Category : C Source Code
Archive   : E_C_STR.ZIP
Filename : _STR2SET.C

/* 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;
}