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/* |
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Data structures for encoding transformations. |
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|
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Perl works internally in either a native 'byte' encoding or |
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in UTF-8 encoded Unicode. We have no immediate need for a "wchar_t" |
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representation. When we do we can use utf8_to_uv(). |
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|
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Most character encodings are either simple byte mappings or |
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variable length multi-byte encodings. UTF-8 can be viewed as a |
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rather extreme case of the latter. |
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|
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So to solve an important part of perl's encode needs we need to solve the |
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"multi-byte -> multi-byte" case. The simple byte forms are then just degenerate |
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case. (Where one of multi-bytes will usually be UTF-8.) |
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|
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The other type of encoding is a shift encoding where a prefix sequence |
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determines what subsequent bytes mean. Such encodings have state. |
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|
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We also need to handle case where a character in one encoding has to be |
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represented as multiple characters in the other. e.g. letter+diacritic. |
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|
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The process can be considered as pseudo perl: |
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|
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my $dst = ''; |
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while (length($src)) |
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{ |
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my $size = $count($src); |
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my $in_seq = substr($src,0,$size,''); |
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my $out_seq = $s2d_hash{$in_seq}; |
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if (defined $out_seq) |
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{ |
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$dst .= $out_seq; |
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} |
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else |
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{ |
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# an error condition |
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} |
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} |
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return $dst; |
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|
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That has the following components: |
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&src_count - a "rule" for how many bytes make up the next character in the |
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source. |
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%s2d_hash - a mapping from input sequences to output sequences |
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|
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The problem with that scheme is that it does not allow the output |
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character repertoire to affect the characters considered from the |
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input. |
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|
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So we use a "trie" representation which can also be considered |
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a state machine: |
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|
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my $dst = ''; |
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my $seq = \@s2d_seq; |
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my $next = \@s2d_next; |
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while (length($src)) |
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{ |
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my $byte = $substr($src,0,1,''); |
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my $out_seq = $seq->[$byte]; |
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if (defined $out_seq) |
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{ |
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$dst .= $out_seq; |
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} |
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else |
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{ |
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# an error condition |
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} |
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($next,$seq) = @$next->[$byte] if $next; |
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} |
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return $dst; |
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|
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There is now a pair of data structures to represent everything. |
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It is valid for output sequence at a particular point to |
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be defined but zero length, that just means "don't know yet". |
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For the single byte case there is no 'next' so new tables will be the same as |
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the original tables. For a multi-byte case a prefix byte will flip to the tables |
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for the next page (adding nothing to the output), then the tables for the page |
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will provide the actual output and set tables back to original base page. |
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|
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This scheme can also handle shift encodings. |
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|
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A slight enhancement to the scheme also allows for look-ahead - if |
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we add a flag to re-add the removed byte to the source we could handle |
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a" -> ä |
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ab -> a (and take b back please) |
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|
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*/ |
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|
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#include <EXTERN.h> |
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#include <perl.h> |
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#define U8 U8 |
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#include "encode.h" |
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|
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int |
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do_encode(encpage_t * enc, const U8 * src, STRLEN * slen, U8 * dst, |
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STRLEN dlen, STRLEN * dout, int approx, const U8 *term, STRLEN tlen) |
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{ |
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const U8 *s = src; |
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const U8 *send = s + *slen; |
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const U8 *last = s; |
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U8 *d = dst; |
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U8 *dend = d + dlen, *dlast = d; |
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int code = 0; |
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while (s < send) { |
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encpage_t *e = enc; |
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U8 byte = *s; |
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while (byte > e->max) |
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e++; |
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if (byte >= e->min && e->slen && (approx || !(e->slen & 0x80))) { |
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const U8 *cend = s + (e->slen & 0x7f); |
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if (cend <= send) { |
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STRLEN n; |
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if ((n = e->dlen)) { |
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const U8 *out = e->seq + n * (byte - e->min); |
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U8 *oend = d + n; |
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if (dst) { |
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if (oend <= dend) { |
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while (d < oend) |
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*d++ = *out++; |
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} |
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else { |
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/* Out of space */ |
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code = ENCODE_NOSPACE; |
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break; |
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} |
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} |
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else |
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d = oend; |
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} |
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enc = e->next; |
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s++; |
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if (s == cend) { |
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if (approx && (e->slen & 0x80)) |
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code = ENCODE_FALLBACK; |
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last = s; |
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if (term && (STRLEN)(d-dlast) == tlen && memEQ(dlast, term, tlen)) { |
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code = ENCODE_FOUND_TERM; |
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break; |
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} |
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dlast = d; |
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} |
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} |
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else { |
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/* partial source character */ |
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code = ENCODE_PARTIAL; |
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break; |
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} |
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} |
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else { |
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/* Cannot represent */ |
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code = ENCODE_NOREP; |
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break; |
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} |
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} |
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*slen = last - src; |
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*dout = d - dst; |
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return code; |
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} |
