Integer-to-string conversion error solved

[Jose E. Marchesi]

> Thanks, Jose, for the quick response about the current incorrect
> string conversions of the most negative integers.
>
> No, I am certainly not suggesting that ga68 change the implementation
> of the Algol 68 MOD operator, which is language defined and must be
> standard.
>
> It is the USE of MOD in integer-to-string conversions that produces an
> incorrect string for -(2**(n - 1)), so it is just the conversion
> routines that need repair.

Yes that's what I was asking.  In our current transput implementation
that would be 'subwhole'.

I will incorporate your fix in our current transput.

Note that Chris Hermansen is working on our new full-fledged transput,
based on van Vliet's, which might or might not provide a new
implementation of `whole'.  In that case, and if the new implementation
uses MOD, we will be careful to use your version so we are compatible
with C number printers.

> Algol 68 parses integers without a sign, and then only afterwards
> applies unary signs.  That is why, as in many programming languages,
> there are subterfuges like the C header file, <limits.h>, with code
> like this:
>
> 	#define INT_MAX 2147483647
> 	#define INT_MIN (-INT_MAX - 1)
>
> A human programmer unfamiliar with details of hardware arithmetic may
> well wonder, and complain about, the refusal of the compiler to accept
> the constant -2147483648.  My view has always been that compilers
> SHOULD accept such constants: all that is needed is digit accumulation
> as a negative value, then attaching the sign after all the digits have
> been collected: grammar rules, alas, may prevent that helpful
> practice.

We actually take care in our implementation to support using -2147483647
(and similar to other type widths) as an integral denotation:

      if (errno == ERANGE
	  || (NEGATED (p) && (val > max_negative))
	  || (!NEGATED (p) && (val > max_positive)))
	{
	  a68_moid_format_token m (moid);
	  a68_error (s, "denotation is too large for %e", &m);
	}

Note the NEGATED attribute which is stored in the AST node corresponding
to the integral denotation.  That is only set if the integral denotation
is immediately passed to the unary minus operator.

> Two's complement arithmetic is essentially universal today: the CDC
> 6x00/7x00 and Univac 11xx systems of the 1960s and 1970s were the last
> major CPUs that (I can recall at the moment) had the symmetric one's
> complement arithmetic, for which INT_MIN = -INT_MAX, but then also
> there is +0 and -0, which complicates comparisons.
>
> My concern is that output of all possible integers as strings, and
> subsequent conversion of those strings back to integers, MUST be
> round-trip correct for all possible numbers.
>
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> - Nelson H. F. Beebe                    Tel: +1 801 581 5254                  -
> - University of Utah                                                          -
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> - 155 S 1400 E RM 233                       beebe@acm.org  beebe@computer.org -
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