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