# File : fz_basic.adb

```   1 ------------------------------------------------------------------------------
2 ------------------------------------------------------------------------------
3 -- This file is part of 'Finite Field Arithmetic', aka 'FFA'.               --
4 --                                                                          --
5 -- (C) 2017 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 Word_Ops; use Word_Ops;
21
22
23 package body FZ_Basic is
24
25    ---------------------------------------------------------------------------
26    -- Fundamental Operations on FZ (finite integers)
27    ---------------------------------------------------------------------------
28
29    -- Determine the Bitness of N
30    function FZ_Bitness(N : in FZ) return Bit_Count is
31    begin
32       return N'Length * Words.Bitness;
33    end FZ_Bitness;
34
35
36    -- Determine the Bitness of the given FZ's Length
37    function FZ_Bitness_Log2(N : in FZ) return Positive is
38       W : Bit_Count := N'Length;
39       R : Positive  := 1;
40    begin
41       while W > 1 loop
42          W := W / 2;
43          R := R + 1;
44       end loop;
45       return R - 1;
46    end FZ_Bitness_Log2;
47
48
49    -- N := 0
50    procedure FZ_Clear(N : out FZ) is
51    begin
52       N := (others => 0);
53    end FZ_Clear;
54
55
56    -- Set given FZ to a given truth value
57    procedure WBool_To_FZ(V : in WBool; N : out FZ) is
58    begin
59       FZ_Clear(N);
60       FZ_Set_Head(N, V);
61    end WBool_To_FZ;
62
63
64    -- First word of N := Source
65    procedure FZ_Set_Head(N : out FZ; Source : in Word) is
66    begin
67       N(N'First) := Source;
68    end FZ_Set_Head;
69
70
71    -- First word of N
72    function FZ_Get_Head(N : in FZ) return Word is
73    begin
74       return N(N'First);
75    end FZ_Get_Head;
76
77
78    -- Exchange X and Y
79    procedure FZ_Swap(X : in out FZ; Y : in out FZ) is
80       T : FZ(X'Range);
81    begin
82       T := X;
83       X := Y;
84       Y := T;
85    end FZ_Swap;
86
87
88    -- Constant-time MUX: Sel = 0: Result := X; Sel = 1: Result := Y
89    procedure FZ_Mux(X : in FZ; Y : in FZ; Result : out FZ; Sel : in WBool) is
90    begin
91       for i in X'Range loop
92          Result(i) := W_Mux(X(i), Y(i), Sel);
93       end loop;
94    end FZ_Mux;
95
96 end FZ_Basic;
```