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1994-05-19
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------------------------------------------------------------------------------
-- --
-- GNAT RUNTIME COMPONENTS --
-- --
-- S Y S T E M . X P --
-- --
-- B o d y --
-- --
-- $Revision: 1.3 $ --
-- --
-- Copyright (c) 1992,1993,1994 NYU, All Rights Reserved --
-- --
-- GNAT is free software; you can redistribute it and/or modify it under --
-- terms of the GNU General Public License as published by the Free Soft- --
-- ware Foundation; either version 2, or (at your option) any later ver- --
-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
-- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
-- for more details. You should have received a copy of the GNU General --
-- Public License distributed with GNAT; see file COPYING. If not, write --
-- to the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. --
-- --
------------------------------------------------------------------------------
package body System.Xp is
--------------------------
-- Exponentiate_Integer --
--------------------------
-- Note that negative exponents get a constraint error because the
-- subtype of the Right argument (the exponent) is Natural.
function Exponentiate_Integer
(Left : Type_Of_Base; Right : Natural) return Type_Of_Base
is
Result : Type_Of_Base := 1;
Factor : Type_Of_Base := Left;
Exp : Natural := Right;
begin
-- We use the standard logarithmic approach, Exp gets shifted right
-- testing successive low order bits and Factor is the value of the
-- base raised to the next power of 2.
-- Note: it is not worth special casing the cases of base values -1,0,+1
-- since the expander does this when the base is a literal, and other
-- cases will be extremely rare.
while Exp /= 0 loop
if Exp rem 2 /= 0 then
Result := Result * Factor;
end if;
Factor := Factor * Factor;
Exp := Exp / 2;
end loop;
return Result;
end Exponentiate_Integer;
------------------------
-- Exponentiate_Float --
------------------------
function Exponentiate_Float
(Left : Type_Of_Base; Right : Integer) return Type_Of_Base
is
Result : Type_Of_Base := 1.0;
Factor : Type_Of_Base := Left;
Exp : Natural := Right;
begin
-- We use the standard logarithmic approach, Exp gets shifted right
-- testing successive low order bits and Factor is the value of the
-- base raised to the next power of 2. For positive exponents we
-- multiply the result by this factor, for negative exponents, we
-- divide by this factor.
if Exp >= 0 then
-- For a positive exponent, if we get a constraint error during
-- this loop, it is an overflow, and the constraint error will
-- simply be passed on to the caller.
while Exp /= 0 loop
if Exp rem 2 /= 0 then
Result := Result * Factor;
end if;
Factor := Factor * Factor;
Exp := Exp / 2;
end loop;
return Result;
else -- Exp < 0 then
-- For the negative exponent case, a constraint error during this
-- calculation happens if Factor gets too large, and the proper
-- response is to return 0.0, since what we essenmtially have is
-- 1.0 / infinity, and the closest model number will be zero.
begin
while Exp /= 0 loop
if Exp rem 2 /= 0 then
Result := Result * Factor;
end if;
Factor := Factor * Factor;
Exp := Exp / 2;
end loop;
return 1.0 / Result;
exception
when Constraint_Error =>
return 0.0;
end;
end if;
end Exponentiate_Float;
end System.Xp;