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// Copyright 2016 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 runtime
import _ "unsafe" // For go:linkname.
// For gccgo, use go:linkname to export compiler-called functions.
//
//go:linkname decoderune
// Numbers fundamental to the encoding.
const (
runeError = '\uFFFD' // the "error" Rune or "Unicode replacement character"
runeSelf = 0x80 // characters below runeSelf are represented as themselves in a single byte.
maxRune = '\U0010FFFF' // Maximum valid Unicode code point.
)
// Code points in the surrogate range are not valid for UTF-8.
const (
surrogateMin = 0xD800
surrogateMax = 0xDFFF
)
const (
t1 = 0x00 // 0000 0000
tx = 0x80 // 1000 0000
t2 = 0xC0 // 1100 0000
t3 = 0xE0 // 1110 0000
t4 = 0xF0 // 1111 0000
t5 = 0xF8 // 1111 1000
maskx = 0x3F // 0011 1111
mask2 = 0x1F // 0001 1111
mask3 = 0x0F // 0000 1111
mask4 = 0x07 // 0000 0111
rune1Max = 1<<7 - 1
rune2Max = 1<<11 - 1
rune3Max = 1<<16 - 1
// The default lowest and highest continuation byte.
locb = 0x80 // 1000 0000
hicb = 0xBF // 1011 1111
)
// countrunes returns the number of runes in s.
func countrunes(s string) int {
n := 0
for range s {
n++
}
return n
}
// decoderune returns the non-ASCII rune at the start of
// s[k:] and the index after the rune in s.
//
// decoderune assumes that caller has checked that
// the to be decoded rune is a non-ASCII rune.
//
// If the string appears to be incomplete or decoding problems
// are encountered (runeerror, k + 1) is returned to ensure
// progress when decoderune is used to iterate over a string.
func decoderune(s string, k int) (r rune, pos int) {
pos = k
if k >= len(s) {
return runeError, k + 1
}
s = s[k:]
switch {
case t2 <= s[0] && s[0] < t3:
// 0080-07FF two byte sequence
if len(s) > 1 && (locb <= s[1] && s[1] <= hicb) {
r = rune(s[0]&mask2)<<6 | rune(s[1]&maskx)
pos += 2
if rune1Max < r {
return
}
}
case t3 <= s[0] && s[0] < t4:
// 0800-FFFF three byte sequence
if len(s) > 2 && (locb <= s[1] && s[1] <= hicb) && (locb <= s[2] && s[2] <= hicb) {
r = rune(s[0]&mask3)<<12 | rune(s[1]&maskx)<<6 | rune(s[2]&maskx)
pos += 3
if rune2Max < r && !(surrogateMin <= r && r <= surrogateMax) {
return
}
}
case t4 <= s[0] && s[0] < t5:
// 10000-1FFFFF four byte sequence
if len(s) > 3 && (locb <= s[1] && s[1] <= hicb) && (locb <= s[2] && s[2] <= hicb) && (locb <= s[3] && s[3] <= hicb) {
r = rune(s[0]&mask4)<<18 | rune(s[1]&maskx)<<12 | rune(s[2]&maskx)<<6 | rune(s[3]&maskx)
pos += 4
if rune3Max < r && r <= maxRune {
return
}
}
}
return runeError, k + 1
}
// encoderune writes into p (which must be large enough) the UTF-8 encoding of the rune.
// It returns the number of bytes written.
func encoderune(p []byte, r rune) int {
// Negative values are erroneous. Making it unsigned addresses the problem.
switch i := uint32(r); {
case i <= rune1Max:
p[0] = byte(r)
return 1
case i <= rune2Max:
_ = p[1] // eliminate bounds checks
p[0] = t2 | byte(r>>6)
p[1] = tx | byte(r)&maskx
return 2
case i > maxRune, surrogateMin <= i && i <= surrogateMax:
r = runeError
fallthrough
case i <= rune3Max:
_ = p[2] // eliminate bounds checks
p[0] = t3 | byte(r>>12)
p[1] = tx | byte(r>>6)&maskx
p[2] = tx | byte(r)&maskx
return 3
default:
_ = p[3] // eliminate bounds checks
p[0] = t4 | byte(r>>18)
p[1] = tx | byte(r>>12)&maskx
p[2] = tx | byte(r>>6)&maskx
p[3] = tx | byte(r)&maskx
return 4
}
}
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