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Source file src/unicode/utf16/utf16.go

Documentation: unicode/utf16

  // Copyright 2010 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 utf16 implements encoding and decoding of UTF-16 sequences.
  package utf16
  
  // The conditions replacementChar==unicode.ReplacementChar and
  // maxRune==unicode.MaxRune are verified in the tests.
  // Defining them locally avoids this package depending on package unicode.
  
  const (
  	replacementChar = '\uFFFD'     // Unicode replacement character
  	maxRune         = '\U0010FFFF' // Maximum valid Unicode code point.
  )
  
  const (
  	// 0xd800-0xdc00 encodes the high 10 bits of a pair.
  	// 0xdc00-0xe000 encodes the low 10 bits of a pair.
  	// the value is those 20 bits plus 0x10000.
  	surr1 = 0xd800
  	surr2 = 0xdc00
  	surr3 = 0xe000
  
  	surrSelf = 0x10000
  )
  
  // IsSurrogate reports whether the specified Unicode code point
  // can appear in a surrogate pair.
  func IsSurrogate(r rune) bool {
  	return surr1 <= r && r < surr3
  }
  
  // DecodeRune returns the UTF-16 decoding of a surrogate pair.
  // If the pair is not a valid UTF-16 surrogate pair, DecodeRune returns
  // the Unicode replacement code point U+FFFD.
  func DecodeRune(r1, r2 rune) rune {
  	if surr1 <= r1 && r1 < surr2 && surr2 <= r2 && r2 < surr3 {
  		return (r1-surr1)<<10 | (r2 - surr2) + surrSelf
  	}
  	return replacementChar
  }
  
  // EncodeRune returns the UTF-16 surrogate pair r1, r2 for the given rune.
  // If the rune is not a valid Unicode code point or does not need encoding,
  // EncodeRune returns U+FFFD, U+FFFD.
  func EncodeRune(r rune) (r1, r2 rune) {
  	if r < surrSelf || r > maxRune {
  		return replacementChar, replacementChar
  	}
  	r -= surrSelf
  	return surr1 + (r>>10)&0x3ff, surr2 + r&0x3ff
  }
  
  // Encode returns the UTF-16 encoding of the Unicode code point sequence s.
  func Encode(s []rune) []uint16 {
  	n := len(s)
  	for _, v := range s {
  		if v >= surrSelf {
  			n++
  		}
  	}
  
  	a := make([]uint16, n)
  	n = 0
  	for _, v := range s {
  		switch {
  		case 0 <= v && v < surr1, surr3 <= v && v < surrSelf:
  			// normal rune
  			a[n] = uint16(v)
  			n++
  		case surrSelf <= v && v <= maxRune:
  			// needs surrogate sequence
  			r1, r2 := EncodeRune(v)
  			a[n] = uint16(r1)
  			a[n+1] = uint16(r2)
  			n += 2
  		default:
  			a[n] = uint16(replacementChar)
  			n++
  		}
  	}
  	return a[:n]
  }
  
  // Decode returns the Unicode code point sequence represented
  // by the UTF-16 encoding s.
  func Decode(s []uint16) []rune {
  	a := make([]rune, len(s))
  	n := 0
  	for i := 0; i < len(s); i++ {
  		switch r := s[i]; {
  		case r < surr1, surr3 <= r:
  			// normal rune
  			a[n] = rune(r)
  		case surr1 <= r && r < surr2 && i+1 < len(s) &&
  			surr2 <= s[i+1] && s[i+1] < surr3:
  			// valid surrogate sequence
  			a[n] = DecodeRune(rune(r), rune(s[i+1]))
  			i++
  		default:
  			// invalid surrogate sequence
  			a[n] = replacementChar
  		}
  		n++
  	}
  	return a[:n]
  }
  

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