/* calc.y
 * YACC desk calculator example, adopted from ALSU, sec. 4.9
 * handles arithmetic on single digits
 *
 * to compile, use:
   yacc calc.y
   gcc -o calc y.tab.c -ly
 */

%{
#include <stdio.h>
%}

%token DIGIT

%%
lines	: lines expr '\n'	{ printf("= %d\n", $2); }
	| lines '\n'
	| /* epsilon */
	;

expr	: expr '+' term		{ $$ = $1 + $3; }
	| expr '-' term		{ $$ = $1 - $3; }
	| term			{ $$ = $1; /* default rule */ }
	;

term	: term '*' factor	{ $$ = $1 * $3; }
	| term '/' factor	{ $$ = $1 / $3; }
	| factor
	;

factor	: '(' expr ')'		{ $$ = $2; }
	| DIGIT
	;

%%

int yylex() {
  int c = getchar();
  if (c >= '0' && c <= '9') {
    yylval = c - '0';
    return DIGIT;
  }
  return c;
}