* Macros to implement UTF-EBCDIC as perl's internal encoding
* Taken from version 7.1 of Unicode Techical Report #16:
* http://www.unicode.org/unicode/reports/tr16
+ *
+ * To summarize, the way it works is:
+ * To convert an EBCDIC character to UTF-EBCDIC:
+ * 1) convert to Unicode. The table in this file that does this is for
+ * EBCDIC bytes is PL_e2a (with inverse PLa2e). The 'a' stands for
+ * ASCIIish, meaning latin1.
+ * 2) convert that to a utf8-like string called I8 with variant characters
+ * occupying multiple bytes. This step is similar to the utf8-creating
+ * step from Unicode, but the details are different. There is a chart
+ * about the bit patterns in a comment later in this file. But
+ * essentially here are the differences:
+ * UTF8 I8
+ * invariant byte starts with 0 starts with 0 or 100
+ * continuation byte starts with 10 starts with 101
+ * start byte same in both: if the code point requires N bytes,
+ * then the leading N bits are 1, followed by a 0. (No
+ * trailing 0 for the very largest possible allocation
+ * in I8, far beyond the current Unicode standard's
+ * max, as shown in the comment later in this file.)
+ * 3) Use the table published in tr16 to convert each byte from step 2 into
+ * final UTF-EBCDIC. The table in this file is PL_utf2e, and its invverse
+ * is PL_e2utf. They are constructed so that all EBCDIC invariants remain
+ * invariant, but no others do. For example, the ordinal value of 'A' is
+ * 193 in EBCDIC, and also is 193 in UTF-EBCDIC. Step 1) converts it to
+ * 65, Step 2 leaves it at 65, and Step 3 converts it back to 193. As an
+ * example of how a variant character works, take LATIN SMALL LETTER Y
+ * WITH DIAERESIS, which is typicially 0xDF in EBCDIC. Step 1 converts it
+ * to the Unicode value, 0xFF. Step 2 converts that to two bytes =
+ * 11000111 10111111 = C7 BF, and Step 3 converts those to 0x47 0xE7
+ *
+ * If you're starting from Unicode, skip step 1. For UTF-EBCDIC to straight
+ * EBCDIC, reverse the steps.
+ *
+ * The EBCDIC invariants have been chosen to be those characters whose Unicode
+ * equivalents have ordinal numbers less than 160, that is the same characters
+ * that are expressible in ASCII, plus the C1 controls. So there are 160
+ * invariants instead of the 128 in UTF-8. (My guess is that this is because
+ * the C1 control NEL (and maybe others) is important in IBM.)
+ *
+ * The purpose of Step 3 is to make the encoding be invariant for the chosen
+ * characters. This messes up the convenient patterns found in step 2, so
+ * generally, one has to undo step 3 into a temporary to use them. However,
+ * a "shadow", or parallel table, PL_utf8skip, has been constructed so that for
+ * each byte, it says how long the sequence is if that byte were to begin it
+ *
+ * There are actually 3 slightly different UTF-EBCDIC encodings in this file,
+ * one for each of the code pages recognized by Perl. That means that there
+ * are actually three different sets of tables, one for each code page. (If
+ * Perl is compiled on platforms using other EBCDIC code pages, it may not
+ * compile, or silently mistake it for one of the three.)
+ *
+ * EBCDIC characters above 0xFF are the same as Unicode in Perl's
+ * implementation of all 3 encodings, so for those Step 1 is trivial.
+ *
+ * (Note that the entries for invariant characters are necessarily the same in
+ * PL_e2a and PLe2f, and the same for their inverses.)
+ *
+ * UTF-EBCDIC strings are the same length or longer than UTF-8 representations
+ * of the same string. The maximum code point representable as 2 bytes in
+ * UTF-EBCDIC is 0x3FFF, instead of 0x7FFF in UTF-8.
*/
START_EXTERN_C
};
#endif
-/* Transform tables from tr16 applied after encoding to render encoding EBCDIC like */
+/* Transform tables from tr16 applied after encoding to render encoding EBCDIC
+ * like, meaning that all the invariants are actually invariant, eg, that 'A'
+ * remains 'A' */
#if '^' == 95 /* if defined(__MVS__) || defined(??) (VM/ESA?) 1047 */
EXTCONST unsigned char PL_utf2e[] = { /* UTF-8-mod to EBCDIC (IBM-1047) */
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