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------------------------------------------------------------------------------
-- --
-- GNAT RUN-TIME COMPONENTS --
-- --
-- S Y S T E M . E X P O N T --
-- --
-- B o d y --
-- --
-- Copyright (C) 1992-2024, Free Software Foundation, Inc. --
-- --
-- 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 3, 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. --
-- --
-- As a special exception under Section 7 of GPL version 3, you are granted --
-- additional permissions described in the GCC Runtime Library Exception, --
-- version 3.1, as published by the Free Software Foundation. --
-- --
-- You should have received a copy of the GNU General Public License and --
-- a copy of the GCC Runtime Library Exception along with this program; --
-- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see --
-- <http://www.gnu.org/licenses/>. --
-- --
-- GNAT was originally developed by the GNAT team at New York University. --
-- Extensive contributions were provided by Ada Core Technologies Inc. --
-- --
------------------------------------------------------------------------------
package body System.Expont
with SPARK_Mode
is
-- Preconditions, postconditions, ghost code, loop invariants and
-- assertions in this unit are meant for analysis only, not for run-time
-- checking, as it would be too costly otherwise. This is enforced by
-- setting the assertion policy to Ignore.
pragma Assertion_Policy (Pre => Ignore,
Post => Ignore,
Ghost => Ignore,
Loop_Invariant => Ignore,
Assert => Ignore);
-- Local lemmas
procedure Lemma_Exp_Expand (A : Big_Integer; Exp : Natural)
with
Ghost,
Pre => A /= 0,
Post =>
(if Exp rem 2 = 0 then
A ** Exp = A ** (Exp / 2) * A ** (Exp / 2)
else
A ** Exp = A ** (Exp / 2) * A ** (Exp / 2) * A);
procedure Lemma_Exp_In_Range (A : Big_Integer; Exp : Positive)
with
Ghost,
Pre => In_Int_Range (A ** Exp * A ** Exp),
Post => In_Int_Range (A * A);
procedure Lemma_Exp_Not_Zero (A : Big_Integer; Exp : Natural)
with
Ghost,
Pre => A /= 0,
Post => A ** Exp /= 0;
procedure Lemma_Exp_Positive (A : Big_Integer; Exp : Natural)
with
Ghost,
Pre => A /= 0
and then Exp rem 2 = 0,
Post => A ** Exp > 0;
procedure Lemma_Mult_In_Range (X, Y, Z : Big_Integer)
with
Ghost,
Pre => Y /= 0
and then not (X = -Big (Int'First) and Y = -1)
and then X * Y = Z
and then In_Int_Range (Z),
Post => In_Int_Range (X);
-----------------------------
-- Local lemma null bodies --
-----------------------------
procedure Lemma_Exp_Not_Zero (A : Big_Integer; Exp : Natural) is null;
procedure Lemma_Mult_In_Range (X, Y, Z : Big_Integer) is null;
-----------
-- Expon --
-----------
function Expon (Left : Int; Right : Natural) return Int is
-- Note that negative exponents get a constraint error because the
-- subtype of the Right argument (the exponent) is Natural.
Result : Int := 1;
Factor : Int := Left;
Exp : Natural := Right;
Rest : Big_Integer with Ghost;
-- Ghost variable to hold Factor**Exp between Exp and Factor updates
begin
pragma Annotate (Gnatcheck, Exempt_On, "Improper_Returns",
"early returns for performance");
-- 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: for compilation only, it is not worth special casing base
-- values -1, 0, +1 since the expander does this when the base is a
-- literal, and other cases will be extremely rare. But for proof,
-- special casing zero in both positions makes ghost code and lemmas
-- simpler, so we do it.
if Right = 0 then
return 1;
elsif Left = 0 then
return 0;
end if;
loop
pragma Loop_Invariant (Exp > 0);
pragma Loop_Invariant (Factor /= 0);
pragma Loop_Invariant
(Big (Result) * Big (Factor) ** Exp = Big (Left) ** Right);
pragma Loop_Variant (Decreases => Exp);
if Exp rem 2 /= 0 then
declare
pragma Unsuppress (Overflow_Check);
begin
pragma Assert
(Big (Factor) ** Exp
= Big (Factor) * Big (Factor) ** (Exp - 1));
Lemma_Exp_Positive (Big (Factor), Exp - 1);
Lemma_Mult_In_Range (Big (Result) * Big (Factor),
Big (Factor) ** (Exp - 1),
Big (Left) ** Right);
Result := Result * Factor;
end;
end if;
Lemma_Exp_Expand (Big (Factor), Exp);
Exp := Exp / 2;
exit when Exp = 0;
Rest := Big (Factor) ** Exp;
pragma Assert
(Big (Result) * (Rest * Rest) = Big (Left) ** Right);
declare
pragma Unsuppress (Overflow_Check);
begin
Lemma_Mult_In_Range (Rest * Rest,
Big (Result),
Big (Left) ** Right);
Lemma_Exp_In_Range (Big (Factor), Exp);
Factor := Factor * Factor;
end;
pragma Assert (Big (Factor) ** Exp = Rest * Rest);
end loop;
pragma Assert (Big (Result) = Big (Left) ** Right);
return Result;
pragma Annotate (Gnatcheck, Exempt_Off, "Improper_Returns");
end Expon;
----------------------
-- Lemma_Exp_Expand --
----------------------
procedure Lemma_Exp_Expand (A : Big_Integer; Exp : Natural) is
begin
if Exp rem 2 = 0 then
pragma Assert (Exp = Exp / 2 + Exp / 2);
else
pragma Assert (Exp = Exp / 2 + Exp / 2 + 1);
pragma Assert (A ** Exp = A ** (Exp / 2) * A ** (Exp / 2 + 1));
pragma Assert (A ** (Exp / 2 + 1) = A ** (Exp / 2) * A);
pragma Assert (A ** Exp = A ** (Exp / 2) * A ** (Exp / 2) * A);
end if;
end Lemma_Exp_Expand;
------------------------
-- Lemma_Exp_In_Range --
------------------------
procedure Lemma_Exp_In_Range (A : Big_Integer; Exp : Positive) is
begin
if A /= 0 and Exp /= 1 then
pragma Assert (A ** Exp = A * A ** (Exp - 1));
Lemma_Mult_In_Range
(A * A, A ** (Exp - 1) * A ** (Exp - 1), A ** Exp * A ** Exp);
end if;
end Lemma_Exp_In_Range;
------------------------
-- Lemma_Exp_Positive --
------------------------
procedure Lemma_Exp_Positive (A : Big_Integer; Exp : Natural) is
begin
if Exp = 0 then
pragma Assert (A ** Exp = 1);
else
pragma Assert (Exp = 2 * (Exp / 2));
pragma Assert (A ** Exp = A ** (Exp / 2) * A ** (Exp / 2));
pragma Assert (A ** Exp = (A ** (Exp / 2)) ** 2);
Lemma_Exp_Not_Zero (A, Exp / 2);
end if;
end Lemma_Exp_Positive;
end System.Expont;
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