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297 lines
8.6 KiB
297 lines
8.6 KiB
// Copyright 2016 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package utf32 provides the UTF-32 Unicode encoding.
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//
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// Please note that support for UTF-32 is discouraged as it is a rare and
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// inefficient encoding, unfit for use as an interchange format. For use
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// on the web, the W3C strongly discourages its use
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// (https://www.w3.org/TR/html5/document-metadata.html#charset)
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// while WHATWG directly prohibits supporting it
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// (https://html.spec.whatwg.org/multipage/syntax.html#character-encodings).
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package utf32 // import "golang.org/x/text/encoding/unicode/utf32"
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import (
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"errors"
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"unicode/utf8"
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"golang.org/x/text/encoding"
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"golang.org/x/text/encoding/internal/identifier"
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"golang.org/x/text/transform"
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)
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// All lists a configuration for each IANA-defined UTF-32 variant.
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var All = []encoding.Encoding{
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UTF32(BigEndian, UseBOM),
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UTF32(BigEndian, IgnoreBOM),
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UTF32(LittleEndian, IgnoreBOM),
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}
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// ErrMissingBOM means that decoding UTF-32 input with ExpectBOM did not
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// find a starting byte order mark.
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var ErrMissingBOM = errors.New("encoding: missing byte order mark")
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// UTF32 returns a UTF-32 Encoding for the given default endianness and
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// byte order mark (BOM) policy.
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//
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// When decoding from UTF-32 to UTF-8, if the BOMPolicy is IgnoreBOM then
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// neither BOMs U+FEFF nor ill-formed code units 0xFFFE0000 in the input
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// stream will affect the endianness used for decoding. Instead BOMs will
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// be output as their standard UTF-8 encoding "\xef\xbb\xbf" while
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// 0xFFFE0000 code units will be output as "\xef\xbf\xbd", the standard
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// UTF-8 encoding for the Unicode replacement character. If the BOMPolicy
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// is UseBOM or ExpectBOM a starting BOM is not written to the UTF-8
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// output. Instead, it overrides the default endianness e for the remainder
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// of the transformation. Any subsequent BOMs U+FEFF or ill-formed code
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// units 0xFFFE0000 will not affect the endianness used, and will instead
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// be output as their standard UTF-8 (replacement) encodings. For UseBOM,
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// if there is no starting BOM, it will proceed with the default
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// Endianness. For ExpectBOM, in that case, the transformation will return
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// early with an ErrMissingBOM error.
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//
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// When encoding from UTF-8 to UTF-32, a BOM will be inserted at the start
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// of the output if the BOMPolicy is UseBOM or ExpectBOM. Otherwise, a BOM
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// will not be inserted. The UTF-8 input does not need to contain a BOM.
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//
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// There is no concept of a 'native' endianness. If the UTF-32 data is
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// produced and consumed in a greater context that implies a certain
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// endianness, use IgnoreBOM. Otherwise, use ExpectBOM and always produce
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// and consume a BOM.
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//
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// In the language of http://www.unicode.org/faq/utf_bom.html#bom10,
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// IgnoreBOM corresponds to "Where the precise type of the data stream is
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// known... the BOM should not be used" and ExpectBOM corresponds to "A
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// particular protocol... may require use of the BOM".
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func UTF32(e Endianness, b BOMPolicy) encoding.Encoding {
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return utf32Encoding{config{e, b}, mibValue[e][b&bomMask]}
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}
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// mibValue maps Endianness and BOMPolicy settings to MIB constants for UTF-32.
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// Note that some configurations map to the same MIB identifier.
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var mibValue = map[Endianness][numBOMValues]identifier.MIB{
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BigEndian: [numBOMValues]identifier.MIB{
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IgnoreBOM: identifier.UTF32BE,
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UseBOM: identifier.UTF32,
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},
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LittleEndian: [numBOMValues]identifier.MIB{
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IgnoreBOM: identifier.UTF32LE,
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UseBOM: identifier.UTF32,
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},
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// ExpectBOM is not widely used and has no valid MIB identifier.
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}
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// BOMPolicy is a UTF-32 encodings's byte order mark policy.
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type BOMPolicy uint8
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const (
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writeBOM BOMPolicy = 0x01
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acceptBOM BOMPolicy = 0x02
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requireBOM BOMPolicy = 0x04
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bomMask BOMPolicy = 0x07
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// HACK: numBOMValues == 8 triggers a bug in the 1.4 compiler (cannot have a
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// map of an array of length 8 of a type that is also used as a key or value
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// in another map). See golang.org/issue/11354.
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// TODO: consider changing this value back to 8 if the use of 1.4.* has
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// been minimized.
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numBOMValues = 8 + 1
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// IgnoreBOM means to ignore any byte order marks.
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IgnoreBOM BOMPolicy = 0
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// Unicode-compliant interpretation for UTF-32BE/LE.
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// UseBOM means that the UTF-32 form may start with a byte order mark,
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// which will be used to override the default encoding.
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UseBOM BOMPolicy = writeBOM | acceptBOM
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// Unicode-compliant interpretation for UTF-32.
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// ExpectBOM means that the UTF-32 form must start with a byte order mark,
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// which will be used to override the default encoding.
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ExpectBOM BOMPolicy = writeBOM | acceptBOM | requireBOM
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// Consistent with BOMPolicy definition in golang.org/x/text/encoding/unicode
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)
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// Endianness is a UTF-32 encoding's default endianness.
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type Endianness bool
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const (
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// BigEndian is UTF-32BE.
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BigEndian Endianness = false
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// LittleEndian is UTF-32LE.
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LittleEndian Endianness = true
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)
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type config struct {
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endianness Endianness
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bomPolicy BOMPolicy
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}
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type utf32Encoding struct {
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config
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mib identifier.MIB
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}
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func (u utf32Encoding) NewDecoder() *encoding.Decoder {
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return &encoding.Decoder{Transformer: &utf32Decoder{
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initial: u.config,
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current: u.config,
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}}
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}
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func (u utf32Encoding) NewEncoder() *encoding.Encoder {
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return &encoding.Encoder{Transformer: &utf32Encoder{
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endianness: u.endianness,
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initialBOMPolicy: u.bomPolicy,
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currentBOMPolicy: u.bomPolicy,
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}}
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}
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func (u utf32Encoding) ID() (mib identifier.MIB, other string) {
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return u.mib, ""
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}
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func (u utf32Encoding) String() string {
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e, b := "B", ""
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if u.endianness == LittleEndian {
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e = "L"
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}
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switch u.bomPolicy {
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case ExpectBOM:
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b = "Expect"
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case UseBOM:
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b = "Use"
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case IgnoreBOM:
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b = "Ignore"
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}
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return "UTF-32" + e + "E (" + b + " BOM)"
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}
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type utf32Decoder struct {
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initial config
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current config
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}
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func (u *utf32Decoder) Reset() {
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u.current = u.initial
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}
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func (u *utf32Decoder) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
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if len(src) == 0 {
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if atEOF && u.current.bomPolicy&requireBOM != 0 {
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return 0, 0, ErrMissingBOM
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}
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return 0, 0, nil
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}
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if u.current.bomPolicy&acceptBOM != 0 {
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if len(src) < 4 {
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return 0, 0, transform.ErrShortSrc
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}
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switch {
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case src[0] == 0x00 && src[1] == 0x00 && src[2] == 0xfe && src[3] == 0xff:
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u.current.endianness = BigEndian
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nSrc = 4
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case src[0] == 0xff && src[1] == 0xfe && src[2] == 0x00 && src[3] == 0x00:
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u.current.endianness = LittleEndian
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nSrc = 4
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default:
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if u.current.bomPolicy&requireBOM != 0 {
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return 0, 0, ErrMissingBOM
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}
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}
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u.current.bomPolicy = IgnoreBOM
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}
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var r rune
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var dSize, sSize int
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for nSrc < len(src) {
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if nSrc+3 < len(src) {
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x := uint32(src[nSrc+0])<<24 | uint32(src[nSrc+1])<<16 |
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uint32(src[nSrc+2])<<8 | uint32(src[nSrc+3])
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if u.current.endianness == LittleEndian {
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x = x>>24 | (x >> 8 & 0x0000FF00) | (x << 8 & 0x00FF0000) | x<<24
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}
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r, sSize = rune(x), 4
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if dSize = utf8.RuneLen(r); dSize < 0 {
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r, dSize = utf8.RuneError, 3
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}
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} else if atEOF {
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// 1..3 trailing bytes.
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r, dSize, sSize = utf8.RuneError, 3, len(src)-nSrc
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} else {
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err = transform.ErrShortSrc
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break
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}
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if nDst+dSize > len(dst) {
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err = transform.ErrShortDst
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break
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}
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nDst += utf8.EncodeRune(dst[nDst:], r)
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nSrc += sSize
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}
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return nDst, nSrc, err
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}
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type utf32Encoder struct {
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endianness Endianness
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initialBOMPolicy BOMPolicy
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currentBOMPolicy BOMPolicy
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}
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func (u *utf32Encoder) Reset() {
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u.currentBOMPolicy = u.initialBOMPolicy
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}
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func (u *utf32Encoder) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
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if u.currentBOMPolicy&writeBOM != 0 {
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if len(dst) < 4 {
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return 0, 0, transform.ErrShortDst
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}
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dst[0], dst[1], dst[2], dst[3] = 0x00, 0x00, 0xfe, 0xff
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u.currentBOMPolicy = IgnoreBOM
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nDst = 4
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}
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r, size := rune(0), 0
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for nSrc < len(src) {
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r = rune(src[nSrc])
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// Decode a 1-byte rune.
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if r < utf8.RuneSelf {
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size = 1
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} else {
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// Decode a multi-byte rune.
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r, size = utf8.DecodeRune(src[nSrc:])
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if size == 1 {
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// All valid runes of size 1 (those below utf8.RuneSelf) were
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// handled above. We have invalid UTF-8 or we haven't seen the
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// full character yet.
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if !atEOF && !utf8.FullRune(src[nSrc:]) {
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err = transform.ErrShortSrc
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break
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}
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}
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}
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if nDst+4 > len(dst) {
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err = transform.ErrShortDst
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break
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}
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dst[nDst+0] = uint8(r >> 24)
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dst[nDst+1] = uint8(r >> 16)
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dst[nDst+2] = uint8(r >> 8)
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dst[nDst+3] = uint8(r)
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nDst += 4
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nSrc += size
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}
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if u.endianness == LittleEndian {
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for i := 0; i < nDst; i += 4 {
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dst[i], dst[i+1], dst[i+2], dst[i+3] = dst[i+3], dst[i+2], dst[i+1], dst[i]
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}
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}
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return nDst, nSrc, err
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}
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