(recode.info)Surfaces

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All about surfaces
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   The "trivial surface" consists of using a fixed number of bits
(often eight) for each character, the bits together hold the integer
value of the index for the character in its charset table.  There are
many kinds of surfaces, beyond the trivial one, all having the purpose
of increasing selected qualities for the storage or transmission.  For
example, surfaces might increase the resistance to channel limits
(`Base64'), the transmission speed (`gzip'), the information privacy
(`DES'), the conformance to operating system conventions (`CR-LF'), the
blocking into records (`VB'), and surely other things as well(1).  Many
surfaces may be applied to a stream of characters from a charset, the
order of application of surfaces is important, and surfaces should be
removed in the reverse order of their application.

   Even if surfaces may generally be applied to various charsets, some
surfaces were specifically designed for a particular charset, and would
not make much sense if applied to other charsets.  In such cases, these
conceptual surfaces have been implemented as `recode' charsets, instead
of as surfaces.  This choice yields to cleaner syntax and usage.  Note:
Universal.

   Surfaces are implemented within `recode' as special charsets which
may only transform to or from the `data' or `tree' special charsets.
Clever users may use this knowledge for writing surface names in
requests exactly as if they were pure charsets, when the only need is
to change surfaces without any kind of recoding between real charsets.
In such contexts, either `data' or `tree' may also be used as if it
were some kind of generic, anonymous charset: the request
`data..SURFACE' merely adds the given SURFACE, while the request
`SURFACE..data' removes it.

   The `recode' library distinguishes between mere data surfaces, and
structural surfaces, also called tree surfaces for short.  Structural
surfaces might allow, in the long run, transformations between a few
specialised representations of structural information like MIME parts,
Perl or Python initialisers, LISP S-expressions, XML, Emacs outlines,
etc.

   We are still experimenting with surfaces in `recode'.  The concept
opens the doors to many avenues; it is not clear yet which ones are
worth pursuing, and which should be abandoned.  In particular,
implementation of structural surfaces is barely starting, there is not
even a commitment that tree surfaces will stay in `recode', if they do
prove to be more cumbersome than useful.  This chapter presents all
surfaces currently available.

Permutations

Permuting groups of bytes

End lines

Representation for end of lines

MIME

MIME contents encodings

Dump

Interpreted character dumps

Test

Artificial data for testing

   ---------- Footnotes ----------

   (1) These are mere examples to explain the concept, `recode' only
has `Base64' and `CR-LF', actually.