 'f':
                value = '\f'
        case 'n':
                value = '\n'
        case 'r':
                value = '\r'
        case 't':
                value = '\t'
        case 'v':
                value = '\v'
        case 'x', 'u', 'U':
                n := 0
                switch c {
                case 'x':
                        n = 2
                case 'u':
                        n = 4
                case 'U':
                        n = 8
                }
                var v rune
                if len(s) < n {
                        err = ErrSyntax
                        return
                }
                for j := 0; j < n; j++ {
                        x, ok := unhex(s[j])
                        if !ok {
                                err = ErrSyntax
                                return
                        }
                        v = v<<4 | x
                }
                s = s[n:]
                if c == 'x' {
                        // single-byte string, possibly not UTF-8
                        value = v
                        break
                }
                if v > unicode.MaxRune {
                        err = ErrSyntax
                        return
                }
                value = v
                multibyte = true
        case '0', '1', '2', '3', '4', '5', '6', '7':
                v := rune(c) - '0'
                if len(s) < 2 {
                        err = ErrSyntax
                        return
                }
                for j := 0; j < 2; j++ { // one digit already; two more
                        x := rune(s[j]) - '0'
                        if x < 0 || x > 7 {
                                err = ErrSyntax
                                return
                        }
                        v = (v << 3) | x
                }
                s = s[2:]
                if v > 255 {
                        err = ErrSyntax
                        return
                }
                value = v
        case '\\':
                value = '\\'
        case '\'', '"':
                if c != quote {
                        err = ErrSyntax
                        return
                }
                value = rune(c)
        default:
                err = ErrSyntax
                return
        }
        tail = s
        return
}

// Unquote interprets s as a single-quoted, double-quoted,
// or backquoted Go string literal, returning the string value
// that s quotes.  (If s is single-quoted, it would be a Go
// character literal; Unquote returns the corresponding
// one-character string.)
func Unquote(s string) (t string, err error) {
        n := len(s)
        if n < 2 {
                return "", ErrSyntax
        }
        quote := s[0]
        if quote != s[n-1] {
                return "", ErrSyntax
        }
        s = s[1 : n-1]

        if quote == '`' {
                if strings.Contains(s, "`") {
                        return "", ErrSyntax
                }
                return s, nil
        }
        if quote != '"' && quote != '\'' {
                return "", ErrSyntax
        }
        if strings.Index(s, "\n") >= 0 {
                return "", ErrSyntax
        }

        // Is it trivial?  Avoid allocation.
        if strings.Index(s, `\`) < 0 && strings.IndexRune(s, rune(quote)) < 0 {
                switch quote {
                case '"':
                        return s, nil
                case '\'':
                        r, size := utf8.DecodeRuneInString(s)
                        if size == len(s) && (r != utf8.RuneError || size != 1) {
                                return s, nil
                        }
                }
        }

        var buf bytes.Buffer
        for len(s) > 0 {
                c, multibyte, ss, err := UnquoteChar(s, quote)
                if err != nil {
                        return "", err
                }
                s = ss
                if c < utf8.RuneSelf || !multibyte {
                        buf.WriteByte(byte(c))
                } else {
                        buf.WriteString(string(c))
                }
                if quote == '\'' && len(s) != 0 {
                        // single-quoted must be single character
                        return "", ErrSyntax
                }
        }
        return buf.String(), nil
}