------------------------------------------------------------------------------
--                                                                          --
--                         GNAT COMPILER COMPONENTS                         --
--                                                                          --
--                  S Y S T E M . A R I T H _ D O U B L E                   --
--                                                                          --
--                                 S p e c                                  --
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--          Copyright (C) 1992-2022, Free Software Foundation, Inc.         --
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-- 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- --
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-- You should have received a copy of the GNU General Public License and    --
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-- <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.      --
--                                                                          --
------------------------------------------------------------------------------

--  This package provides software routines for doing arithmetic on "double"
--  signed integer values in cases where either overflow checking is required,
--  or intermediate results are longer than the result type.

with Ada.Numerics.Big_Numbers.Big_Integers_Ghost;
use Ada.Numerics.Big_Numbers.Big_Integers_Ghost;

generic

   type Double_Int is range <>;

   type Double_Uns is mod <>;

   type Single_Uns is mod <>;

   with function Shift_Left (A : Double_Uns; B : Natural) return Double_Uns
     is <>;

   with function Shift_Right (A : Double_Uns; B : Natural) return Double_Uns
     is <>;

   with function Shift_Left (A : Single_Uns; B : Natural) return Single_Uns
     is <>;

package System.Arith_Double
  with Pure, SPARK_Mode
is
   --  Preconditions in this unit are meant for analysis only, not for run-time
   --  checking, so that the expected exceptions are raised. This is enforced
   --  by setting the corresponding assertion policy to Ignore. Postconditions
   --  and contract cases should not be executed at runtime as well, in order
   --  not to slow down the execution of these functions.

   pragma Assertion_Policy (Pre            => Ignore,
                            Post           => Ignore,
                            Contract_Cases => Ignore,
                            Ghost          => Ignore);

   package Signed_Conversion is new Signed_Conversions (Int => Double_Int);

   function Big (Arg : Double_Int) return Big_Integer is
     (Signed_Conversion.To_Big_Integer (Arg))
   with Ghost;

   package Unsigned_Conversion is new Unsigned_Conversions (Int => Double_Uns);

   function Big (Arg : Double_Uns) return Big_Integer is
     (Unsigned_Conversion.To_Big_Integer (Arg))
   with Ghost;

   function In_Double_Int_Range (Arg : Big_Integer) return Boolean is
     (In_Range (Arg, Big (Double_Int'First), Big (Double_Int'Last)))
   with Ghost;

   function Add_With_Ovflo_Check (X, Y : Double_Int) return Double_Int
   with
     Pre  => In_Double_Int_Range (Big (X) + Big (Y)),
     Post => Add_With_Ovflo_Check'Result = X + Y;
   --  Raises Constraint_Error if sum of operands overflows Double_Int,
   --  otherwise returns the signed integer sum.

   function Subtract_With_Ovflo_Check (X, Y : Double_Int) return Double_Int
   with
     Pre  => In_Double_Int_Range (Big (X) - Big (Y)),
     Post => Subtract_With_Ovflo_Check'Result = X - Y;
   --  Raises Constraint_Error if difference of operands overflows Double_Int,
   --  otherwise returns the signed integer difference.

   function Multiply_With_Ovflo_Check (X, Y : Double_Int) return Double_Int
   with
     Pre  => In_Double_Int_Range (Big (X) * Big (Y)),
     Post => Multiply_With_Ovflo_Check'Result = X * Y;
   pragma Convention (C, Multiply_With_Ovflo_Check);
   --  Raises Constraint_Error if product of operands overflows Double_Int,
   --  otherwise returns the signed integer product. Gigi may also call this
   --  routine directly.

   function Same_Sign (X, Y : Big_Integer) return Boolean is
     (X = Big (Double_Int'(0))
        or else Y = Big (Double_Int'(0))
        or else (X < Big (Double_Int'(0))) = (Y < Big (Double_Int'(0))))
   with Ghost;

   function Round_Quotient (X, Y, Q, R : Big_Integer) return Big_Integer is
     (if abs R > (abs Y - Big (Double_Int'(1))) / Big (Double_Int'(2)) then
       (if Same_Sign (X, Y) then Q + Big (Double_Int'(1))
        else Q - Big (Double_Int'(1)))
      else
        Q)
   with
     Ghost,
     Pre => Y /= 0 and then Q = X / Y and then R = X rem Y;

   procedure Scaled_Divide
     (X, Y, Z : Double_Int;
      Q, R    : out Double_Int;
      Round   : Boolean)
   with
     Pre  => Z /= 0
       and then In_Double_Int_Range
         (if Round then Round_Quotient (Big (X) * Big (Y), Big (Z),
                                        Big (X) * Big (Y) / Big (Z),
                                        Big (X) * Big (Y) rem Big (Z))
          else Big (X) * Big (Y) / Big (Z)),
     Post => Big (R) = Big (X) * Big (Y) rem Big (Z)
       and then
         (if Round then
            Big (Q) = Round_Quotient (Big (X) * Big (Y), Big (Z),
                                      Big (X) * Big (Y) / Big (Z), Big (R))
          else
            Big (Q) = Big (X) * Big (Y) / Big (Z));
   --  Performs the division of (X * Y) / Z, storing the quotient in Q
   --  and the remainder in R. Constraint_Error is raised if Z is zero,
   --  or if the quotient does not fit in Double_Int. Round indicates if
   --  the result should be rounded. If Round is False, then Q, R are
   --  the normal quotient and remainder from a truncating division.
   --  If Round is True, then Q is the rounded quotient. The remainder
   --  R is not affected by the setting of the Round flag.

   procedure Double_Divide
     (X, Y, Z : Double_Int;
      Q, R    : out Double_Int;
      Round   : Boolean)
   with
     Pre  => Y /= 0
       and then Z /= 0
       and then In_Double_Int_Range
         (if Round then Round_Quotient (Big (X), Big (Y) * Big (Z),
                                        Big (X) / (Big (Y) * Big (Z)),
                                        Big (X) rem (Big (Y) * Big (Z)))
          else Big (X) / (Big (Y) * Big (Z))),
     Post => Big (R) = Big (X) rem (Big (Y) * Big (Z))
       and then
         (if Round then
            Big (Q) = Round_Quotient (Big (X), Big (Y) * Big (Z),
                                      Big (X) / (Big (Y) * Big (Z)), Big (R))
          else
            Big (Q) = Big (X) / (Big (Y) * Big (Z)));
   --  Performs the division X / (Y * Z), storing the quotient in Q and
   --  the remainder in R. Constraint_Error is raised if Y or Z is zero,
   --  or if the quotient does not fit in Double_Int. Round indicates if the
   --  result should be rounded. If Round is False, then Q, R are the normal
   --  quotient and remainder from a truncating division. If Round is True,
   --  then Q is the rounded quotient. The remainder R is not affected by the
   --  setting of the Round flag.

end System.Arith_Double;
