Re: File chicken counts eggs

["Nick Roberts" <Nick.Roberts@dial.pipex.com>]

Mark Kambites <mek100@york.ac.uk> wrote in article
<5kqku3$bch$1@netty.york.ac.uk>...
> Hello fellow Ada users!
> 
> I'm attempting to implement a B-tree-type structure on disk. This
requires
> that each node in the tree contains file indices, indicating where in the
> file its children can be found. Such indices are implemented in Ada as
the
> 'count' type which is declared in direct_io. Unfortunately, nothing in
> direct_io is visible until direct_io has been instantiated, and I need
> to instantiate it with my node as the generic type parameter. Have you
> spotted the problem yet?
> 
> It seems that the size of my node must be known to the compiler before
> direct_io can be instantiated, and the size of the node is dependant upon
> the size of 'count', which is unknown UNTIL direct_io HAS BEEN
instantiated.


I think it is generally acknowledged that the COUNT type declared in
SEQUENTIAL_IO, DIRECT_IO, and TEXT_IO (in Ada 83) was a mistake. A
predefined integer type should have been used instead; among other things,
this would have obviated the problem you encounter.

Since you are (presumably) using multiple instantiations of DIRECT_IO, each
instantiation will give rise to a (nominally) different type COUNT. Thus,
you cannot avoid explicit type conversions.

Technically, given instantiations such as:

package FILE_1_IO is new DIRECT_IO(TYPE_1);
package FILE_2_IO is new DIRECT_IO(TYPE_2);
package FILE_3_IO is new DIRECT_IO(TYPE_3);

you should declare a new type:

type OVERALL_COUNT is range 0 .. MAX(MAX(FILE_1_IO.COUNT'LAST,
FILE_2_IO.COUNT'LAST), FILE_3_IO.COUNT'LAST);

where MIN and MAX are declared as appropriate.

However, this is so clumsy, you may want to simply declare OVERALL_COUNT as
0 .. SYSTEM.MAX_INT, or some other big range which works.

Either way, you will have to declare components of type OVERALL_COUNT in
your tree, and convert explicitly as required.

Nick.