utf8.go

Documentation: runtime

		 1  // Copyright 2016 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 runtime
		 6  
		 7  // Numbers fundamental to the encoding.
		 8  const (
		 9  	runeError = '\uFFFD'		 // the "error" Rune or "Unicode replacement character"
		10  	runeSelf	= 0x80				 // characters below runeSelf are represented as themselves in a single byte.
		11  	maxRune	 = '\U0010FFFF' // Maximum valid Unicode code point.
		12  )
		13  
		14  // Code points in the surrogate range are not valid for UTF-8.
		15  const (
		16  	surrogateMin = 0xD800
		17  	surrogateMax = 0xDFFF
		18  )
		19  
		20  const (
		21  	t1 = 0x00 // 0000 0000
		22  	tx = 0x80 // 1000 0000
		23  	t2 = 0xC0 // 1100 0000
		24  	t3 = 0xE0 // 1110 0000
		25  	t4 = 0xF0 // 1111 0000
		26  	t5 = 0xF8 // 1111 1000
		27  
		28  	maskx = 0x3F // 0011 1111
		29  	mask2 = 0x1F // 0001 1111
		30  	mask3 = 0x0F // 0000 1111
		31  	mask4 = 0x07 // 0000 0111
		32  
		33  	rune1Max = 1<<7 - 1
		34  	rune2Max = 1<<11 - 1
		35  	rune3Max = 1<<16 - 1
		36  
		37  	// The default lowest and highest continuation byte.
		38  	locb = 0x80 // 1000 0000
		39  	hicb = 0xBF // 1011 1111
		40  )
		41  
		42  // countrunes returns the number of runes in s.
		43  func countrunes(s string) int {
		44  	n := 0
		45  	for range s {
		46  		n++
		47  	}
		48  	return n
		49  }
		50  
		51  // decoderune returns the non-ASCII rune at the start of
		52  // s[k:] and the index after the rune in s.
		53  //
		54  // decoderune assumes that caller has checked that
		55  // the to be decoded rune is a non-ASCII rune.
		56  //
		57  // If the string appears to be incomplete or decoding problems
		58  // are encountered (runeerror, k + 1) is returned to ensure
		59  // progress when decoderune is used to iterate over a string.
		60  func decoderune(s string, k int) (r rune, pos int) {
		61  	pos = k
		62  
		63  	if k >= len(s) {
		64  		return runeError, k + 1
		65  	}
		66  
		67  	s = s[k:]
		68  
		69  	switch {
		70  	case t2 <= s[0] && s[0] < t3:
		71  		// 0080-07FF two byte sequence
		72  		if len(s) > 1 && (locb <= s[1] && s[1] <= hicb) {
		73  			r = rune(s[0]&mask2)<<6 | rune(s[1]&maskx)
		74  			pos += 2
		75  			if rune1Max < r {
		76  				return
		77  			}
		78  		}
		79  	case t3 <= s[0] && s[0] < t4:
		80  		// 0800-FFFF three byte sequence
		81  		if len(s) > 2 && (locb <= s[1] && s[1] <= hicb) && (locb <= s[2] && s[2] <= hicb) {
		82  			r = rune(s[0]&mask3)<<12 | rune(s[1]&maskx)<<6 | rune(s[2]&maskx)
		83  			pos += 3
		84  			if rune2Max < r && !(surrogateMin <= r && r <= surrogateMax) {
		85  				return
		86  			}
		87  		}
		88  	case t4 <= s[0] && s[0] < t5:
		89  		// 10000-1FFFFF four byte sequence
		90  		if len(s) > 3 && (locb <= s[1] && s[1] <= hicb) && (locb <= s[2] && s[2] <= hicb) && (locb <= s[3] && s[3] <= hicb) {
		91  			r = rune(s[0]&mask4)<<18 | rune(s[1]&maskx)<<12 | rune(s[2]&maskx)<<6 | rune(s[3]&maskx)
		92  			pos += 4
		93  			if rune3Max < r && r <= maxRune {
		94  				return
		95  			}
		96  		}
		97  	}
		98  
		99  	return runeError, k + 1
	 100  }
	 101  
	 102  // encoderune writes into p (which must be large enough) the UTF-8 encoding of the rune.
	 103  // It returns the number of bytes written.
	 104  func encoderune(p []byte, r rune) int {
	 105  	// Negative values are erroneous. Making it unsigned addresses the problem.
	 106  	switch i := uint32(r); {
	 107  	case i <= rune1Max:
	 108  		p[0] = byte(r)
	 109  		return 1
	 110  	case i <= rune2Max:
	 111  		_ = p[1] // eliminate bounds checks
	 112  		p[0] = t2 | byte(r>>6)
	 113  		p[1] = tx | byte(r)&maskx
	 114  		return 2
	 115  	case i > maxRune, surrogateMin <= i && i <= surrogateMax:
	 116  		r = runeError
	 117  		fallthrough
	 118  	case i <= rune3Max:
	 119  		_ = p[2] // eliminate bounds checks
	 120  		p[0] = t3 | byte(r>>12)
	 121  		p[1] = tx | byte(r>>6)&maskx
	 122  		p[2] = tx | byte(r)&maskx
	 123  		return 3
	 124  	default:
	 125  		_ = p[3] // eliminate bounds checks
	 126  		p[0] = t4 | byte(r>>18)
	 127  		p[1] = tx | byte(r>>12)&maskx
	 128  		p[2] = tx | byte(r>>6)&maskx
	 129  		p[3] = tx | byte(r)&maskx
	 130  		return 4
	 131  	}
	 132  }
	 133
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