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From: rsalz@bbn.com (Rich Salz)
Newsgroups: comp.lang.c
Subject: Re: Bit map algorithms needed
Message-ID: <2599@litchi.bbn.com>
Date: 1 Jun 90 22:03:07 GMT
References: <90051107045355@masnet.uucp>
Organization: BBN Systems and Technology, Inc.
Lines: 62

Someone posted the following code:

>{
>   unsigned char test_val;
>   int index;
> 
>   index=bit_pos/8;      /* element of array to test */
>   switch(bit_pos%8) {   /* bit in indexth element to get, set, or clear */
>   case 0: test_val=0x01; break;
>   case 1: test_val=0x02; break;
>   case 2: test_val=0x04; break;
>   case 3: test_val=0x08; break;
>   case 4: test_val=0x10; break;
>   case 5: test_val=0x20; break;
>   case 6: test_val=0x40; break;
>   case 7: test_val=0x80; break;
>   }   
>   switch(action) {
>   case GET:
>      if((array[index]&test_val)==test_val)
>         return(TRUE);
>      return(FALSE);
>   case SET:
>      array[index]|=test_val;
>      return(TRUE);
>   case CLR:
>      array[index]&=(0xff-test_val);
>      return(FALSE);
>   }
>}

I think this is better.  Style aside, it avoids the gross unportability
used in the CLR case, and the switch used to set test_val.
	index = bit_pos >> 3;
	test_val = 1 << (bit_pos & 7);
	switch (action) {
	case GET:
	    return array[index] & test_val;
	case SET:
	    array[index] |= test_value;
	    break;
	case CLR:
	    array[index] &= ~test_val;
	    break;
	}
	return TRUE;

If you're doing general bitmap stuff, you're probably best off turning
the above routine into speciale-purpose macros, like the BSD fdset stuff,
or setbit/clrbit in <sys/param.h>:
    #define setbit(a,i)     ((a)[(i)/NBBY] |= 1<<((i)%NBBY))
    #define clrbit(a,i)     ((a)[(i)/NBBY] &= ~(1<<((i)%NBBY)))
    #define isset(a,i)      ((a)[(i)/NBBY] & (1<<((i)%NBBY)))
    #define isclr(a,i)      (((a)[(i)/NBBY] & (1<<((i)%NBBY))) == 0)
NBBY is the number of bits in a byte, and is typically 8, so
    #define setbit(a,i)     ((a)[(i) >> 3] |= 1<<((i) & 7))
    #define clrbit(a,i)     ((a)[(i) >> 3] &= ~(1<<((i) & 7)))
    #define isset(a,i)      ((a)[(i) >> 3] & (1<<((i) & 7)))
    #define isclr(a,i)      (((a)[(i) >> 3] & (1<<((i) & 7))) == 0)
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