.. mode: -*- rst -*-

Transliterating the alphabet into hexadecimal
=============================================

:Tag: guide.hex.trans
:Status: incomplete documentation
:Author: Gavin Matthews
:Organization: Harlequin
:Date: 1997-04-11
:Revision: $Id: //info.ravenbrook.com/project/mps/version/1.113/design/guide.hex.trans.txt#1 $
:Copyright: See section `Copyright and License`_.
:Index terms: pair: hexadecimal; transliterating


Introduction
------------

_`.scope`: This document explains how to represent the alphabet as
hexadecimal digits.

_`.readership`: This document is intended for anyone devising
arbitrary constants which may appear in hex-dumps.

_`.sources`: This transliteration was supplied by RichardK in
mail.richardk.1997-04-07.13-44.


Transliteration
---------------

_`.forward`: The chosen transliteration is as follows::

    ABCDEFGHIJKLMNOPQRSTUVWXYZ
    ABCDEF9811C7340BC6520F3812

_`.backward`: The backwards transliteration is as follows::

    0 OU
    1 IJY
    2 TZ
    3 MW
    4 N
    5 S
    6 R
    7 L
    8 HX
    9 G
    A A
    B BP
    C CKQ
    D D
    E E
    F FV

_`.pad`: If padding is required (to fill a hex constant length), you
should use 9's, because G is rare and can usually be inferred from
context.

_`.punc`: There is no formal scheme for spaces, or punctuation. It is
suggested that you use 9 (as `.pad`_).


Justification
--------------

_`.letters`: The hexadecimal letters (A-F) are all formed by
similarity of sound. B and P sound similar, as do F and V, and C, K, &
Q can all sound similar.

_`.numbers`: The numbers (0-9) are all formed by similarity of shape
(but see `.trans.t`_). Nevertheless, 1=IJY retains some similarity of
sound.

_`.trans.t`: T is an exception to `.numbers`_, but is such a common
letter that it deserves it.


4. Notes
--------

_`.change`: This transliteration differs from the old transliteration
used for signatures (see design.mps.sig(0)), as follows: J:6->1;
L:1->7; N:9->4; R:4->6; W:8->3; X:5->8; Y:E->I.

_`.problem.mw`: There is a known problem that M and W are both common,
map to the same digit (3), and are hard to distinguish in context.

_`.find.c`: It is possible to find all 8-digit hexadecimal constants
and how many times they're used in C files, using the following Perl
script::

    perl5 -n -e 'BEGIN { %C=(); } if(/0x([0-9A-Fa-f]{8})/) { $C{$1} = +[] if(
    !defined($C{$1})); push(@{$C{$1}}, $ARGV); } END { foreach $H (sort(keys(%C)))
    { printf "%3d %s %s\n", scalar(@{$C{$H}}), $H, join(", ", @{@C{$H}}); } }' *.c
    *.h

_`.comment`: It is a good idea to add a comment to any constant
declaration indicating the English version and which letters were
selected (by capitalisation), e.g.::

    #define SpaceSig        ((Sig)0x5195BACE) /* SIGnature SPACE */


Document History
----------------
2013-05-10  RB_  Converted to reStructuredText and imported to MPS design.

.. _RB: http://www.ravenbrook.com/consultants/rb/


Copyright and License
---------------------

Copyright © 2013-2014 Ravenbrook Limited. All rights reserved.
<http://www.ravenbrook.com/>. This is an open source license. Contact
Ravenbrook for commercial licensing options.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

#. Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.

#. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.

#. Redistributions in any form must be accompanied by information on how
   to obtain complete source code for this software and any
   accompanying software that uses this software.  The source code must
   either be included in the distribution or be available for no more than
   the cost of distribution plus a nominal fee, and must be freely
   redistributable under reasonable conditions.  For an executable file,
   complete source code means the source code for all modules it contains.
   It does not include source code for modules or files that typically
   accompany the major components of the operating system on which the
   executable file runs.

**This software is provided by the copyright holders and contributors
"as is" and any express or implied warranties, including, but not
limited to, the implied warranties of merchantability, fitness for a
particular purpose, or non-infringement, are disclaimed.  In no event
shall the copyright holders and contributors be liable for any direct,
indirect, incidental, special, exemplary, or consequential damages
(including, but not limited to, procurement of substitute goods or
services; loss of use, data, or profits; or business interruption)
however caused and on any theory of liability, whether in contract,
strict liability, or tort (including negligence or otherwise) arising in
any way out of the use of this software, even if advised of the
possibility of such damage.**
