_str2pat.c

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

/*  File   : _str2pat.c
    Author : Richard A. O'Keefe.
    Updated: 2 June 1984
    Defines: _pat_lim, _pat_vec[], _str2pat()

    Searching in this package is done by an algorithm due  to  R.  Nigel
    Hospool,  described  in  Software  Practice & Experience 1980, p505.
    Elsewhere I have a version of it which does  exact  case  or  either
    case  match,  word  more or literal mode, forwards or backwards, and
    will look for the Nth instance.  For most applications that  is  too
    much  and  a  simple  exact  case forward search will do.  Hospool's
    algorithm is a simplification of  the  Boyer-Moore  algorithm  which
    doesn't  guarantee linear time, but in practice is very good indeed.

    _str2pat(pat) builds a search table for the string pat.  As usual in
    this pacakge, if pat == NullS, the table is not changed and the last
    search string is re-used.  To support  this,  _str2pat  returns  the
    actual search string.
*/

#include "strings.h"
#include "_str2pat.h"

int	_pat_lim;
int	_pat_vec[_AlphabetSize];
static	_char_	*oldPat = "";


_char_ *_str2pat(pat)
    register _char_ *pat;
    {
	register int L, i;

	if (pat == NullS) pat = oldPat; else oldPat = pat;
	for (L = 0; *pat++; L++) ;
	for (i = _AlphabetSize; --i >= 0; _pat_vec[i] = L) ;
	for (pat = oldPat, i = L; --i > 0; _pat_vec[*pat++] = i) ;
	_pat_lim = --L;
	return oldPat;
    }