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From: kaleb@mars.jpl.nasa.gov (Kaleb Keithley)
Newsgroups: comp.lang.c
Subject: bitmask lookup table creation
Message-ID: <3947@jato.Jpl.Nasa.Gov>
Date: 6 Jun 90 00:00:21 GMT
Sender: news@jato.Jpl.Nasa.Gov
Reply-To: kaleb@mars.jpl.nasa.gov (Kaleb Keithley)
Organization: Jet Propulsion Laboratory, Pasadena, CA.
Lines: 27

I want to create the following bitmask lookup table:
    unsigned short masks[] = {
        0x0000, 0x0001, 0x0003, 0x0007,
        0x000F, 0x001F, 0x003F, 0x007F,
        0x00FF, 0x01FF, 0x03FF, 0x07FF,
        0x0FFF, 0x1FFF, 0x3FFF, 0x7FFF 
    };

in a machine independent way.

In /usr/include/values.h, the following are #defined:
#define BITS(type)  (BITSPERBYTE * (int)sizeof(type)
#define HIBITS  ((short)(1 << BITS(short) - 1))
#define MAXSHORT    ((short)~HIBITS)

which are 0x8000 and 0x7FFF respectively on my machine, which happens to
be a Sun Sparc.

It is nearly trivial to do it at run-time, and the above definitely works
on my Sun.  Does anyone have any neat trick to generate this table at 
compile time in such a way as to be completely machine independent?

adva-thanks-nce
kaleb@thyme.jpl.nasa.gov            Jet Propeller Labs
Kaleb Keithley

"So that's what an invisible barrier looks like"