Re: CRC in Ada?

[dbrown@vigra.com (David Brown)]

dewar@merv.cs.nyu.edu (Robert Dewar) writes:

> The proper way to read this file in Ada 95 is to use Stream_IO into
> a stream element buffer. The imagined "prime number" restriction
> does not exist, and in practice this should be highly efficient.

Ok, so I am reading the file data into a stream element buffer
(Stream_Element_Array).  My file, however, really consists of some
small objects (16 bit signed integers).  What is the "proper" way to
get these integers out of this buffer?  I want there to be as little
copying of these numbers as possible.  Here are some solutions I've
come up with:

1.  Have code to extract and insert my objects out of a stream element
    array.  These routines could be inlined and the access would end
    up fast.  However, they would be cumbersome to call because the
    item wouldn't quite be an array.

2.  Use for Foo'Address clauses or address to access conversions to
    alias an array of my data on top of the stream element buffer.
    Aside from the general feeling of grossness about this, I have
    heard mentioned on this group that this is not the proper way of
    aliasing two data items.  Is there a "proper" way of doing this?

3.  Use Ada.Storage_IO to convert the buffer into a buffer of my
    data.  This will cause a copy.

4.  Just do my I/O by calling my underlying operating system calls.
    This would probably be most efficient.  With the GNAT runtime, the
    stream IO read and write just call fread which on my OS doesn't do
    a copy if the buffer is large enough.  In other words, if I could
    somehow avoid the copy in Ada, the Stream IO code would be
    efficient enough.

Dave Brown
dbrown@vigra.com