1 /*
2 Data structures for encoding transformations.
3
4 Perl works internally in either a native 'byte' encoding or
5 in UTF-8 encoded Unicode. We have no immediate need for a "wchar_t"
6 representation. When we do we can use utf8_to_uv().
7
8 Most character encodings are either simple byte mappings or
9 variable length multi-byte encodings. UTF-8 can be viewed as a
10 rather extreme case of the latter.
11
12 So to solve an important part of perl's encode needs we need to solve the
13 "multi-byte -> multi-byte" case. The simple byte forms are then just degenerate
14 case. (Where one of multi-bytes will usually be UTF-8.)
15
16 The other type of encoding is a shift encoding where a prefix sequence
17 determines what subsequent bytes mean. Such encodings have state.
18
19 We also need to handle case where a character in one encoding has to be
20 represented as multiple characters in the other. e.g. letter+diacritic.
21
22 The process can be considered as pseudo perl:
23
24 my $dst = '';
25 while (length($src))
26 {
27 my $size = src_count($src);
28 my $in_seq = substr($src,0,$size,'');
29 my $out_seq = $s2d_hash{$in_seq};
30 if (defined $out_seq)
31 {
32 $dst .= $out_seq;
33 }
34 else
35 {
36 # an error condition
37 }
38 }
39 return $dst;
40
41 That has the following components:
42 &src_count - a "rule" for how many bytes make up the next character in the
43 source.
44 %s2d_hash - a mapping from input sequences to output sequences
45
46 The problem with that scheme is that it does not allow the output
47 character repertoire to affect the characters considered from the
48 input.
49
50 So we use a "trie" representation which can also be considered
51 a state machine:
52
53 my $dst = '';
54 my $seq = \@s2d_seq;
55 my $next = \@s2d_next;
56 while (length($src))
57 {
58 my $byte = $substr($src,0,1,'');
59 my $out_seq = $seq->[$byte];
60 if (defined $out_seq)
61 {
62 $dst .= $out_seq;
63 }
64 else
65 {
66 # an error condition
67 }
68 ($next,$seq) = @$next->[$byte] if $next;
69 }
70 return $dst;
71
72 There is now a pair of data structures to represent everything.
73 It is valid for output sequence at a particular point to
74 be defined but zero length, that just means "don't know yet".
75 For the single byte case there is no 'next' so new tables will be the same as
76 the original tables. For a multi-byte case a prefix byte will flip to the tables
77 for the next page (adding nothing to the output), then the tables for the page
78 will provide the actual output and set tables back to original base page.
79
80 This scheme can also handle shift encodings.
81
82 A slight enhancement to the scheme also allows for look-ahead - if
83 we add a flag to re-add the removed byte to the source we could handle
84 a" -> U+00E4 (LATIN SMALL LETTER A WITH DIAERESIS)
85 ab -> a (and take b back please)
86
87 */
88
89 #define PERL_NO_GET_CONTEXT
90 #include <EXTERN.h>
91 #include <perl.h>
92 #include "encode.h"
93
94 int
do_encode(const encpage_t * enc,const U8 * src,STRLEN * slen,U8 * dst,STRLEN dlen,STRLEN * dout,int approx,const U8 * term,STRLEN tlen)95 do_encode(const encpage_t * enc, const U8 * src, STRLEN * slen, U8 * dst,
96 STRLEN dlen, STRLEN * dout, int approx, const U8 *term, STRLEN tlen)
97 {
98 const U8 *s = src;
99 const U8 *send = s + *slen;
100 const U8 *last = s;
101 U8 *d = dst;
102 U8 *dend = d + dlen, *dlast = d;
103 int code = 0;
104 if (!dst)
105 return ENCODE_NOSPACE;
106 while (s < send) {
107 const encpage_t *e = enc;
108 U8 byte = *s;
109 while (byte > e->max)
110 e++;
111 if (byte >= e->min && e->slen && (approx || !(e->slen & 0x80))) {
112 const U8 *cend = s + (e->slen & 0x7f);
113 if (cend <= send) {
114 STRLEN n;
115 if ((n = e->dlen)) {
116 const U8 *out = e->seq + n * (byte - e->min);
117 U8 *oend = d + n;
118 if (dst) {
119 if (oend <= dend) {
120 while (d < oend)
121 *d++ = *out++;
122 }
123 else {
124 /* Out of space */
125 code = ENCODE_NOSPACE;
126 break;
127 }
128 }
129 else
130 d = oend;
131 }
132 enc = e->next;
133 s++;
134 if (s == cend) {
135 if (approx && (e->slen & 0x80))
136 code = ENCODE_FALLBACK;
137 last = s;
138 if (term && (STRLEN)(d-dlast) == tlen && memEQ(dlast, term, tlen)) {
139 code = ENCODE_FOUND_TERM;
140 break;
141 }
142 dlast = d;
143 }
144 }
145 else {
146 /* partial source character */
147 code = ENCODE_PARTIAL;
148 break;
149 }
150 }
151 else {
152 /* Cannot represent */
153 code = ENCODE_NOREP;
154 break;
155 }
156 }
157 *slen = last - src;
158 *dout = d - dst;
159 return code;
160 }
161