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{
Construct the position table, as well as first position sets.
The position table stores symbol positions in regular expressions of
the Lex grammar. It also allows to store the corresponding first
and follow sets. By this means, the position table represents an eps-
free nondeterministic automaton for the regular expressions of the
Lex grammar (cf. Aho/Sethi/Ullman, Compilers : Principles, Techniques
and Tools, 1986, Section 3.9, on constructing NFA's from regular
expressions using position tables).
Copyright (c) 1990-92 Albert Graef <ag@muwiinfa.geschichte.uni-mainz.de>
Copyright (C) 1996 Berend de Boer <berend@pobox.com>
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
$Revision: 1.2 $
$Modtime: 96-08-01 6:30 $
$History: LEXPOS.PAS $
*
* ***************** Version 2 *****************
* User: Berend Date: 96-10-10 Time: 21:16
* Updated in $/Lex and Yacc/tply
* Updated for protected mode, windows and Delphi 1.X and 2.X.
}
unit LexPos;
interface
uses LexBase, LexTable;
procedure addExpr(r : RegExpr; var FIRST : IntSet);
(* Add the positions in r to the position table, and return the set of
first positions of r. *)
implementation
procedure eval(r : RegExpr;
var FIRST, LAST : IntSet;
var nullable : Boolean);
(* Evaluates the expression r, adding the positions in r to the position
table and assigning FIRST, LAST and FOLLOW sets accordingly (cf. Aho/
Sethi/Ullman, Compilers : Principles, Techniques and Tools, Section 3.9).
Returns:
- FIRST: the set of first positions in r
- LAST: the set of last positions in r
- nullable: denotes whether the r is nullable (i.e. is matched by the
empty string). *)
var
c : Char;
str : StrPtr;
cc : CClassPtr;
rule, pos : Integer;
r1, r2 : RegExpr;
FIRST1, LAST1 : IntSet;
nullable1 : Boolean;
i : integer;
begin
if is_epsExpr(r) then
begin
empty(FIRST); empty(LAST);
nullable := true
end
else if is_markExpr(r, rule, pos) then
begin
addMarkPos(rule, pos);
singleton(FIRST, n_pos); singleton(LAST, n_pos);
nullable := true
end
else if is_charExpr(r, c) then
begin
addCharPos(c);
singleton(FIRST, n_pos); singleton(LAST, n_pos);
nullable := false
end
else if is_strExpr(r, str) then
if length(str^)=0 then
(* empty string is treated as empty expression *)
begin
empty(FIRST); empty(LAST);
nullable := true
end
else
begin
addCharPos(str^[1]);
singleton(FIRST, n_pos);
for i := 2 to length(str^) do
begin
addCharPos(str^[i]);
singleton(pos_table^[pred(n_pos)].follow_pos^, n_pos);
end;
singleton(LAST, n_pos);
nullable := false
end
else if is_CClassExpr(r, cc) then
begin
addCClassPos(cc);
singleton(FIRST, n_pos); singleton(LAST, n_pos);
nullable := false
end
else if is_starExpr(r, r1) then
begin
eval(r1, FIRST, LAST, nullable);
for i := 1 to size(LAST) do
setunion(pos_table^[LAST[i]].follow_pos^, FIRST);
nullable := true
end
else if is_plusExpr(r, r1) then
begin
eval(r1, FIRST, LAST, nullable);
for i := 1 to size(LAST) do
setunion(pos_table^[LAST[i]].follow_pos^, FIRST);
end
else if is_optExpr(r, r1) then
begin
eval(r1, FIRST, LAST, nullable);
nullable := true
end
else if is_catExpr(r, r1, r2) then
begin
eval(r1, FIRST, LAST1, nullable);
eval(r2, FIRST1, LAST, nullable1);
for i := 1 to size(LAST1) do
setunion(pos_table^[LAST1[i]].follow_pos^, FIRST1);
if nullable then setunion(FIRST, FIRST1);
if nullable1 then setunion(LAST, LAST1);
nullable := nullable and nullable1
end
else if is_altExpr(r, r1, r2) then
begin
eval(r1, FIRST, LAST, nullable);
eval(r2, FIRST1, LAST1, nullable1);
setunion(FIRST, FIRST1);
setunion(LAST, LAST1);
nullable := nullable or nullable1
end
end(*eval*);
procedure addExpr(r : RegExpr; var FIRST : IntSet);
var LAST : IntSet;
nullable : Boolean;
begin
eval(r, FIRST, LAST, nullable);
end(*addExpr*);
end(*LexPos*).
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