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// Copyright 2015 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 unicode
import (
"golang.org/x/text/transform"
)
// BOMOverride returns a new decoder transformer that is identical to fallback,
// except that the presence of a Byte Order Mark at the start of the input
// causes it to switch to the corresponding Unicode decoding. It will only
// consider BOMs for UTF-8, UTF-16BE, and UTF-16LE.
//
// This differs from using ExpectBOM by allowing a BOM to switch to UTF-8, not
// just UTF-16 variants, and allowing falling back to any encoding scheme.
//
// This technique is recommended by the W3C for use in HTML 5: "For
// compatibility with deployed content, the byte order mark (also known as BOM)
// is considered more authoritative than anything else."
// http://www.w3.org/TR/encoding/#specification-hooks
//
// Using BOMOverride is mostly intended for use cases where the first characters
// of a fallback encoding are known to not be a BOM, for example, for valid HTML
// and most encodings.
func BOMOverride(fallback transform.Transformer) transform.Transformer {
// TODO: possibly allow a variadic argument of unicode encodings to allow
// specifying details of which fallbacks are supported as well as
// specifying the details of the implementations. This would also allow for
// support for UTF-32, which should not be supported by default.
return &bomOverride{fallback: fallback}
}
type bomOverride struct {
fallback transform.Transformer
current transform.Transformer
}
func (d *bomOverride) Reset() {
d.current = nil
d.fallback.Reset()
}
var (
// TODO: we could use decode functions here, instead of allocating a new
// decoder on every NewDecoder as IgnoreBOM decoders can be stateless.
utf16le = UTF16(LittleEndian, IgnoreBOM)
utf16be = UTF16(BigEndian, IgnoreBOM)
)
const utf8BOM = "\ufeff"
func (d *bomOverride) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
if d.current != nil {
return d.current.Transform(dst, src, atEOF)
}
if len(src) < 3 && !atEOF {
return 0, 0, transform.ErrShortSrc
}
d.current = d.fallback
bomSize := 0
if len(src) >= 2 {
if src[0] == 0xFF && src[1] == 0xFE {
d.current = utf16le.NewDecoder()
bomSize = 2
} else if src[0] == 0xFE && src[1] == 0xFF {
d.current = utf16be.NewDecoder()
bomSize = 2
} else if len(src) >= 3 &&
src[0] == utf8BOM[0] &&
src[1] == utf8BOM[1] &&
src[2] == utf8BOM[2] {
d.current = transform.Nop
bomSize = 3
}
}
if bomSize < len(src) {
nDst, nSrc, err = d.current.Transform(dst, src[bomSize:], atEOF)
}
return nDst, nSrc + bomSize, err
}