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[perl5.git] / ext / Encode / encengine.c
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017e2add
NIS
1/*
2Data structures for encoding transformations.
3
4Perl works internally in either a native 'byte' encoding or
5in UTF-8 encoded Unicode. We have no immediate need for a "wchar_t"
6representation. When we do we can use utf8_to_uv().
7
8Most character encodings are either simple byte mappings or
9variable length multi-byte encodings. UTF-8 can be viewed as a
10rather extreme case of the latter.
11
12So 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
14case. (Where one of multi-bytes will usually be UTF-8.)
15
16The other type of encoding is a shift encoding where a prefix sequence
17determines what subsequent bytes mean. Such encodings have state.
18
19We also need to handle case where a character in one encoding has to be
20represented as multiple characters in the other. e.g. letter+diacritic.
21
22The process can be considered as pseudo perl:
23
24my $dst = '';
25while (length($src))
26 {
27 my $size = $count($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 }
39return $dst;
40
41That 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
46The problem with that scheme is that it does not allow the output
47character repertoire to affect the characters considered from the
48input.
49
50So we use a "trie" representation which can also be considered
51a state machine:
52
53my $dst = '';
54my $seq = \@s2d_seq;
55my $next = \@s2d_next;
56while (length($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 }
70return $dst;
71
72There is now a pair of data structures to represent everything.
73It is valid for output sequence at a particular point to
74be defined but zero length, that just means "don't know yet".
75For the single byte case there is no 'next' so new tables will be the same as
76the original tables. For a multi-byte case a prefix byte will flip to the tables
77for the next page (adding nothing to the output), then the tables for the page
78will provide the actual output and set tables back to original base page.
79
80This scheme can also handle shift encodings.
81
82A slight enhancement to the scheme also allows for look-ahead - if
83we add a flag to re-add the removed byte to the source we could handle
84 a" -> รค
85 ab -> a (and take b back please)
86
87*/
88
89#include <EXTERN.h>
90#include <perl.h>
91#define U8 U8
92#include "encode.h"
93
2f2b4ff2 94int
dd88d393
JH
95do_encode(encpage_t * enc, const U8 * src, STRLEN * slen, U8 * dst,
96 STRLEN dlen, STRLEN * dout, int approx)
017e2add 97{
dd88d393
JH
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;
103 int code = 0;
104 while (s < send) {
105 encpage_t *e = enc;
106 U8 byte = *s;
107 while (byte > e->max)
108 e++;
109 if (byte >= e->min && e->slen && (approx || !(e->slen & 0x80))) {
110 const U8 *cend = s + (e->slen & 0x7f);
111 if (cend <= send) {
112 STRLEN n;
113 if ((n = e->dlen)) {
114 const U8 *out = e->seq + n * (byte - e->min);
115 U8 *oend = d + n;
116 if (dst) {
117 if (oend <= dend) {
118 while (d < oend)
119 *d++ = *out++;
120 }
121 else {
122 /* Out of space */
123 code = ENCODE_NOSPACE;
124 break;
125 }
126 }
127 else
128 d = oend;
129 }
130 enc = e->next;
131 s++;
132 if (s == cend) {
133 if (approx && (e->slen & 0x80))
134 code = ENCODE_FALLBACK;
135 last = s;
136 }
137 }
138 else {
139 /* partial source character */
140 code = ENCODE_PARTIAL;
141 break;
142 }
143 }
144 else {
145 /* Cannot represent */
146 code = ENCODE_NOREP;
147 break;
148 }
017e2add 149 }
dd88d393
JH
150 *slen = last - src;
151 *dout = d - dst;
152 return code;
017e2add 153}