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string_util.go
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string_util.go
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// 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
}