Source file src/pkg/unicode/utf16/utf16.go

     1	// Copyright 2010 The Go Authors.  All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	// Package utf16 implements encoding and decoding of UTF-16 sequences.
     6	package utf16
     7	
     8	// The conditions replacementChar==unicode.ReplacementChar and
     9	// maxRune==unicode.MaxRune are verified in the tests.
    10	// Defining them locally avoids this package depending on package unicode.
    11	
    12	const (
    13		replacementChar = '\uFFFD'     // Unicode replacement character
    14		maxRune         = '\U0010FFFF' // Maximum valid Unicode code point.
    15	)
    16	
    17	const (
    18		// 0xd800-0xdc00 encodes the high 10 bits of a pair.
    19		// 0xdc00-0xe000 encodes the low 10 bits of a pair.
    20		// the value is those 20 bits plus 0x10000.
    21		surr1 = 0xd800
    22		surr2 = 0xdc00
    23		surr3 = 0xe000
    24	
    25		surrSelf = 0x10000
    26	)
    27	
    28	// IsSurrogate returns true if the specified Unicode code point
    29	// can appear in a surrogate pair.
    30	func IsSurrogate(r rune) bool {
    31		return surr1 <= r && r < surr3
    32	}
    33	
    34	// DecodeRune returns the UTF-16 decoding of a surrogate pair.
    35	// If the pair is not a valid UTF-16 surrogate pair, DecodeRune returns
    36	// the Unicode replacement code point U+FFFD.
    37	func DecodeRune(r1, r2 rune) rune {
    38		if surr1 <= r1 && r1 < surr2 && surr2 <= r2 && r2 < surr3 {
    39			return (rune(r1)-surr1)<<10 | (rune(r2) - surr2) + 0x10000
    40		}
    41		return replacementChar
    42	}
    43	
    44	// EncodeRune returns the UTF-16 surrogate pair r1, r2 for the given rune.
    45	// If the rune is not a valid Unicode code point or does not need encoding,
    46	// EncodeRune returns U+FFFD, U+FFFD.
    47	func EncodeRune(r rune) (r1, r2 rune) {
    48		if r < surrSelf || r > maxRune || IsSurrogate(r) {
    49			return replacementChar, replacementChar
    50		}
    51		r -= surrSelf
    52		return surr1 + (r>>10)&0x3ff, surr2 + r&0x3ff
    53	}
    54	
    55	// Encode returns the UTF-16 encoding of the Unicode code point sequence s.
    56	func Encode(s []rune) []uint16 {
    57		n := len(s)
    58		for _, v := range s {
    59			if v >= surrSelf {
    60				n++
    61			}
    62		}
    63	
    64		a := make([]uint16, n)
    65		n = 0
    66		for _, v := range s {
    67			switch {
    68			case v < 0, surr1 <= v && v < surr3, v > maxRune:
    69				v = replacementChar
    70				fallthrough
    71			case v < surrSelf:
    72				a[n] = uint16(v)
    73				n++
    74			default:
    75				r1, r2 := EncodeRune(v)
    76				a[n] = uint16(r1)
    77				a[n+1] = uint16(r2)
    78				n += 2
    79			}
    80		}
    81		return a[0:n]
    82	}
    83	
    84	// Decode returns the Unicode code point sequence represented
    85	// by the UTF-16 encoding s.
    86	func Decode(s []uint16) []rune {
    87		a := make([]rune, len(s))
    88		n := 0
    89		for i := 0; i < len(s); i++ {
    90			switch r := s[i]; {
    91			case surr1 <= r && r < surr2 && i+1 < len(s) &&
    92				surr2 <= s[i+1] && s[i+1] < surr3:
    93				// valid surrogate sequence
    94				a[n] = DecodeRune(rune(r), rune(s[i+1]))
    95				i++
    96				n++
    97			case surr1 <= r && r < surr3:
    98				// invalid surrogate sequence
    99				a[n] = replacementChar
   100				n++
   101			default:
   102				// normal rune
   103				a[n] = rune(r)
   104				n++
   105			}
   106		}
   107		return a[0:n]
   108	}