// Copyright 2015 PingCAP, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.

package stringutil

import (
	"strings"
	"unicode/utf8"

	"github.com/juju/errors"
)

// ErrSyntax indicates that a value does not have the right syntax for the target type.
var ErrSyntax = errors.New("invalid syntax")

// Reverse returns its argument string reversed rune-wise left to right.
func Reverse(s string) string {
	r := []rune(s)
	for i, j := 0, len(r)-1; i < len(r)/2; i, j = i+1, j-1 {
		r[i], r[j] = r[j], r[i]
	}
	return string(r)
}

// 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.
// Different with strconv.UnquoteChar, it permits unnecessary backslash.
func UnquoteChar(s string, quote byte) (value []byte, tail string, err error) {
	// easy cases
	switch c := s[0]; {
	case c == quote:
		err = errors.Trace(ErrSyntax)
		return
	case c >= utf8.RuneSelf:
		r, size := utf8.DecodeRuneInString(s)
		if r == utf8.RuneError {
			value = append(value, c)
			return value, s[1:], nil
		}
		value = append(value, string(r)...)
		return value, s[size:], nil
	case c != '\\':
		value = append(value, c)
		return value, s[1:], nil
	}
	// hard case: c is backslash
	if len(s) <= 1 {
		err = errors.Trace(ErrSyntax)
		return
	}
	c := s[1]
	s = s[2:]
	switch c {
	case 'b':
		value = append(value, '\b')
	case 'n':
		value = append(value, '\n')
	case 'r':
		value = append(value, '\r')
	case 't':
		value = append(value, '\t')
	case 'Z':
		value = append(value, '\032')
	case '0':
		value = append(value, '\000')
	case '_', '%':
		value = append(value, '\\')
		value = append(value, c)
	case '\\':
		value = append(value, '\\')
	case '\'', '"':
		value = append(value, c)
	default:
		value = append(value, c)
	}
	tail = s
	return
}

// Unquote interprets s as a single-quoted, double-quoted,
// or backquoted Go string literal, returning the string value
// that s quotes. For example: test=`"\"\n"` (hex: 22 5c 22 5c 6e 22)
// should be converted to `"\n` (hex: 22 0a).
func Unquote(s string) (t string, err error) {
	n := len(s)
	if n < 2 {
		return "", errors.Trace(ErrSyntax)
	}
	quote := s[0]
	if quote != s[n-1] {
		return "", errors.Trace(ErrSyntax)
	}
	s = s[1 : n-1]
	if quote != '"' && quote != '\'' {
		return "", errors.Trace(ErrSyntax)
	}
	// Avoid allocation. No need to convert if there is no '\'
	if strings.IndexByte(s, '\\') == -1 && strings.IndexByte(s, quote) == -1 {
		return s, nil
	}
	buf := make([]byte, 0, 3*len(s)/2) // Try to avoid more allocations.
	for len(s) > 0 {
		mb, ss, err := UnquoteChar(s, quote)
		if err != nil {
			return "", errors.Trace(err)
		}
		s = ss
		buf = append(buf, mb...)
	}
	return string(buf), nil
}
