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/* Coding system handler (conversion, detection, and etc).
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   Copyright (C) 1995, 1997, 1998 Electrotechnical Laboratory, JAPAN.
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   Licensed to the Free Software Foundation.
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This file is part of GNU Emacs.

GNU Emacs is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.
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GNU Emacs is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
along with GNU Emacs; see the file COPYING.  If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.  */
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/*** TABLE OF CONTENTS ***

  1. Preamble
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  2. Emacs' internal format (emacs-mule) handlers
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  3. ISO2022 handlers
  4. Shift-JIS and BIG5 handlers
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  5. CCL handlers
  6. End-of-line handlers
  7. C library functions
  8. Emacs Lisp library functions
  9. Post-amble
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*/

/*** GENERAL NOTE on CODING SYSTEM ***

  Coding system is an encoding mechanism of one or more character
  sets.  Here's a list of coding systems which Emacs can handle.  When
  we say "decode", it means converting some other coding system to
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  Emacs' internal format (emacs-internal), and when we say "encode",
  it means converting the coding system emacs-mule to some other
  coding system.
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  0. Emacs' internal format (emacs-mule)
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  Emacs itself holds a multi-lingual character in a buffer and a string
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  in a special format.  Details are described in section 2.
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  1. ISO2022

  The most famous coding system for multiple character sets.  X's
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  Compound Text, various EUCs (Extended Unix Code), and coding
  systems used in Internet communication such as ISO-2022-JP are
  all variants of ISO2022.  Details are described in section 3.
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  2. SJIS (or Shift-JIS or MS-Kanji-Code)
   
  A coding system to encode character sets: ASCII, JISX0201, and
  JISX0208.  Widely used for PC's in Japan.  Details are described in
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  section 4.
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  3. BIG5

  A coding system to encode character sets: ASCII and Big5.  Widely
  used by Chinese (mainly in Taiwan and Hong Kong).  Details are
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  described in section 4.  In this file, when we write "BIG5"
  (all uppercase), we mean the coding system, and when we write
  "Big5" (capitalized), we mean the character set.
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  4. Raw text

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  A coding system for a text containing random 8-bit code.  Emacs does
  no code conversion on such a text except for end-of-line format.
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  5. Other
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  If a user wants to read/write a text encoded in a coding system not
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  listed above, he can supply a decoder and an encoder for it in CCL
  (Code Conversion Language) programs.  Emacs executes the CCL program
  while reading/writing.

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  Emacs represents a coding system by a Lisp symbol that has a property
  `coding-system'.  But, before actually using the coding system, the
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  information about it is set in a structure of type `struct
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  coding_system' for rapid processing.  See section 6 for more details.
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*/

/*** GENERAL NOTES on END-OF-LINE FORMAT ***

  How end-of-line of a text is encoded depends on a system.  For
  instance, Unix's format is just one byte of `line-feed' code,
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  whereas DOS's format is two-byte sequence of `carriage-return' and
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  `line-feed' codes.  MacOS's format is usually one byte of
  `carriage-return'.
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  Since text characters encoding and end-of-line encoding are
  independent, any coding system described above can take
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  any format of end-of-line.  So, Emacs has information of format of
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  end-of-line in each coding-system.  See section 6 for more details.
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*/

/*** GENERAL NOTES on `detect_coding_XXX ()' functions ***

  These functions check if a text between SRC and SRC_END is encoded
  in the coding system category XXX.  Each returns an integer value in
  which appropriate flag bits for the category XXX is set.  The flag
  bits are defined in macros CODING_CATEGORY_MASK_XXX.  Below is the
  template of these functions.  */
#if 0
int
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detect_coding_emacs_mule (src, src_end)
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     unsigned char *src, *src_end;
{
  ...
}
#endif

/*** GENERAL NOTES on `decode_coding_XXX ()' functions ***

  These functions decode SRC_BYTES length text at SOURCE encoded in
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  CODING to Emacs' internal format (emacs-mule).  The resulting text
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  goes to a place pointed to by DESTINATION, the length of which
  should not exceed DST_BYTES.  These functions set the information of
  original and decoded texts in the members produced, produced_char,
  consumed, and consumed_char of the structure *CODING.

  The return value is an integer (CODING_FINISH_XXX) indicating how
  the decoding finished.

  DST_BYTES zero means that source area and destination area are
  overlapped, which means that we can produce a decoded text until it
  reaches at the head of not-yet-decoded source text.

  Below is a template of these functions.  */
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#if 0
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decode_coding_XXX (coding, source, destination, src_bytes, dst_bytes)
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     struct coding_system *coding;
     unsigned char *source, *destination;
     int src_bytes, dst_bytes;
{
  ...
}
#endif

/*** GENERAL NOTES on `encode_coding_XXX ()' functions ***

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  These functions encode SRC_BYTES length text at SOURCE of Emacs'
  internal format (emacs-mule) to CODING.  The resulting text goes to
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  a place pointed to by DESTINATION, the length of which should not
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  exceed DST_BYTES.  These functions set the information of
  original and encoded texts in the members produced, produced_char,
  consumed, and consumed_char of the structure *CODING.

  The return value is an integer (CODING_FINISH_XXX) indicating how
  the encoding finished.

  DST_BYTES zero means that source area and destination area are
  overlapped, which means that we can produce a decoded text until it
  reaches at the head of not-yet-decoded source text.

  Below is a template of these functions.  */
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#if 0
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encode_coding_XXX (coding, source, destination, src_bytes, dst_bytes)
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     struct coding_system *coding;
     unsigned char *source, *destination;
     int src_bytes, dst_bytes;
{
  ...
}
#endif

/*** COMMONLY USED MACROS ***/

/* The following three macros ONE_MORE_BYTE, TWO_MORE_BYTES, and
   THREE_MORE_BYTES safely get one, two, and three bytes from the
   source text respectively.  If there are not enough bytes in the
   source, they jump to `label_end_of_loop'.  The caller should set
   variables `src' and `src_end' to appropriate areas in advance.  */

#define ONE_MORE_BYTE(c1)   	\
  do {			     	\
    if (src < src_end)	     	\
      c1 = *src++;	     	\
    else		     	\
      goto label_end_of_loop;	\
  } while (0)

#define TWO_MORE_BYTES(c1, c2) 	\
  do {			       	\
    if (src + 1 < src_end)     	\
      c1 = *src++, c2 = *src++;	\
    else		       	\
      goto label_end_of_loop;  	\
  } while (0)

#define THREE_MORE_BYTES(c1, c2, c3)	    	\
  do {					    	\
    if (src + 2 < src_end)		    	\
      c1 = *src++, c2 = *src++, c3 = *src++;	\
    else				    	\
      goto label_end_of_loop;		    	\
  } while (0)

/* The following three macros DECODE_CHARACTER_ASCII,
   DECODE_CHARACTER_DIMENSION1, and DECODE_CHARACTER_DIMENSION2 put
   the multi-byte form of a character of each class at the place
   pointed by `dst'.  The caller should set the variable `dst' to
   point to an appropriate area and the variable `coding' to point to
   the coding-system of the currently decoding text in advance.  */

/* Decode one ASCII character C.  */

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#define DECODE_CHARACTER_ASCII(c)	\
  do {					\
    *dst++ = (c) & 0x7F;		\
    coding->produced_char++;		\
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  } while (0)

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/* Decode one DIMENSION1 character whose charset is CHARSET and whose
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   position-code is C.  */

#define DECODE_CHARACTER_DIMENSION1(charset, c)				\
  do {									\
    unsigned char leading_code = CHARSET_LEADING_CODE_BASE (charset);	\
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    									\
    *dst++ = leading_code;						\
    if ((leading_code = CHARSET_LEADING_CODE_EXT (charset)) > 0)	\
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      *dst++ = leading_code;						\
    *dst++ = (c) | 0x80;						\
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    coding->produced_char++;						\
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  } while (0)

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/* Decode one DIMENSION2 character whose charset is CHARSET and whose
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   position-codes are C1 and C2.  */

#define DECODE_CHARACTER_DIMENSION2(charset, c1, c2)	\
  do {							\
    DECODE_CHARACTER_DIMENSION1 (charset, c1);		\
    *dst++ = (c2) | 0x80;				\
  } while (0)


/*** 1. Preamble ***/

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#ifdef emacs
#include <config.h>
#endif

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#include <stdio.h>

#ifdef emacs

#include "lisp.h"
#include "buffer.h"
#include "charset.h"
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#include "composite.h"
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#include "ccl.h"
#include "coding.h"
#include "window.h"

#else  /* not emacs */

#include "mulelib.h"

#endif /* not emacs */

Lisp_Object Qcoding_system, Qeol_type;
Lisp_Object Qbuffer_file_coding_system;
Lisp_Object Qpost_read_conversion, Qpre_write_conversion;
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Lisp_Object Qno_conversion, Qundecided;
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Lisp_Object Qcoding_system_history;
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Lisp_Object Qsafe_charsets;
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Lisp_Object Qvalid_codes;
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extern Lisp_Object Qinsert_file_contents, Qwrite_region;
Lisp_Object Qcall_process, Qcall_process_region, Qprocess_argument;
Lisp_Object Qstart_process, Qopen_network_stream;
Lisp_Object Qtarget_idx;

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Lisp_Object Vselect_safe_coding_system_function;

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/* Mnemonic string for each format of end-of-line.  */
Lisp_Object eol_mnemonic_unix, eol_mnemonic_dos, eol_mnemonic_mac;
/* Mnemonic string to indicate format of end-of-line is not yet
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   decided.  */
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Lisp_Object eol_mnemonic_undecided;
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/* Format of end-of-line decided by system.  This is CODING_EOL_LF on
   Unix, CODING_EOL_CRLF on DOS/Windows, and CODING_EOL_CR on Mac.  */
int system_eol_type;

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#ifdef emacs

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Lisp_Object Vcoding_system_list, Vcoding_system_alist;

Lisp_Object Qcoding_system_p, Qcoding_system_error;
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/* Coding system emacs-mule and raw-text are for converting only
   end-of-line format.  */
Lisp_Object Qemacs_mule, Qraw_text;
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/* Coding-systems are handed between Emacs Lisp programs and C internal
   routines by the following three variables.  */
/* Coding-system for reading files and receiving data from process.  */
Lisp_Object Vcoding_system_for_read;
/* Coding-system for writing files and sending data to process.  */
Lisp_Object Vcoding_system_for_write;
/* Coding-system actually used in the latest I/O.  */
Lisp_Object Vlast_coding_system_used;

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/* A vector of length 256 which contains information about special
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   Latin codes (especially for dealing with Microsoft codes).  */
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Lisp_Object Vlatin_extra_code_table;
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/* Flag to inhibit code conversion of end-of-line format.  */
int inhibit_eol_conversion;

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/* Flag to make buffer-file-coding-system inherit from process-coding.  */
int inherit_process_coding_system;

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/* Coding system to be used to encode text for terminal display.  */
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struct coding_system terminal_coding;

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/* Coding system to be used to encode text for terminal display when
   terminal coding system is nil.  */
struct coding_system safe_terminal_coding;

/* Coding system of what is sent from terminal keyboard.  */
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struct coding_system keyboard_coding;

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/* Default coding system to be used to write a file.  */
struct coding_system default_buffer_file_coding;

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Lisp_Object Vfile_coding_system_alist;
Lisp_Object Vprocess_coding_system_alist;
Lisp_Object Vnetwork_coding_system_alist;
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Lisp_Object Vlocale_coding_system;

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#endif /* emacs */

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Lisp_Object Qcoding_category, Qcoding_category_index;
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/* List of symbols `coding-category-xxx' ordered by priority.  */
Lisp_Object Vcoding_category_list;

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/* Table of coding categories (Lisp symbols).  */
Lisp_Object Vcoding_category_table;
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/* Table of names of symbol for each coding-category.  */
char *coding_category_name[CODING_CATEGORY_IDX_MAX] = {
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  "coding-category-emacs-mule",
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  "coding-category-sjis",
  "coding-category-iso-7",
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  "coding-category-iso-7-tight",
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  "coding-category-iso-8-1",
  "coding-category-iso-8-2",
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  "coding-category-iso-7-else",
  "coding-category-iso-8-else",
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  "coding-category-ccl",
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  "coding-category-big5",
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  "coding-category-raw-text",
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  "coding-category-binary"
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};

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/* Table of pointers to coding systems corresponding to each coding
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   categories.  */
struct coding_system *coding_system_table[CODING_CATEGORY_IDX_MAX];

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/* Table of coding category masks.  Nth element is a mask for a coding
   cateogry of which priority is Nth.  */
static
int coding_priorities[CODING_CATEGORY_IDX_MAX];

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/* Flag to tell if we look up translation table on character code
   conversion.  */
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Lisp_Object Venable_character_translation;
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/* Standard translation table to look up on decoding (reading).  */
Lisp_Object Vstandard_translation_table_for_decode;
/* Standard translation table to look up on encoding (writing).  */
Lisp_Object Vstandard_translation_table_for_encode;
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Lisp_Object Qtranslation_table;
Lisp_Object Qtranslation_table_id;
Lisp_Object Qtranslation_table_for_decode;
Lisp_Object Qtranslation_table_for_encode;
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/* Alist of charsets vs revision number.  */
Lisp_Object Vcharset_revision_alist;

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/* Default coding systems used for process I/O.  */
Lisp_Object Vdefault_process_coding_system;

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/* Global flag to tell that we can't call post-read-conversion and
   pre-write-conversion functions.  Usually the value is zero, but it
   is set to 1 temporarily while such functions are running.  This is
   to avoid infinite recursive call.  */
static int inhibit_pre_post_conversion;

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/*** 2. Emacs internal format (emacs-mule) handlers ***/
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/* Emacs' internal format for encoding multiple character sets is a
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   kind of multi-byte encoding, i.e. characters are encoded by
   variable-length sequences of one-byte codes.  ASCII characters
   and control characters (e.g. `tab', `newline') are represented by
   one-byte sequences which are their ASCII codes, in the range 0x00
   through 0x7F.  The other characters are represented by a sequence
   of `base leading-code', optional `extended leading-code', and one
   or two `position-code's.  The length of the sequence is determined
   by the base leading-code.  Leading-code takes the range 0x80
   through 0x9F, whereas extended leading-code and position-code take
   the range 0xA0 through 0xFF.  See `charset.h' for more details
   about leading-code and position-code.

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   --- CODE RANGE of Emacs' internal format ---
   (character set)	(range)
   ASCII		0x00 .. 0x7F
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   ELSE (1st byte)	0x81 .. 0x9F
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	(rest bytes)	0xA0 .. 0xFF
   ---------------------------------------------

  */

enum emacs_code_class_type emacs_code_class[256];

/* Go to the next statement only if *SRC is accessible and the code is
   greater than 0xA0.  */
#define CHECK_CODE_RANGE_A0_FF 	\
  do {			       	\
    if (src >= src_end)	       	\
      goto label_end_of_switch;	\
    else if (*src++ < 0xA0)    	\
      return 0;		       	\
  } while (0)

/* See the above "GENERAL NOTES on `detect_coding_XXX ()' functions".
   Check if a text is encoded in Emacs' internal format.  If it is,
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   return CODING_CATEGORY_MASK_EMACS_MULE, else return 0.  */
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int
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detect_coding_emacs_mule (src, src_end)
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     unsigned char *src, *src_end;
{
  unsigned char c;
  int composing = 0;

  while (src < src_end)
    {
      c = *src++;

      if (composing)
	{
	  if (c < 0xA0)
	    composing = 0;
	  else
	    c -= 0x20;
	}

      switch (emacs_code_class[c])
	{
	case EMACS_ascii_code:
	case EMACS_linefeed_code:
	  break;

	case EMACS_control_code:
	  if (c == ISO_CODE_ESC || c == ISO_CODE_SI || c == ISO_CODE_SO)
	    return 0;
	  break;

	case EMACS_invalid_code:
	  return 0;

	case EMACS_leading_code_4:
	  CHECK_CODE_RANGE_A0_FF;
	  /* fall down to check it two more times ...  */

	case EMACS_leading_code_3:
	  CHECK_CODE_RANGE_A0_FF;
	  /* fall down to check it one more time ...  */

	case EMACS_leading_code_2:
	  CHECK_CODE_RANGE_A0_FF;
	  break;

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	case 0x80:	/* Old leading code for a composite character.  */
	  if (composing)
	    CHECK_CODE_RANGE_A0_FF;
	  else
	    composing = 1;
	  break;

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	default:
	label_end_of_switch:
	  break;
	}
    }
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  return CODING_CATEGORY_MASK_EMACS_MULE;
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}


/*** 3. ISO2022 handlers ***/

/* The following note describes the coding system ISO2022 briefly.
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   Since the intention of this note is to help understand the
   functions in this file, some parts are NOT ACCURATE or OVERLY
   SIMPLIFIED.  For thorough understanding, please refer to the
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   original document of ISO2022.

   ISO2022 provides many mechanisms to encode several character sets
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   in 7-bit and 8-bit environments.  For 7-bite environments, all text
   is encoded using bytes less than 128.  This may make the encoded
   text a little bit longer, but the text passes more easily through
   several gateways, some of which strip off MSB (Most Signigant Bit).
 
   There are two kinds of character sets: control character set and
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   graphic character set.  The former contains control characters such
   as `newline' and `escape' to provide control functions (control
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   functions are also provided by escape sequences).  The latter
   contains graphic characters such as 'A' and '-'.  Emacs recognizes
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   two control character sets and many graphic character sets.

   Graphic character sets are classified into one of the following
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   four classes, according to the number of bytes (DIMENSION) and
   number of characters in one dimension (CHARS) of the set:
   - DIMENSION1_CHARS94
   - DIMENSION1_CHARS96
   - DIMENSION2_CHARS94
   - DIMENSION2_CHARS96

   In addition, each character set is assigned an identification tag,
   unique for each set, called "final character" (denoted as <F>
   hereafter).  The <F> of each character set is decided by ECMA(*)
   when it is registered in ISO.  The code range of <F> is 0x30..0x7F
   (0x30..0x3F are for private use only).
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   Note (*): ECMA = European Computer Manufacturers Association

   Here are examples of graphic character set [NAME(<F>)]:
	o DIMENSION1_CHARS94 -- ASCII('B'), right-half-of-JISX0201('I'), ...
	o DIMENSION1_CHARS96 -- right-half-of-ISO8859-1('A'), ...
	o DIMENSION2_CHARS94 -- GB2312('A'), JISX0208('B'), ...
	o DIMENSION2_CHARS96 -- none for the moment

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   A code area (1 byte=8 bits) is divided into 4 areas, C0, GL, C1, and GR.
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	C0 [0x00..0x1F] -- control character plane 0
	GL [0x20..0x7F] -- graphic character plane 0
	C1 [0x80..0x9F] -- control character plane 1
	GR [0xA0..0xFF] -- graphic character plane 1

   A control character set is directly designated and invoked to C0 or
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   C1 by an escape sequence.  The most common case is that:
   - ISO646's  control character set is designated/invoked to C0, and
   - ISO6429's control character set is designated/invoked to C1,
   and usually these designations/invocations are omitted in encoded
   text.  In a 7-bit environment, only C0 can be used, and a control
   character for C1 is encoded by an appropriate escape sequence to
   fit into the environment.  All control characters for C1 are
   defined to have corresponding escape sequences.
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   A graphic character set is at first designated to one of four
   graphic registers (G0 through G3), then these graphic registers are
   invoked to GL or GR.  These designations and invocations can be
   done independently.  The most common case is that G0 is invoked to
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   GL, G1 is invoked to GR, and ASCII is designated to G0.  Usually
   these invocations and designations are omitted in encoded text.
   In a 7-bit environment, only GL can be used.
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   When a graphic character set of CHARS94 is invoked to GL, codes
   0x20 and 0x7F of the GL area work as control characters SPACE and
   DEL respectively, and codes 0xA0 and 0xFF of the GR area should not
   be used.
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   There are two ways of invocation: locking-shift and single-shift.
   With locking-shift, the invocation lasts until the next different
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   invocation, whereas with single-shift, the invocation affects the
   following character only and doesn't affect the locking-shift
   state.  Invocations are done by the following control characters or
   escape sequences:
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   ----------------------------------------------------------------------
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   abbrev  function	             cntrl escape seq	description
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   ----------------------------------------------------------------------
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   SI/LS0  (shift-in)		     0x0F  none		invoke G0 into GL
   SO/LS1  (shift-out)		     0x0E  none		invoke G1 into GL
   LS2     (locking-shift-2)	     none  ESC 'n'	invoke G2 into GL
   LS3     (locking-shift-3)	     none  ESC 'o'	invoke G3 into GL
   LS1R    (locking-shift-1 right)   none  ESC '~'      invoke G1 into GR (*)
   LS2R    (locking-shift-2 right)   none  ESC '}'      invoke G2 into GR (*)
   LS3R    (locking-shift 3 right)   none  ESC '|'      invoke G3 into GR (*)
   SS2     (single-shift-2)	     0x8E  ESC 'N'	invoke G2 for one char
   SS3     (single-shift-3)	     0x8F  ESC 'O'	invoke G3 for one char
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   ----------------------------------------------------------------------
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   (*) These are not used by any known coding system.

   Control characters for these functions are defined by macros
   ISO_CODE_XXX in `coding.h'.
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   Designations are done by the following escape sequences:
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   ----------------------------------------------------------------------
   escape sequence	description
   ----------------------------------------------------------------------
   ESC '(' <F>		designate DIMENSION1_CHARS94<F> to G0
   ESC ')' <F>		designate DIMENSION1_CHARS94<F> to G1
   ESC '*' <F>		designate DIMENSION1_CHARS94<F> to G2
   ESC '+' <F>		designate DIMENSION1_CHARS94<F> to G3
   ESC ',' <F>		designate DIMENSION1_CHARS96<F> to G0 (*)
   ESC '-' <F>		designate DIMENSION1_CHARS96<F> to G1
   ESC '.' <F>		designate DIMENSION1_CHARS96<F> to G2
   ESC '/' <F>		designate DIMENSION1_CHARS96<F> to G3
   ESC '$' '(' <F>	designate DIMENSION2_CHARS94<F> to G0 (**)
   ESC '$' ')' <F>	designate DIMENSION2_CHARS94<F> to G1
   ESC '$' '*' <F>	designate DIMENSION2_CHARS94<F> to G2
   ESC '$' '+' <F>	designate DIMENSION2_CHARS94<F> to G3
   ESC '$' ',' <F>	designate DIMENSION2_CHARS96<F> to G0 (*)
   ESC '$' '-' <F>	designate DIMENSION2_CHARS96<F> to G1
   ESC '$' '.' <F>	designate DIMENSION2_CHARS96<F> to G2
   ESC '$' '/' <F>	designate DIMENSION2_CHARS96<F> to G3
   ----------------------------------------------------------------------

   In this list, "DIMENSION1_CHARS94<F>" means a graphic character set
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   of dimension 1, chars 94, and final character <F>, etc...
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   Note (*): Although these designations are not allowed in ISO2022,
   Emacs accepts them on decoding, and produces them on encoding
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   CHARS96 character sets in a coding system which is characterized as
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   7-bit environment, non-locking-shift, and non-single-shift.

   Note (**): If <F> is '@', 'A', or 'B', the intermediate character
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   '(' can be omitted.  We refer to this as "short-form" hereafter.
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   Now you may notice that there are a lot of ways for encoding the
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   same multilingual text in ISO2022.  Actually, there exist many
   coding systems such as Compound Text (used in X11's inter client
   communication, ISO-2022-JP (used in Japanese internet), ISO-2022-KR
   (used in Korean internet), EUC (Extended UNIX Code, used in Asian
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   localized platforms), and all of these are variants of ISO2022.

   In addition to the above, Emacs handles two more kinds of escape
   sequences: ISO6429's direction specification and Emacs' private
   sequence for specifying character composition.

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   ISO6429's direction specification takes the following form:
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	o CSI ']'      -- end of the current direction
	o CSI '0' ']'  -- end of the current direction
	o CSI '1' ']'  -- start of left-to-right text
	o CSI '2' ']'  -- start of right-to-left text
   The control character CSI (0x9B: control sequence introducer) is
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   abbreviated to the escape sequence ESC '[' in a 7-bit environment.

   Character composition specification takes the following form:
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	o ESC '0' -- start relative composition
	o ESC '1' -- end composition
	o ESC '2' -- start rule-base composition (*)
	o ESC '3' -- start relative composition with alternate chars  (**)
	o ESC '4' -- start rule-base composition with alternate chars  (**)
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   Since these are not standard escape sequences of any ISO standard,
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   the use of them for these meaning is restricted to Emacs only.

   (*) This form is used only in Emacs 20.5 and the older versions,
   but the newer versions can safely decode it.
   (**) This form is used only in Emacs 21.1 and the newer versions,
   and the older versions can't decode it.

   Here's a list of examples usages of these composition escape
   sequences (categorized by `enum composition_method').

   COMPOSITION_RELATIVE:
	ESC 0 CHAR [ CHAR ] ESC 1
   COMPOSITOIN_WITH_RULE:
	ESC 2 CHAR [ RULE CHAR ] ESC 1
   COMPOSITION_WITH_ALTCHARS:
	ESC 3 ALTCHAR [ ALTCHAR ] ESC 0 CHAR [ CHAR ] ESC 1
   COMPOSITION_WITH_RULE_ALTCHARS:
	ESC 4 ALTCHAR [ RULE ALTCHAR ] ESC 0 CHAR [ CHAR ] ESC 1 */
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enum iso_code_class_type iso_code_class[256];

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#define CHARSET_OK(idx, charset)				\
  (coding_system_table[idx]					\
   && (coding_system_table[idx]->safe_charsets[charset]		\
       || (CODING_SPEC_ISO_REQUESTED_DESIGNATION		\
            (coding_system_table[idx], charset)			\
           != CODING_SPEC_ISO_NO_REQUESTED_DESIGNATION)))
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#define SHIFT_OUT_OK(idx) \
  (CODING_SPEC_ISO_INITIAL_DESIGNATION (coding_system_table[idx], 1) >= 0)

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/* See the above "GENERAL NOTES on `detect_coding_XXX ()' functions".
   Check if a text is encoded in ISO2022.  If it is, returns an
   integer in which appropriate flag bits any of:
	CODING_CATEGORY_MASK_ISO_7
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	CODING_CATEGORY_MASK_ISO_7_TIGHT
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	CODING_CATEGORY_MASK_ISO_8_1
	CODING_CATEGORY_MASK_ISO_8_2
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	CODING_CATEGORY_MASK_ISO_7_ELSE
	CODING_CATEGORY_MASK_ISO_8_ELSE
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   are set.  If a code which should never appear in ISO2022 is found,
   returns 0.  */

int
detect_coding_iso2022 (src, src_end)
     unsigned char *src, *src_end;
{
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  int mask = CODING_CATEGORY_MASK_ISO;
  int mask_found = 0;
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  int reg[4], shift_out = 0, single_shifting = 0;
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  int c, c1, i, charset;
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  reg[0] = CHARSET_ASCII, reg[1] = reg[2] = reg[3] = -1;
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  while (mask && src < src_end)
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    {
      c = *src++;
      switch (c)
	{
	case ISO_CODE_ESC:
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	  single_shifting = 0;
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	  if (src >= src_end)
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	    break;
	  c = *src++;
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	  if (c >= '(' && c <= '/')
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	    {
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	      /* Designation sequence for a charset of dimension 1.  */
	      if (src >= src_end)
		break;
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	      c1 = *src++;
	      if (c1 < ' ' || c1 >= 0x80
		  || (charset = iso_charset_table[0][c >= ','][c1]) < 0)
		/* Invalid designation sequence.  Just ignore.  */
		break;
	      reg[(c - '(') % 4] = charset;
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	    }
	  else if (c == '$')
	    {
	      /* Designation sequence for a charset of dimension 2.  */
	      if (src >= src_end)
		break;
	      c = *src++;
	      if (c >= '@' && c <= 'B')
		/* Designation for JISX0208.1978, GB2312, or JISX0208.  */
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		reg[0] = charset = iso_charset_table[1][0][c];
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	      else if (c >= '(' && c <= '/')
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		{
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		  if (src >= src_end)
		    break;
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		  c1 = *src++;
		  if (c1 < ' ' || c1 >= 0x80
		      || (charset = iso_charset_table[1][c >= ','][c1]) < 0)
		    /* Invalid designation sequence.  Just ignore.  */
		    break;
		  reg[(c - '(') % 4] = charset;
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		}
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	      else
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		/* Invalid designation sequence.  Just ignore.  */
		break;
	    }
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	  else if (c == 'N' || c == 'O')
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	    {
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	      /* ESC <Fe> for SS2 or SS3.  */
	      mask &= CODING_CATEGORY_MASK_ISO_7_ELSE;
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	      break;
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	    }
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	  else if (c >= '0' && c <= '4')
	    {
	      /* ESC <Fp> for start/end composition.  */
	      mask_found |= CODING_CATEGORY_MASK_ISO;
	      break;
	    }
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	  else
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	    /* Invalid escape sequence.  Just ignore.  */
	    break;

	  /* We found a valid designation sequence for CHARSET.  */
	  mask &= ~CODING_CATEGORY_MASK_ISO_8BIT;
	  if (CHARSET_OK (CODING_CATEGORY_IDX_ISO_7, charset))
	    mask_found |= CODING_CATEGORY_MASK_ISO_7;
	  else
	    mask &= ~CODING_CATEGORY_MASK_ISO_7;
	  if (CHARSET_OK (CODING_CATEGORY_IDX_ISO_7_TIGHT, charset))
	    mask_found |= CODING_CATEGORY_MASK_ISO_7_TIGHT;
	  else
	    mask &= ~CODING_CATEGORY_MASK_ISO_7_TIGHT;
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	  if (CHARSET_OK (CODING_CATEGORY_IDX_ISO_7_ELSE, charset))
	    mask_found |= CODING_CATEGORY_MASK_ISO_7_ELSE;
	  else
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	    mask &= ~CODING_CATEGORY_MASK_ISO_7_ELSE;
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	  if (CHARSET_OK (CODING_CATEGORY_IDX_ISO_8_ELSE, charset))
	    mask_found |= CODING_CATEGORY_MASK_ISO_8_ELSE;
	  else
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	    mask &= ~CODING_CATEGORY_MASK_ISO_8_ELSE;
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	  break;

	case ISO_CODE_SO:
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	  single_shifting = 0;
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	  if (shift_out == 0
	      && (reg[1] >= 0
		  || SHIFT_OUT_OK (CODING_CATEGORY_IDX_ISO_7_ELSE)
		  || SHIFT_OUT_OK (CODING_CATEGORY_IDX_ISO_8_ELSE)))
	    {
	      /* Locking shift out.  */
	      mask &= ~CODING_CATEGORY_MASK_ISO_7BIT;
	      mask_found |= CODING_CATEGORY_MASK_ISO_SHIFT;
	    }
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	  break;
	  
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	case ISO_CODE_SI:
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	  single_shifting = 0;
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	  if (shift_out == 1)
	    {
	      /* Locking shift in.  */
	      mask &= ~CODING_CATEGORY_MASK_ISO_7BIT;
	      mask_found |= CODING_CATEGORY_MASK_ISO_SHIFT;
	    }
	  break;

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	case ISO_CODE_CSI:
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	  single_shifting = 0;
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	case ISO_CODE_SS2:
	case ISO_CODE_SS3:
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	  {
	    int newmask = CODING_CATEGORY_MASK_ISO_8_ELSE;

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	    if (c != ISO_CODE_CSI)
	      {
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		if (coding_system_table[CODING_CATEGORY_IDX_ISO_8_1]->flags
		    & CODING_FLAG_ISO_SINGLE_SHIFT)
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		  newmask |= CODING_CATEGORY_MASK_ISO_8_1;
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		if (coding_system_table[CODING_CATEGORY_IDX_ISO_8_2]->flags
		    & CODING_FLAG_ISO_SINGLE_SHIFT)
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		  newmask |= CODING_CATEGORY_MASK_ISO_8_2;
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		single_shifting = 1;
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	      }
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	    if (VECTORP (Vlatin_extra_code_table)
		&& !NILP (XVECTOR (Vlatin_extra_code_table)->contents[c]))
	      {
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		if (coding_system_table[CODING_CATEGORY_IDX_ISO_8_1]->flags
		    & CODING_FLAG_ISO_LATIN_EXTRA)
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		  newmask |= CODING_CATEGORY_MASK_ISO_8_1;
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		if (coding_system_table[CODING_CATEGORY_IDX_ISO_8_2]->flags
		    & CODING_FLAG_ISO_LATIN_EXTRA)
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		  newmask |= CODING_CATEGORY_MASK_ISO_8_2;
	      }
	    mask &= newmask;
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	    mask_found |= newmask;
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	  }
	  break;
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	default:
	  if (c < 0x80)
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	    {
	      single_shifting = 0;
	      break;
	    }
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	  else if (c < 0xA0)
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	    {
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	      single_shifting = 0;
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	      if (VECTORP (Vlatin_extra_code_table)
		  && !NILP (XVECTOR (Vlatin_extra_code_table)->contents[c]))
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		{
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		  int newmask = 0;

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		  if (coding_system_table[CODING_CATEGORY_IDX_ISO_8_1]->flags
		      & CODING_FLAG_ISO_LATIN_EXTRA)
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		    newmask |= CODING_CATEGORY_MASK_ISO_8_1;
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		  if (coding_system_table[CODING_CATEGORY_IDX_ISO_8_2]->flags
		      & CODING_FLAG_ISO_LATIN_EXTRA)
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		    newmask |= CODING_CATEGORY_MASK_ISO_8_2;
		  mask &= newmask;
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		  mask_found |= newmask;
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		}
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	      else
		return 0;
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	    }
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	  else
	    {
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	      unsigned char *src_begin = src;
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	      mask &= ~(CODING_CATEGORY_MASK_ISO_7BIT
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			| CODING_CATEGORY_MASK_ISO_7_ELSE);
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	      mask_found |= CODING_CATEGORY_MASK_ISO_8_1;
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	      /* Check the length of succeeding codes of the range
                 0xA0..0FF.  If the byte length is odd, we exclude
                 CODING_CATEGORY_MASK_ISO_8_2.  We can check this only
                 when we are not single shifting.  */
	      if (!single_shifting)
		{
		  while (src < src_end && *src >= 0xA0)
		    src++;
		  if ((src - src_begin - 1) & 1 && src < src_end)
		    mask &= ~CODING_CATEGORY_MASK_ISO_8_2;
		  else
		    mask_found |= CODING_CATEGORY_MASK_ISO_8_2;
		}
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	    }
	  break;
	}
    }

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  return (mask & mask_found);
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}

/* Decode a character of which charset is CHARSET and the 1st position
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   code is C1.  If dimension of CHARSET is 2, the 2nd position code is
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   fetched from SRC and set to C2.  If CHARSET is negative, it means
   that we are decoding ill formed text, and what we can do is just to
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   read C1 as is.

   If we are now in the middle of composition sequence, the decoded
   character may be ALTCHAR (see the comment above).  In that case,
   the character goes to coding->cmp_data->data instead of DST.  */

#define DECODE_ISO_CHARACTER(charset, c1)				  \
  do {									  \
    int c_alt = -1, charset_alt = (charset);				  \
    if (charset_alt >= 0)						  \
      {									  \
	if (CHARSET_DIMENSION (charset_alt) == 2)			  \
	  {								  \
	    ONE_MORE_BYTE (c2);						  \
	    if (iso_code_class[(c2) & 0x7F] != ISO_0x20_or_0x7F		  \
		&& iso_code_class[(c2) & 0x7F] != ISO_graphic_plane_0)	  \
	      {								  \
		src--;							  \
		charset_alt = CHARSET_ASCII;				  \
	      }								  \
	  }								  \
	if (!NILP (translation_table)					  \
	    && ((c_alt = translate_char (translation_table,		  \
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					 -1, charset_alt, c1, c2)) >= 0)) \
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	  SPLIT_CHAR (c_alt, charset_alt, c1, c2);			  \
      }									  \
    if (! COMPOSING_P (coding)						  \
	|| coding->composing == COMPOSITION_RELATIVE			  \
	|| coding->composing == COMPOSITION_WITH_RULE)			  \
      {									  \
	if (charset_alt == CHARSET_ASCII || charset_alt < 0)		  \
	  DECODE_CHARACTER_ASCII (c1);					  \
	else if (CHARSET_DIMENSION (charset_alt) == 1)			  \
	  DECODE_CHARACTER_DIMENSION1 (charset_alt, c1);		  \
	else								  \
	  DECODE_CHARACTER_DIMENSION2 (charset_alt, c1, c2);		  \
      }									  \
    if (COMPOSING_P (coding)						  \
	&& coding->composing != COMPOSITION_RELATIVE)			  \
      {									  \
	if (c_alt < 0)							  \
	  c_alt = MAKE_CHAR (charset_alt, c1, c2);			  \
	CODING_ADD_COMPOSITION_COMPONENT (coding, c_alt);		  \
	coding->composition_rule_follows				  \
	  = coding->composing != COMPOSITION_WITH_ALTCHARS;		  \
      }									  \
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  } while (0)

/* Set designation state into CODING.  */
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#define DECODE_DESIGNATION(reg, dimension, chars, final_char)		   \
  do {									   \
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    int charset;							   \
    									   \
    if (final_char < '0' || final_char >= 128)				   \
      goto label_invalid_code;						   \
    charset = ISO_CHARSET_TABLE (make_number (dimension),		   \
				 make_number (chars),			   \
				 make_number (final_char));		   \
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    if (charset >= 0							   \
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	&& (CODING_SPEC_ISO_REQUESTED_DESIGNATION (coding, charset) == reg \
	    || coding->safe_charsets[charset]))				   \
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      {									   \
	if (coding->spec.iso2022.last_invalid_designation_register == 0	   \
	    && reg == 0							   \
	    && charset == CHARSET_ASCII)				   \
	  {								   \
	    /* We should insert this designation sequence as is so	   \
               that it is surely written back to a file.  */		   \
	    coding->spec.iso2022.last_invalid_designation_register = -1;   \
	    goto label_invalid_code;					   \
	  }								   \
	coding->spec.iso2022.last_invalid_designation_register = -1;	   \
        if ((coding->mode & CODING_MODE_DIRECTION)			   \
	    && CHARSET_REVERSE_CHARSET (charset) >= 0)			   \
          charset = CHARSET_REVERSE_CHARSET (charset);			   \
        CODING_SPEC_ISO_DESIGNATION (coding, reg) = charset;		   \
      }									   \
    else								   \
      {									   \
	coding->spec.iso2022.last_invalid_designation_register = reg;	   \
	goto label_invalid_code;					   \
      }									   \
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  } while (0)

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/* Allocate a memory block for storing information about compositions.
   The block is chained to the already allocated blocks.  */
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static void
coding_allocate_composition_data (coding, char_offset)
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     struct coding_system *coding;
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     int char_offset;
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{
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  struct composition_data *cmp_data
    = (struct composition_data *) xmalloc (sizeof *cmp_data);

  cmp_data->char_offset = char_offset;
  cmp_data->used = 0;
  cmp_data->prev = coding->cmp_data;
  cmp_data->next = NULL;
  if (coding->cmp_data)
    coding->cmp_data->next = cmp_data;
  coding->cmp_data = cmp_data;
  coding->cmp_data_start = 0;
}
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/* Record the starting position START and METHOD of one composition.  */

#define CODING_ADD_COMPOSITION_START(coding, start, method)	\
  do {								\
    struct composition_data *cmp_data = coding->cmp_data;	\
    int *data = cmp_data->data + cmp_data->used;		\
    coding->cmp_data_start = cmp_data->used;			\
    data[0] = -1;						\
    data[1] = cmp_data->char_offset + start;			\
    data[3] = (int) method;					\
    cmp_data->used += 4;					\
  } while (0)

/* Record the ending position END of the current composition.  */

#define CODING_ADD_COMPOSITION_END(coding, end)			\
  do {								\
    struct composition_data *cmp_data = coding->cmp_data;	\
    int *data = cmp_data->data + coding->cmp_data_start;	\
    data[0] = cmp_data->used - coding->cmp_data_start;		\
    data[2] = cmp_data->char_offset + end;			\
  } while (0)

/* Record one COMPONENT (alternate character or composition rule).  */

#define CODING_ADD_COMPOSITION_COMPONENT(coding, component)	\
  (coding->cmp_data->data[coding->cmp_data->used++] = component)

/* Handle compositoin start sequence ESC 0, ESC 2, ESC 3, or ESC 4.  */

#define DECODE_COMPOSITION_START(c1)					\
  do {									\
    if (coding->composing == COMPOSITION_DISABLED)			\
      {									\
      	*dst++ = ISO_CODE_ESC;						\
	*dst++ = c1 & 0x7f;						\
	coding->produced_char += 2;					\
      }									\
    else if (!COMPOSING_P (coding))					\
      {									\
	/* This is surely the start of a composition.  We must be sure	\
           that coding->cmp_data has enough space to store the		\
           information about the composition.  If not, terminate the	\
           current decoding loop, allocate one more memory block for	\
           coding->cmp_data in the calller, then start the decoding	\
           loop again.  We can't allocate memory here directly because	\
           it may cause buffer/string relocation.  */			\
	if (coding->cmp_data->used + COMPOSITION_DATA_MAX_BUNCH_LENGTH	\
	    >= COMPOSITION_DATA_SIZE)					\
	  {								\
	    result = CODING_FINISH_INSUFFICIENT_CMP;			\
	    goto label_end_of_loop_2;					\
	  }								\
	coding->composing = (c1 == '0' ? COMPOSITION_RELATIVE		\
			     : c1 == '2' ? COMPOSITION_WITH_RULE	\
			     : c1 == '3' ? COMPOSITION_WITH_ALTCHARS	\
			     : COMPOSITION_WITH_RULE_ALTCHARS);		\
	CODING_ADD_COMPOSITION_START (coding, coding->produced_char,	\
				      coding->composing);		\
	coding->composition_rule_follows = 0;				\
      }									\
    else								\
      {									\
	/* We are already handling a composition.  If the method is	\
           the following two, the codes following the current escape	\
           sequence are actual characters stored in a buffer.  */	\
	if (coding->composing == COMPOSITION_WITH_ALTCHARS		\
	    || coding->composing == COMPOSITION_WITH_RULE_ALTCHARS)	\
	  {								\
	    coding->composing = COMPOSITION_RELATIVE;			\
	    coding->composition_rule_follows = 0;			\
	  }								\
      }									\
  } while (0)

/* Handle compositoin end sequence ESC 1.  */

#define DECODE_COMPOSITION_END(c1)					\
  do {									\
    if (coding->composing == COMPOSITION_DISABLED)			\
      {									\
	*dst++ = ISO_CODE_ESC;						\
	*dst++ = c1;							\
	coding->produced_char += 2;					\
      }									\
    else								\
      {									\
	CODING_ADD_COMPOSITION_END (coding, coding->produced_char);	\
	coding->composing = COMPOSITION_NO;				\
      }									\
  } while (0)

/* Decode a composition rule from the byte C1 (and maybe one more byte
   from SRC) and store one encoded composition rule in
   coding->cmp_data.  */

#define DECODE_COMPOSITION_RULE(c1)					\
  do {									\
    int rule = 0;							\
    (c1) -= 32;								\
    if (c1 < 81)		/* old format (before ver.21) */	\
      {									\
	int gref = (c1) / 9;						\
	int nref = (c1) % 9;						\
	if (gref == 4) gref = 10;					\
	if (nref == 4) nref = 10;					\
	rule = COMPOSITION_ENCODE_RULE (gref, nref);			\
      }									\
    else if (c1 < 93)		/* new format (after ver.21 */		\
      {									\
	ONE_MORE_BYTE (c2);						\
	rule = COMPOSITION_ENCODE_RULE (c1 - 81, c2 - 32);		\
      }									\
    CODING_ADD_COMPOSITION_COMPONENT (coding, rule);			\
    coding->composition_rule_follows = 0;				\
  } while (0)
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/* See the above "GENERAL NOTES on `decode_coding_XXX ()' functions".  */

int
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decode_coding_iso2022 (coding, source, destination, src_bytes, dst_bytes)
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     struct coding_system *coding;
     unsigned char *source, *destination;
     int src_bytes, dst_bytes;
{
  unsigned char *src = source;
  unsigned char *src_end = source + src_bytes;
  unsigned char *dst = destination;
  unsigned char *dst_end = destination + dst_bytes;
  /* Since the maximum bytes produced by each loop is 7, we subtract 6
     from DST_END to assure that overflow checking is necessary only
     at the head of loop.  */
  unsigned char *adjusted_dst_end = dst_end - 6;
  int charset;
  /* Charsets invoked to graphic plane 0 and 1 respectively.  */
  int charset0 = CODING_SPEC_ISO_PLANE_CHARSET (coding, 0);
  int charset1 = CODING_SPEC_ISO_PLANE_CHARSET (coding, 1);
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  Lisp_Object translation_table
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    = coding->translation_table_for_decode;
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  int result = CODING_FINISH_NORMAL;
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  if (!NILP (Venable_character_translation) && NILP (translation_table))
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    translation_table = Vstandard_translation_table_for_decode;
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  coding->produced_char = 0;
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  coding->fake_multibyte = 0;
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  while (src < src_end && (dst_bytes
			   ? (dst < adjusted_dst_end)
			   : (dst < src - 6)))
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    {
      /* SRC_BASE remembers the start position in source in each loop.
	 The loop will be exited when there's not enough source text
	 to analyze long escape sequence or 2-byte code (within macros
	 ONE_MORE_BYTE or TWO_MORE_BYTES).  In that case, SRC is reset
	 to SRC_BASE before exiting.  */
      unsigned char *src_base = src;
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      int c1 = *src++, c2;
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      /* We produce no character or one character.  */
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      switch (iso_code_class [c1])
	{
	case ISO_0x20_or_0x7F:
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	  if (COMPOSING_P (coding) && coding->composition_rule_follows)
	    {
	      DECODE_COMPOSITION_RULE (c1);
	      break;
	    }
	  if (charset0 < 0 || CHARSET_CHARS (charset0) == 94)
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	    {
	      /* This is SPACE or DEL.  */
	      *dst++ = c1;
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	      coding->produced_char++;
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	      break;
	    }
	  /* This is a graphic character, we fall down ...  */

	case ISO_graphic_plane_0:
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	  if (COMPOSING_P (coding) && coding->composition_rule_follows)
	    DECODE_COMPOSITION_RULE (c1);
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	  else
	    DECODE_ISO_CHARACTER (charset0, c1);
	  break;

	case ISO_0xA0_or_0xFF:
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	  if (charset1 < 0 || CHARSET_CHARS (charset1) == 94
	      || coding->flags & CODING_FLAG_ISO_SEVEN_BITS)
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	    goto label_invalid_code;
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	  /* This is a graphic character, we fall down ... */

	case ISO_graphic_plane_1:
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	  if (coding->flags & CODING_FLAG_ISO_SEVEN_BITS)
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	    goto label_invalid_code;
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	  DECODE_ISO_CHARACTER (charset1, c1);
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	  break;

	case ISO_control_code:
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	  if (COMPOSING_P (coding))
	    DECODE_COMPOSITION_END ('1');

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	  /* All ISO2022 control characters in this class have the
             same representation in Emacs internal format.  */
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	  if (c1 == '\n'
	      && (coding->mode & CODING_MODE_INHIBIT_INCONSISTENT_EOL)
	      && (coding->eol_type == CODING_EOL_CR
		  ||