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
5package runtime
6
7// Numbers fundamental to the encoding.
8const (
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.
15const (
16	surrogateMin = 0xD800
17	surrogateMax = 0xDFFF
18)
19
20const (
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.
43func 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.
60func 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.
104func 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