File : fz_measr.adb
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2 ------------------------------------------------------------------------------
3 -- This file is part of 'Finite Field Arithmetic', aka 'FFA'. --
4 -- --
5 -- (C) 2018 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 . --
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18 ------------------------------------------------------------------------------
19
20 with Word_Ops; use Word_Ops;
21 with W_Pred; use W_Pred;
22 with W_Shifts; use W_Shifts;
23
24
25 package body FZ_Measr is
26
27 -- Find the index of eldest nonzero bit ( 0 if none, or 1 .. FZBitness )
28 function FZ_Measure(N : in FZ) return Word is
29
30 -- The result (default : 0, will remain 0 if N is in fact zero)
31 Index : Word := 0;
32
33 -- The currently-examined Word, starting from the junior-most
34 Ni : Word;
35
36 -- The most recently-seen nonzero Word, if indeed any exist
37 W : Word := 0;
38
39 -- 1 if currently-examined Word is zero, otherwise 0
40 NiZ : WBool;
41
42 begin
43
44 -- Find, in constant time, eldest non-zero Word:
45 for i in 0 .. Indices(N'Length - 1) loop
46 Ni := N(N'First + i); -- Ni := current Word;
47 NiZ := W_ZeroP(Ni); -- ... is this Word = 0?
48 Index := W_Mux(Word(i), Index, NiZ); -- If NO, save the Index,
49 W := W_Mux(Ni, W, NiZ); -- ... and save that Word.
50 end loop;
51
52 -- Set Index to be the bit-position of the eldest non-zero Word:
53 Index := Shift_Left(Index, BitnessLog2); -- Index := Index * Bitness
54
55 -- Find, in constant time, eldest non-zero bit in that Word:
56 for b in 1 .. Bitness loop
57 -- If W is non-zero, advance the Index...
58 Index := W_Mux(Index + 1, Index, W_ZeroP(W));
59 -- ... in either case, advance W:
60 W := Shift_Right(W, 1);
61 end loop;
62
63 -- If N = 0, result will be 0; otherwise: index of the eldest 1 bit.
64 return Index;
65
66 end FZ_Measure;
67
68 end FZ_Measr;