------------------------------------------------------------------------------ | |

-- -- | |

-- GNAT COMPILER COMPONENTS -- | |

-- -- | |

-- S Y S T E M . V A L U E _ F -- | |

-- -- | |

-- B o d y -- | |

-- -- | |

-- Copyright (C) 2020-2021, 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. -- | |

-- -- | |

------------------------------------------------------------------------------ | |

with System.Unsigned_Types; use System.Unsigned_Types; | |

with System.Val_Util; use System.Val_Util; | |

with System.Value_R; | |

package body System.Value_F is | |

-- The prerequisite of the implementation is that the computation of the | |

-- operands of the scaled divide does not unduly overflow when the small | |

-- is neither an integer nor the reciprocal of an integer, which means | |

-- that its numerator and denominator must be both not larger than the | |

-- smallest divide 2**(Int'Size - 1) / Base where Base ranges over the | |

-- supported values for the base of the literal. Given that the largest | |

-- supported base is 16, this gives a limit of 2**(Int'Size - 5). | |

pragma Assert (Int'Size <= Uns'Size); | |

-- We need an unsigned type large enough to represent the mantissa | |

package Impl is new Value_R (Uns, 2**(Int'Size - 1), Round => True); | |

-- We use the Extra digit for ordinary fixed-point types | |

function Integer_To_Fixed | |

(Str : String; | |

Val : Uns; | |

Base : Unsigned; | |

ScaleB : Integer; | |

Extra : Unsigned; | |

Minus : Boolean; | |

Num : Int; | |

Den : Int) return Int; | |

-- Convert the real value from integer to fixed point representation | |

-- The goal is to compute Val * (Base ** ScaleB) / (Num / Den) with correct | |

-- rounding for all decimal values output by Typ'Image, that is to say up | |

-- to Typ'Aft decimal digits. Unlike for the output, the RM does not say | |

-- what the rounding must be for the input, but a reasonable exegesis of | |

-- the intent is that Typ'Value o Typ'Image should be the identity, which | |

-- is made possible because 'Aft is defined such that 'Image is injective. | |

-- For a type with a mantissa of M bits including the sign, the number N1 | |

-- of decimal digits required to represent all the numbers is given by: | |

-- N1 = ceil ((M - 1) * log 2 / log 10) [N1 = 10/19/39 for M = 32/64/128] | |

-- but this mantissa can represent any set of contiguous numbers with only | |

-- N2 different decimal digits where: | |

-- N2 = floor ((M - 1) * log 2 / log 10) [N2 = 9/18/38 for M = 32/64/128] | |

-- Of course N1 = N2 + 1 holds, which means both that Val may not contain | |

-- enough significant bits to represent all the values of the type and that | |

-- 1 extra decimal digit contains the information for the missing bits. | |

-- Therefore the actual computation to be performed is | |

-- V = (Val * Base + Extra) * (Base ** (ScaleB - 1)) / (Num / Den) | |

-- using two steps of scaled divide if Extra is positive and ScaleB too | |

-- (1) Val * (Den * (Base ** ScaleB)) = Q1 * Num + R1 | |

-- (2) Extra * (Den * (Base ** ScaleB)) = Q2 * -Base + R2 | |

-- which yields after dividing (1) by Num and (2) by Num * Base and summing | |

-- V = Q1 + (R1 - Q2) / Num + R2 / (Num * Base) | |

-- but we get rid of the third term by using a rounding divide for (2). | |

-- This works only if Den * (Base ** ScaleB) does not overflow for inputs | |

-- corresponding to 'Image. Let S = Num / Den, B = Base and N the scale in | |

-- base B of S, i.e. the smallest integer such that B**N * S >= 1. Then, | |

-- for X a positive of the mantissa, i.e. 1 <= X <= 2**(M-1), we have | |

-- 1/B <= X * S * B**(N-1) < 2**(M-1) | |

-- which means that the inputs corresponding to the output of 'Image have a | |

-- ScaleB equal either to 1 - N or (after multiplying the inequality by B) | |

-- to -N, possibly after renormalizing X, i.e. multiplying it by a suitable | |

-- power of B. Therefore | |

-- Den * (Base ** ScaleB) <= Den * (B ** (1 - N)) < Num * B | |

-- which means that the product does not overflow if Num <= 2**(M-1) / B. | |

-- On the other hand, if Extra is positive and ScaleB negative, the above | |

-- two steps are | |

-- (1b) Val * Den = Q1 * (Num * (Base ** -ScaleB)) + R1 | |

-- (2b) Extra * Den = Q2 * -Base + R2 | |

-- which yields after dividing (1b) by Num * (Base ** -ScaleB) and (2b) by | |

-- Num * (Base ** (1 - ScaleB)) and summing | |

-- V = Q1 + (R1 - Q2) / (Num * (Base ** -ScaleB)) + R2 / ... | |

-- but we get rid of the third term by using a rounding divide for (2b). | |

-- This works only if Num * (Base ** -ScaleB) does not overflow for inputs | |

-- corresponding to 'Image. With the determination of ScaleB above, we have | |

-- Num * (Base ** -ScaleB) <= Num * (B ** N) < Den * B | |

-- which means that the product does not overflow if Den <= 2**(M-1) / B. | |

---------------------- | |

-- Integer_To_Fixed -- | |

---------------------- | |

function Integer_To_Fixed | |

(Str : String; | |

Val : Uns; | |

Base : Unsigned; | |

ScaleB : Integer; | |

Extra : Unsigned; | |

Minus : Boolean; | |

Num : Int; | |

Den : Int) return Int | |

is | |

pragma Assert (Base in 2 .. 16); | |

pragma Assert (Extra < Base); | |

-- Accept only one extra digit after those used for Val | |

pragma Assert (Num < 0 and then Den < 0); | |

-- Accept only negative numbers to allow -2**(Int'Size - 1) | |

function Safe_Expont | |

(Base : Int; | |

Exp : in out Natural; | |

Factor : Int) return Int; | |

-- Return (Base ** Exp) * Factor if the computation does not overflow, | |

-- or else the number of the form (Base ** K) * Factor with the largest | |

-- magnitude if the former computation overflows. In both cases, Exp is | |

-- updated to contain the remaining power in the computation. Note that | |

-- Factor is expected to be negative in this context. | |

function Unsigned_To_Signed (Val : Uns) return Int; | |

-- Convert an integer value from unsigned to signed representation | |

----------------- | |

-- Safe_Expont -- | |

----------------- | |

function Safe_Expont | |

(Base : Int; | |

Exp : in out Natural; | |

Factor : Int) return Int | |

is | |

pragma Assert (Base /= 0 and then Factor < 0); | |

Min : constant Int := Int'First / Base; | |

Result : Int := Factor; | |

begin | |

while Exp > 0 and then Result >= Min loop | |

Result := Result * Base; | |

Exp := Exp - 1; | |

end loop; | |

return Result; | |

end Safe_Expont; | |

------------------------ | |

-- Unsigned_To_Signed -- | |

------------------------ | |

function Unsigned_To_Signed (Val : Uns) return Int is | |

begin | |

-- Deal with overflow cases, and also with largest negative number | |

if Val > Uns (Int'Last) then | |

if Minus and then Val = Uns (-(Int'First)) then | |

return Int'First; | |

else | |

Bad_Value (Str); | |

end if; | |

-- Negative values | |

elsif Minus then | |

return -(Int (Val)); | |

-- Positive values | |

else | |

return Int (Val); | |

end if; | |

end Unsigned_To_Signed; | |

-- Local variables | |

B : constant Int := Int (Base); | |

V : Uns := Val; | |

E : Uns := Uns (Extra); | |

Y, Z, Q1, R1, Q2, R2 : Int; | |

begin | |

-- We will use a scaled divide operation for which we must control the | |

-- magnitude of operands so that an overflow exception is not unduly | |

-- raised during the computation. The only real concern is the exponent. | |

-- If ScaleB is too negative, then drop trailing digits, but preserve | |

-- the last dropped digit. | |

if ScaleB < 0 then | |

declare | |

LS : Integer := -ScaleB; | |

begin | |

Y := Den; | |

Z := Safe_Expont (B, LS, Num); | |

for J in 1 .. LS loop | |

E := V rem Uns (B); | |

V := V / Uns (B); | |

end loop; | |

end; | |

-- If ScaleB is too positive, then scale V up, which may then overflow | |

elsif ScaleB > 0 then | |

declare | |

LS : Integer := ScaleB; | |

begin | |

Y := Safe_Expont (B, LS, Den); | |

Z := Num; | |

for J in 1 .. LS loop | |

if V <= (Uns'Last - E) / Uns (B) then | |

V := V * Uns (B) + E; | |

E := 0; | |

else | |

Bad_Value (Str); | |

end if; | |

end loop; | |

end; | |

-- If ScaleB is zero, then proceed directly | |

else | |

Y := Den; | |

Z := Num; | |

end if; | |

-- Perform a scaled divide operation with final rounding to match Image | |

-- using two steps if there is an extra digit available. The second and | |

-- third operands are always negative so the sign of the quotient is the | |

-- sign of the first operand and the sign of the remainder the opposite. | |

if E > 0 then | |

Scaled_Divide (Unsigned_To_Signed (V), Y, Z, Q1, R1, Round => False); | |

Scaled_Divide (Unsigned_To_Signed (E), Y, -B, Q2, R2, Round => True); | |

-- Avoid an overflow during the subtraction. Note that Q2 is smaller | |

-- than Y and R1 smaller than Z in magnitude, so it is safe to take | |

-- their absolute value. | |

if abs Q2 >= 2 ** (Int'Size - 2) | |

or else abs R1 >= 2 ** (Int'Size - 2) | |

then | |

declare | |

Bit : constant Int := Q2 rem 2; | |

begin | |

Q2 := (Q2 - Bit) / 2; | |

R1 := (R1 - Bit) / 2; | |

Y := -2; | |

end; | |

else | |

Y := -1; | |

end if; | |

Scaled_Divide (Q2 - R1, Y, Z, Q2, R2, Round => True); | |

return Q1 + Q2; | |

else | |

Scaled_Divide (Unsigned_To_Signed (V), Y, Z, Q1, R1, Round => True); | |

return Q1; | |

end if; | |

exception | |

when Constraint_Error => Bad_Value (Str); | |

end Integer_To_Fixed; | |

---------------- | |

-- Scan_Fixed -- | |

---------------- | |

function Scan_Fixed | |

(Str : String; | |

Ptr : not null access Integer; | |

Max : Integer; | |

Num : Int; | |

Den : Int) return Int | |

is | |

Base : Unsigned; | |

ScaleB : Integer; | |

Extra : Unsigned; | |

Minus : Boolean; | |

Val : Uns; | |

begin | |

Val := Impl.Scan_Raw_Real (Str, Ptr, Max, Base, ScaleB, Extra, Minus); | |

return Integer_To_Fixed (Str, Val, Base, ScaleB, Extra, Minus, Num, Den); | |

end Scan_Fixed; | |

----------------- | |

-- Value_Fixed -- | |

----------------- | |

function Value_Fixed | |

(Str : String; | |

Num : Int; | |

Den : Int) return Int | |

is | |

Base : Unsigned; | |

ScaleB : Integer; | |

Extra : Unsigned; | |

Minus : Boolean; | |

Val : Uns; | |

begin | |

Val := Impl.Value_Raw_Real (Str, Base, ScaleB, Extra, Minus); | |

return Integer_To_Fixed (Str, Val, Base, ScaleB, Extra, Minus, Num, Den); | |

end Value_Fixed; | |

end System.Value_F; |