_str2set.c

Many C samples. It is a good sample for students to learn C language.

/*  File   : _str2set.c
    Author : Richard A. O'Keefe.
    Updated: 20 April 1984
    Defines: _set_ctr, _set_vec[], _str2set().
    Purpose: Convert a character string to a set.
*/

/*  The obvious way of representing a set of characters  is  as  a
    vector  of 0s and 1s.  The snag with that is that to convert a
    string to such a vector, we have to clear all the elements  to
    0,  and  then  set the elements corresponding to characters in
    the string to 1, so the cost is  O(|alphabet|+|string|).  This
    package  uses another method, where there is a vector of small
    numbers and a counter.  A character is in the current  set  if
    and  only  if the corresponding element of the vector is equal
    to the current value of  the  counter.   Every  so  often  the
    vector  elements  would  overflow  and  we  have  to clear the
    vector, but the cost is reduced to O(|string|+1).

    Note that NUL ('\0') will never be in any set built by str2set.

    While this method reduces the cost of building a set, it would
    be useful to avoid it entirely.  So when the "set" argument is
    NullS the set is not changed.  Use NullS to mean "the same set
    as before."  MaxPosChar is the largest integer value which can
    be stored in a "char".  Although we might get a slightly wider
    range by using "unsigned char", "char" may be cheaper (as on a
    PDP-11).  By all means change the number from 127 if your C is
    one of those that treats char as unsigned, but don't change it
    just because _AlphabetSize is 256, the two are unrelated.  And
    don't dare change it on a VAX: it is built into the asm code!
*/

#include "strings.h"
#include "_str2set.h"

#if	CharsAreSigned
#define	MaxPosChar	127
#else  ~CharsAreSigned
#define MaxPosChar	255
#endif	CharsAreSigned

int  _set_ctr = MaxPosChar;
char _set_vec[_AlphabetSize];


void _str2set(set)
    register char *set;
    {
	if (set == NullS) return;
	if (++_set_ctr == MaxPosChar+1) {
#if	VaxAsm
	    asm("movc5 $0,4(ap),$0,$128,__set_vec");
#else  ~VaxAsm
	    register char *w = &_set_vec[_AlphabetSize];
	    do *--w = NUL; while (w != &_set_vec[0]);
#endif	VaxAsm
	    _set_ctr = 1;
	}
	while (*set) _set_vec[*set++] = _set_ctr;
    }