From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: a07f3367d7,a97feceab5b57877
X-Google-Attributes: gida07f3367d7,public,usenet
X-Google-NewGroupId: yes
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news2.google.com!news1.google.com!news.glorb.com!newsfeed1.swip.net!newsfeed1.funet.fi!newsfeeds.funet.fi!feeder1.news.saunalahti.fi!feeder2.news.saunalahti.fi!fi.sn.net!newsfeed2.tdcnet.fi!news.song.fi!not-for-mail
Date: Wed, 29 Jul 2009 22:14:44 +0300
From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Organization: Tidorum Ltd
User-Agent: Mozilla-Thunderbird 2.0.0.22 (X11/20090706)
MIME-Version: 1.0
Newsgroups: comp.lang.ada
Subject: Re: Saturation arithmetic woes.
References: 
 <1a16b458-0201-4320-9787-2836ed58f991@e27g2000yqm.googlegroups.com>
In-Reply-To: 
 <1a16b458-0201-4320-9787-2836ed58f991@e27g2000yqm.googlegroups.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Message-ID: <4a709faf$0$6282$4f793bc4@news.tdc.fi>
NNTP-Posting-Host: 81.17.205.61
X-Trace: 1248894895 news.tdc.fi 6282 81.17.205.61:42337
X-Complaints-To: abuse@tdcnet.fi
Xref: g2news2.google.com comp.lang.ada:7423
Date: 2009-07-29T22:14:44+03:00
List-Id: <comp.lang.ada>

In addition to the replies from Martin and Dmitry regarding the absence 
of a "+" operator for Discrete_Type, I would like to comment on the 
logic of your saturated-"+":

xorque wrote:
> package body Saturation is
> 
>   function "+"
>     (Left  : Saturated_Type;
>      Right : Saturated_Type) return Saturated_Type
>   is
>     Temp_Left  : constant Discrete_Type := Discrete_Type (Left);
>     Temp_Right : constant Discrete_Type := Discrete_Type (Right);
>   begin
>     if Temp_Left + Temp_Right > Discrete_Type'Last then

Depending on the actual type associated with Discrete_Type (and assuming 
you use the predefined "+" for that type), you risk getting an overflow 
on Temp_Left + Temp_Right. If you have overflow checks disabled (as Gnat 
has by default) the sum may "wrap around" and seem to be less than 
Discrete_Type'Last, giving you the wrong result. If you have overflow 
checks enabled, you get a Constraint_Error instead of 
Discrete_Type'Last. Ditto for underflow and Discrete_Type'First.

One option is to have two formal types, one that defines the range for 
saturation (as Discrete_Type in your code) and another that is wide 
enough to compute the sum of Left and Right without under- or overflow.

If the other type is called Wide_Type, you could then replace Temp_Left 
and Temp_Right with

    Sum : constant Wide_Type := Wide_Type (Left) + Wide_Type (Right);

which would simplify your code a bit. Or just write one expression:

    return Discrete_Type (
       Wide_Type'Min (Wide_Type (Discrete_Type'Last),
       Wide_Type'Max (Wide_Type (Discrete_Type'First),
                      Wide_Type (Left) + Wide_Type (Right))));

That assumes that the formal Wide_Type is defined as an integer type, 
not just as any discrete type.

HTH,

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .