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NNTP-Posting-Date: Mon, 04 Jun 2007 23:10:59 -0500
From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Puzzling small piece of code - exception block essentially used
 as goto
References: <Q049i.5409$nj4.313@trndny06>
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Date: Mon, 04 Jun 2007 23:10:59 -0500
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Date: 2007-06-04T23:10:59-05:00
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> > Without knowing the details of the types of the variables used, I
> > can't say.
>
> Here are the ranges for the variables (all are integers):
>
> Hour_Delta: -9999*60..9999*60
> New_Hour:          0..2^32
> Hour:              0..2^32
>
> Relative_Time : constant := 0;
  And what is the value of Max_Hour_Delta?

Another way to look at that code is to observe that the only way out of
the loop is to not raise Constraint_Error, ie, New_Hour is within
+-9999*60 of Hour (that *60 rather suggests the units are minutes, not
hours as one might assume fron the names).  If that's not true at first,
the code adjusts Hour by steps of Max_Delta_Hour, hopefully in the
direction of New_Hour.  If it doesn't overshoot, Hour will eventually be
in range, at which time it will be set to the value of New_Hour.  So the
code essentially says:

if Max_Delta_Hour < 0 then
  burn some CPU time;      -- move Hour ever farther from New_Hour
  raise Constraint_Error;  -- till it hits a limit
elsif Max_Delta_Hour = 0 then
  sit forever in infinite loop, never changing Hour;
elsif Max_Delta_Hour in > 2 * 9999*60
                          + 1
                          + ( abs(New_Hour-Hour) mod (9999*60+1) ) then
  oscillate forever between Hour too positive and Hour too negative;
else
  burn some CPU time;
end if;
Hour := New_Hour;
(if I did my algebra correctly).

So replacing the whole thing by "Hour := New_Hour;" probably does
what the original programmer wanted.