reversing a mask

Geoff Kuenning geoff at desint.UUCP
Mon Nov 26 09:14:41 AEST 1984

In article <691 at gloria.UUCP> colonel at gloria.UUCP (George Sicherman) writes:

>What's the fastest way to reverse a 16-bit mask in C?  In 68000 asl?
>(Maybe this should be in net.puzzle!)

Try this:

    static unsigned char revtab[256] = {0x80, 0x40, 0xC0, 0x20, 0xA0,...};

    short bitrev (mask)
	register unsigned short	mask;
	return (revtab[mask & 0xFF] << 8) | revtab[mask >> 8];

Note:  I haven't actually tried compiling this program.  It depends on the
compiler generating a zero-filling right-shift for unsigned shorts.  If yours
does not, you will need to use:

	return (revtab[mask & 0xFF] << 8) | revtab[(mask >> 8) & 0xFF];

	Geoff Kuenning

More information about the Comp.lang.c mailing list