Info Catalog elisp: Scanning Charsets elisp: Non-ASCII Characters elisp: Coding Systems

elisp: Translation of Characters

 
 32.9 Translation of Characters
 ==============================
 
 A “translation table” is a char-table (Char-Tables) that
 specifies a mapping of characters into characters.  These tables are
 used in encoding and decoding, and for other purposes.  Some coding
 systems specify their own particular translation tables; there are also
 default translation tables which apply to all other coding systems.
 
    A translation table has two extra slots.  The first is either ‘nil’
 or a translation table that performs the reverse translation; the second
 is the maximum number of characters to look up for translating sequences
 of characters (see the description of
 ‘make-translation-table-from-alist’ below).
 
  -- Function: make-translation-table &rest translations
      This function returns a translation table based on the argument
      TRANSLATIONS.  Each element of TRANSLATIONS should be a list of
      elements of the form ‘(FROM . TO)’; this says to translate the
      character FROM into TO.
 
      The arguments and the forms in each argument are processed in
      order, and if a previous form already translates TO to some other
      character, say TO-ALT, FROM is also translated to TO-ALT.
 
    During decoding, the translation table’s translations are applied to
 the characters that result from ordinary decoding.  If a coding system
 has the property ‘:decode-translation-table’, that specifies the
 translation table to use, or a list of translation tables to apply in
 sequence.  (This is a property of the coding system, as returned by
 ‘coding-system-get’, not a property of the symbol that is the coding
 system’s name.  Basic Concepts of Coding Systems Coding System
 Basics.)  Finally, if ‘standard-translation-table-for-decode’ is
 non-‘nil’, the resulting characters are translated by that table.
 
    During encoding, the translation table’s translations are applied to
 the characters in the buffer, and the result of translation is actually
 encoded.  If a coding system has property ‘:encode-translation-table’,
 that specifies the translation table to use, or a list of translation
 tables to apply in sequence.  In addition, if the variable
 ‘standard-translation-table-for-encode’ is non-‘nil’, it specifies the
 translation table to use for translating the result.
 
  -- Variable: standard-translation-table-for-decode
      This is the default translation table for decoding.  If a coding
      systems specifies its own translation tables, the table that is the
      value of this variable, if non-‘nil’, is applied after them.
 
  -- Variable: standard-translation-table-for-encode
      This is the default translation table for encoding.  If a coding
      systems specifies its own translation tables, the table that is the
      value of this variable, if non-‘nil’, is applied after them.
 
  -- Variable: translation-table-for-input
      Self-inserting characters are translated through this translation
      table before they are inserted.  Search commands also translate
      their input through this table, so they can compare more reliably
      with what’s in the buffer.
 
      This variable automatically becomes buffer-local when set.
 
  -- Function: make-translation-table-from-vector vec
      This function returns a translation table made from VEC that is an
      array of 256 elements to map bytes (values 0 through #xFF) to
      characters.  Elements may be ‘nil’ for untranslated bytes.  The
      returned table has a translation table for reverse mapping in the
      first extra slot, and the value ‘1’ in the second extra slot.
 
      This function provides an easy way to make a private coding system
      that maps each byte to a specific character.  You can specify the
      returned table and the reverse translation table using the
      properties ‘:decode-translation-table’ and
      ‘:encode-translation-table’ respectively in the PROPS argument to
      ‘define-coding-system’.
 
  -- Function: make-translation-table-from-alist alist
      This function is similar to ‘make-translation-table’ but returns a
      complex translation table rather than a simple one-to-one mapping.
      Each element of ALIST is of the form ‘(FROM . TO)’, where FROM and
      TO are either characters or vectors specifying a sequence of
      characters.  If FROM is a character, that character is translated
      to TO (i.e., to a character or a character sequence).  If FROM is a
      vector of characters, that sequence is translated to TO.  The
      returned table has a translation table for reverse mapping in the
      first extra slot, and the maximum length of all the FROM character
      sequences in the second extra slot.
 

Info Catalog elisp: Scanning Charsets elisp: Non-ASCII Characters elisp: Coding Systems