File : words.ads

   1 ------------------------------------------------------------------------------
   2 ------------------------------------------------------------------------------
   3 -- This file is part of 'Finite Field Arithmetic', aka 'FFA'.               --
   4 --                                                                          --
   5 -- (C) 2019 Stanislav Datskovskiy ( www.loper-os.org )                      --
   6 -- http://wot.deedbot.org/17215D118B7239507FAFED98B98228A001ABFFC7.html     --
   7 --                                                                          --
   8 -- You do not have, nor can you ever acquire the right to use, copy or      --
   9 -- distribute this software ; Should you use this software for any purpose, --
  10 -- or copy and distribute it to anyone or in any manner, you are breaking   --
  11 -- the laws of whatever soi-disant jurisdiction, and you promise to         --
  12 -- continue doing so for the indefinite future. In any case, please         --
  13 -- always : read and understand any software ; verify any PGP signatures    --
  14 -- that you use - for any purpose.                                          --
  15 --                                                                          --
  16 -- See also http://trilema.com/2015/a-new-software-licensing-paradigm .     --
  17 ------------------------------------------------------------------------------
  18 ------------------------------------------------------------------------------
  19 
  20 with Iron;
  21 
  22 package Words is
  23    
  24    pragma Pure;
  25    
  26    -- The most fundamental fact about Word: width.
  27    Bitness     : constant Positive := Iron.MachineBitness;
  28    
  29    -- It is possible to calculate BitnessLog2 at elaboration time,
  30    -- but we will avoid having ~any~ elaboration at all in FFA.
  31    BitnessLog2 : constant Positive := Iron.MachineBitnessLog2;
  32    
  33    -- The Word width, expressed in ~bytes~:
  34    Byteness    : constant Positive := Bitness / Iron.ByteBits;
  35    
  36    -- Nibbles per Word:
  37    Nibbleness  : constant Positive := Byteness * 2;
  38    
  39    -- What kind of words to use. Must be machine-word or smaller.
  40    type Word is mod 2**Bitness;
  41    for Word'Size use Bitness;
  42    
  43    -- The very same Word, but its only legal values are 0 and 1.
  44    subtype WBool is Word range 0 .. 1;
  45    
  46    -- Word, restricted to Nibble range.
  47    subtype Nibble is Word range 0 .. 16#F#;
  48    
  49    -- When we must refer to individual bit positions of a machine word:
  50    subtype WBit_Index is Natural range 0 .. Bitness - 1;
  51    
  52    -- For when a computed Word is mandatorily expected to equal zero:
  53    subtype WZeroOrDie is Word range 0 .. 0;
  54    
  55 end Words;