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From: Paul Graham <pgraham@cadence.com>
Newsgroups: comp.lang.ada
Subject: Wraparound on multiply overflow
Date: Wed, 08 Aug 2001 14:16:03 -0700
Organization: Cadence Design System, Inc.
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Xref: archiver1.google.com comp.lang.ada:11688
Date: 2001-08-08T14:16:03-07:00
List-Id: <comp.lang.ada>

In C it is defined that the result of multiplying two unsigned integers
is the mathematical result modulo the value (int_max + 1), where int_max
is the maximum unsigned integer.  I'm interested in porting to Ada  a
random number generator which relies on this rule.  The code looks like:

    unsigned int X[10];
    ...
    X[0] = seed;
    for (i = 1; i <= 10; i++)
        X[i] = X[i-1] * 12345;

The idea is that a table of random looking numbers is created by
repeated multiplication of a seed by some constant.  This sequence is
guaranteed to be the same across conforming C implementations, because
of  the modulo rule I mentioned above.

Can this code be written in Ada?  A few iterations of the loop would
result in a range error or overflow.  I could suppress checks, but would
the result of an overflowing multiply be defined in that case?  Is there
any other way of emulating C's behavior for this program?  I know that
Ada has modular types, but these don't work with multiply, do they?

Paul