Arbitrary length integer in Ada

ada

Solution

The Ada Crypto Library supports big unsigned numbers (`Big_Numbers`). You can download the lib from http://sourceforge.net/projects/libadacrypt-dev/. I recommend checking out the svn. The Big_Numbers multiplication function of the current release has a minor bug.

You can compile the lib with the current GNAT compiler from the AdaCore Libre site.

The lib will not compile under gcc-4.3 or gcc-4.4 because of a bug in gcc.

Finally, I will give you a small exmple how to multiply two 512-bit Big_Numbers from the LibAdaCrypt.

package Test.Big_Numbers is

with Crypto.Types.Big_Numbers;

pragma Elaborate_All(Crypto.Types.Big_Numbers);

package Big is new Crypto.Types.Big_Numbers(512);
    use Big;
    use Big.Utils;
end Test.Big_Numbers;



package body Test.Big_Numbers is

x : Big_Unsigned := To_Big_Unsigned("16#57C19F8F7866F8633AC1D25B92FC83B4#");
Y : Big_Unsigned := To_Big_Unsigned("16#FFFFFFFFFFFFFFFFFFFFFFFFFFFFFF60#");

x := X * Y;
Put_Line(X);

end Test.Big_Numbers;

 
Best regards
   Christian

Problem

I am currently teaching myself Ada and though I could begin by tackling some of the more conventional problems to get started. More specifically I try to calculate the factorial n!, whereas n>100. My implementation so far is: ``` with Ada.Text_IO; with Ada.Integer_Text_IO; use Ada.Text_IO; procedure Factorial is -- define a type covering the range beginning at 1 up to which faculty is to -- be computed. subtype Argument is Long_Long_Integer range 1..100; -- define a type that is large enough to hold the result subtype Result is Long_Long_Integer range 1..Long_Long_Integer'Last; package Result_IO is new Ada.Text_IO.Integer_IO(Result); use Result_IO; -- variable holding the faculty calculated. fac : Result := 1; begin -- loop over whole range of ARGUMENT and calculate n! for n in ARGUMENT loop fac := (fac * n); end loop; end; ``` The problem is obviously that even Long_Long_Integer is may too small for this and throws a CONTRAINT_ERROR exception for n>20. Is there a package that implements arbitrary-sized integers? Thanks! PS: I did opt against recursion because I wanted to explore loops in this exercise. But otherwise please comment on all aspects of the code (style, best-practices, error..)

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