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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Pascal Calling Convention
Date: Mon, 28 Mar 2011 14:16:28 -0700 (PDT)
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On Mar 28, 1:29=A0pm, Hyman Rosen <hyro...@mail.com> wrote:
> On 3/26/2011 4:46 PM, Robert A Duff wrote:
>
> > It doesn't seem reasonable to me. =A0Uninitialized variables do not
> > contain random data. =A0They contain arbitrary data. =A0And it's quite
> > likely that such arbitrary data happens to be the same every time,
> > or has some other non-random pattern. =A0Bad Guys sometimes exploit
> > such patterns to cause security problems.
>
> Time for my favorite data structure! This is a structure which
> represents an unordered set of the integers [0,N) and which has
> constant time initialization, insertion, deletion, and membership
> test. It relies on reading uninitialized data as some arbitrary
> value. Compilers which regard doing this as undefined behavior
> will break the code. I'm going to write it in C++, but it should
> be straightforward to translate to Ada:

I hope your "favorite data structure" comment was sarcastic.  I would
expect this code to behave incorrectly a large portion of the time;
specifically, if I read the code right, once you add any element to
the set, the program is likely to think 0 is in the set whether you've
added it or not.

                        -- Adam



> template <unsigned N>
> class set
> {
> =A0 =A0 =A0unsigned n; =A0 =A0// number of members in set
> =A0 =A0 =A0unsigned d[N]; // members are in d[0..n-1]
> =A0 =A0 =A0unsigned s[N]; // d[s[k]] =3D=3D k if k is a member
> =A0 =A0 =A0// unspecified elements of d and s are arbitrary
>
> public:
> =A0 =A0 =A0set() : n(0) { }
> =A0 =A0 =A0bool has(unsigned k)
> =A0 =A0 =A0 =A0 =A0{ return k < N && s[k] < n && d[s[k]] =3D=3D k; }
> =A0 =A0 =A0void add(unsigned k)
> =A0 =A0 =A0 =A0 =A0{ if (k < N && !has(k)) { d[n] =3D k; s[k] =3D n++; }
> =A0 =A0 =A0void del(unsigned k)
> =A0 =A0 =A0 =A0 =A0{ if (has(k)) { unsigned i =3D s[k]; d[i] =3D d[--n]; =
s[d[i]] =3D i; }
> =A0 =A0 =A0void clr() { n =3D 0; }
>
>
>
> };