I have a quite old C corporate parser / compiler code that was generated from an ancient Yacc and the original grammar source is lost (as the intermediate files) the only result the ytab.c parser generated file. That legacy piece of code need revamping but I cannot afford to recode it from scratch.
By "ancient Yacc", I mean that the parser is using tables named yyact, yypact, yypgo, yyr1, yyr2, yytoks, yyexca, yychk, yydef.
Could it be possible to mechanically retrieve / regenerate the parsing rules by deduction of the parsing tables in order to reconstruct the grammar?
Example with a little sample of an expression parser that I can process with the same ancient Yacc:
yytabelem yyexca[] ={
-1, 1,
0, -1,
-2, 0,
-1, 21,
261, 0,
-2, 8,
};
yytabelem yyact[]={
13, 9, 10, 11, 12, 23, 8, 22, 13, 9,
10, 11, 12, 9, 10, 11, 12, 1, 2, 11,
12, 6, 7, 4, 3, 0, 16, 5, 0, 14,
15, 0, 0, 0, 17, 18, 19, 20, 21, 0,
0, 24 };
yytabelem yypact[]={
-248, -1000, -236, -261, -236, -236, -1000, -1000, -248, -236,
-236, -236, -236, -236, -253, -1000, -263, -245, -245, -1000,
-1000, -249, -1000, -248, -1000 };
yytabelem yypgo[]={
0, 17, 24 };
yytabelem yyr1[]={
0, 1, 1, 1, 2, 2, 2, 2, 2, 2,
2, 2, 2 };
yytabelem yyr2[]={
0, 8, 12, 0, 6, 6, 6, 6, 6, 6,
4, 2, 2 };
yytabelem yychk[]={
-1000, -1, 266, -2, 259, 263, 257, 258, 267, 262,
263, 264, 265, 261, -2, -2, -1, -2, -2, -2,
-2, -2, 260, 268, -1 };
yytabelem yydef[]={
3, -2, 0, 0, 0, 0, 11, 12, 3, 0,
0, 0, 0, 0, 0, 10, 1, 4, 5, 6,
7, -2, 9, 3, 2 };
yytoktype yytoks[] =
{
"NAME", 257,
"NUMBER", 258,
"LPAREN", 259,
"RPAREN", 260,
"EQUAL", 261,
"PLUS", 262,
"MINUS", 263,
"TIMES", 264,
"DIVIDE", 265,
"IF", 266,
"THEN", 267,
"ELSE", 268,
"LOW", 269,
"UMINUS", 270,
"-unknown-", -1 /* ends search */
};
I am looking to retrieve
stmt : IF exp THEN stmt
| IF exp THEN stmt ELSE stmt
| /*others*/
;
exp : exp PLUS exp
| exp MINUS exp
| exp TIMES exp
| exp DIVIDE exp
| exp EQUAL exp
| LPAREN exp RPAREN
| MINUS exp
| NAME
| NUMBER
;
The full parser for this example is available as a gist, it is showing a yyreds table containing the rules as a debug info that is not in the parser that I am trying to reverse.
Note : I have asked this formerly on SO, and was suggested to ask it in RE. Any help?
PS: i am not familiar with the RE question tags, feel free to correct this.
