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 Words; use Words; 21 with Word_Ops; use Word_Ops; 22 with W_Pred; use W_Pred; 23 with W_Shifts; use W_Shifts; 24 25 26 package body FZ_Measr is 27 28 -- Find the index of eldest nonzero bit ( 0 if none, or 1 .. FZBitness ) 29 function FZ_Measure(N : in FZ) return FZBit_Index is 30 31 -- The result (default : 0, will remain 0 if N is in fact zero) 32 Index : Word := 0; 33 34 -- The currently-examined Word, starting from the junior-most 35 Ni : Word; 36 37 -- The most recently-seen nonzero Word, if indeed any exist 38 W : Word := 0; 39 40 -- 1 if currently-examined Word is zero, otherwise 0 41 NiZ : WBool; 42 43 begin 44 45 -- Find, in constant time, eldest non-zero Word: 46 for i in 0 .. Indices(N'Length - 1) loop 47 Ni := N(N'First + i); -- Ni := current Word; 48 NiZ := W_ZeroP(Ni); -- ... is this Word = 0? 49 Index := W_Mux(Word(i), Index, NiZ); -- If NO, save the Index, 50 W := W_Mux(Ni, W, NiZ); -- ... and save that Word. 51 end loop; 52 53 -- Set Index to be the bit-position of the eldest non-zero Word: 54 Index := Shift_Left(Index, BitnessLog2); -- Index := Index * Bitness 55 56 -- Find, in constant time, eldest non-zero bit in that Word: 57 for b in 1 .. Bitness loop 58 59 -- If W is non-zero, advance the Index... 60 Index := Index + W_NZeroP(W); 61 62 -- ... in either case, advance W: 63 W := Shift_Right(W, 1); 64 65 end loop; 66 67 -- If N = 0, result will be 0; otherwise: index of the eldest 1 bit. 68 return FZBit_Index(Index); 69 70 end FZ_Measure; 71 72 end FZ_Measr;