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go-mtree/pkg/govis/unvis.go

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/*
* govis: unicode aware vis(3) encoding implementation
* Copyright (C) 2017 SUSE LLC.
*
* 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,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package govis
import (
"fmt"
"strconv"
"unicode"
)
// unvisParser stores the current state of the token parser.
type unvisParser struct {
tokens []rune
idx int
flag VisFlag
}
// Next moves the index to the next character.
func (p *unvisParser) Next() {
p.idx++
}
// Peek gets the current token.
func (p *unvisParser) Peek() (rune, error) {
if p.idx >= len(p.tokens) {
return unicode.ReplacementChar, fmt.Errorf("tried to read past end of token list")
}
return p.tokens[p.idx], nil
}
// End returns whether all of the tokens have been consumed.
func (p *unvisParser) End() bool {
return p.idx >= len(p.tokens)
}
func newParser(input string, flag VisFlag) *unvisParser {
return &unvisParser{
tokens: []rune(input),
idx: 0,
flag: flag,
}
}
// While a recursive descent parser is overkill for parsing simple escape
// codes, this is IMO much easier to read than the ugly 80s coroutine code used
// by the original unvis(3) parser. Here's the EBNF for an unvis sequence:
//
// <input> ::= (<rune>)*
// <rune> ::= ("\" <escape-sequence>) | ("%" <escape-hex>) | <plain-rune>
// <plain-rune> ::= any rune
// <escape-sequence> ::= ("x" <escape-hex>) | ("M" <escape-meta>) | ("^" <escape-ctrl) | <escape-cstyle> | <escape-octal>
// <escape-meta> ::= ("-" <escape-meta1>) | ("^" <escape-ctrl>)
// <escape-meta1> ::= any rune
// <escape-ctrl> ::= "?" | any rune
// <escape-cstyle> ::= "\" | "n" | "r" | "b" | "a" | "v" | "t" | "f"
// <escape-hex> ::= [0-9a-f] [0-9a-f]
// <escape-octal> ::= [0-7] ([0-7] ([0-7])?)?
func unvisPlainRune(p *unvisParser) ([]byte, error) {
ch, err := p.Peek()
if err != nil {
return nil, fmt.Errorf("plain rune: %c", ch)
}
p.Next()
// XXX: Maybe we should not be converting to runes and then back to strings
// here. Are we sure that the byte-for-byte representation is the
// same? If the bytes change, then using these strings for paths will
// break...
str := string(ch)
return []byte(str), nil
}
func unvisEscapeCStyle(p *unvisParser) ([]byte, error) {
ch, err := p.Peek()
if err != nil {
return nil, fmt.Errorf("escape hex: %s", err)
}
output := ""
switch ch {
case 'n':
output = "\n"
case 'r':
output = "\r"
case 'b':
output = "\b"
case 'a':
output = "\x07"
case 'v':
output = "\v"
case 't':
output = "\t"
case 'f':
output = "\f"
case 's':
output = " "
case 'E':
output = "\x1b"
case '\n':
// Hidden newline.
case '$':
// Hidden marker.
default:
// XXX: We should probably allow falling through and return "\" here...
return nil, fmt.Errorf("escape cstyle: unknown escape character: %q", ch)
}
p.Next()
return []byte(output), nil
}
func unvisEscapeDigits(p *unvisParser, base int, force bool) ([]byte, error) {
var code int
for i := int(0xFF); i > 0; i /= base {
ch, err := p.Peek()
if err != nil {
if !force && i != 0xFF {
break
}
return nil, fmt.Errorf("escape base %d: %s", base, err)
}
digit, err := strconv.ParseInt(string(ch), base, 8)
if err != nil {
if !force && i != 0xFF {
break
}
return nil, fmt.Errorf("escape base %d: could not parse digit: %s", base, err)
}
code = (code * base) + int(digit)
p.Next()
}
if code > unicode.MaxLatin1 {
return nil, fmt.Errorf("escape base %d: code %q outside latin-1 encoding", base, code)
}
char := byte(code & 0xFF)
return []byte{char}, nil
}
func unvisEscapeCtrl(p *unvisParser, mask byte) ([]byte, error) {
ch, err := p.Peek()
if err != nil {
return nil, fmt.Errorf("escape ctrl: %s", err)
}
if ch > unicode.MaxLatin1 {
return nil, fmt.Errorf("escape ctrl: code %q outside latin-1 encoding", ch)
}
char := byte(ch) & 0x1f
if ch == '?' {
char = 0x7f
}
p.Next()
return []byte{mask | char}, nil
}
func unvisEscapeMeta(p *unvisParser) ([]byte, error) {
ch, err := p.Peek()
if err != nil {
return nil, fmt.Errorf("escape meta: %s", err)
}
mask := byte(0x80)
switch ch {
case '^':
// The same as "\^..." except we apply a mask.
p.Next()
return unvisEscapeCtrl(p, mask)
case '-':
p.Next()
ch, err := p.Peek()
if err != nil {
return nil, fmt.Errorf("escape meta1: %s", err)
}
if ch > unicode.MaxLatin1 {
return nil, fmt.Errorf("escape meta1: code %q outside latin-1 encoding", ch)
}
// Add mask to character.
p.Next()
return []byte{mask | byte(ch)}, nil
}
return nil, fmt.Errorf("escape meta: unknown escape char: %s", err)
}
func unvisEscapeSequence(p *unvisParser) ([]byte, error) {
ch, err := p.Peek()
if err != nil {
return nil, fmt.Errorf("escape sequence: %s", err)
}
switch ch {
case '\\':
p.Next()
return []byte("\\"), nil
case '0', '1', '2', '3', '4', '5', '6', '7':
return unvisEscapeDigits(p, 8, false)
case 'x':
p.Next()
return unvisEscapeDigits(p, 16, true)
case '^':
p.Next()
return unvisEscapeCtrl(p, 0x00)
case 'M':
p.Next()
return unvisEscapeMeta(p)
default:
return unvisEscapeCStyle(p)
}
}
func unvisRune(p *unvisParser) ([]byte, error) {
ch, err := p.Peek()
if err != nil {
return nil, fmt.Errorf("rune: %s", err)
}
switch ch {
case '\\':
p.Next()
return unvisEscapeSequence(p)
case '%':
// % HEX HEX only applies to HTTPStyle encodings.
if p.flag&VisHTTPStyle == VisHTTPStyle {
p.Next()
return unvisEscapeDigits(p, 16, true)
}
fallthrough
default:
return unvisPlainRune(p)
}
}
func unvis(p *unvisParser) (string, error) {
var output []byte
for !p.End() {
ch, err := unvisRune(p)
if err != nil {
return "", fmt.Errorf("input: %s", err)
}
output = append(output, ch...)
}
return string(output), nil
}
// Unvis takes a string formatted with the given Vis flags (though only the
// VisHTTPStyle flag is checked) and output the un-encoded version of the
// encoded string. An error is returned if any escape sequences in the input
// string were invalid.
func Unvis(input string, flag VisFlag) (string, error) {
// TODO: Check all of the VisFlag bits.
p := newParser(input, flag)
output, err := unvis(p)
if err != nil {
return "", fmt.Errorf("unvis: %s", err)
}
if !p.End() {
return "", fmt.Errorf("unvis: trailing characters at end of input")
}
return output, nil
}