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LEXPOS.PAS
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Pascal/Delphi Source File
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1993-01-24
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4KB
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139 lines
unit LexPos;
(* 2-5-91 AG *)
(* Copyright (c) 1990,91 by Albert Graef, Schillerstr. 18,
6509 Schornsheim/Germany
All rights reserved *)
interface
uses LexBase, LexTables;
(* 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). *)
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*).