/* calc_eqn.y   for interfacing with  qm  Quine_McClusky.c */
%{
#include <stdio.h>
#include <ctype.h>

extern int  regs[26];   /* values of variables a, b, c, ... */
extern int  eqn_result; /* returned equation value */
extern char line[];     /* equation line to parse  */ 
extern int  line_index; /* index, initialize to zero, each equation */

static void yyerror(char *message);
static int yylex();

%}

%start list

%token DIGIT LETTER

/* do without precedence */

%%   /* beginning of rules section */

list	:	/* empty */
	|	list stat ';'
	|	list stat '\n'
	|	list '\n'
	|	list error '\n'
			{	yyerrok; }
	;
stat	:	expr
			{eqn_result=$1; }
	|	LETTER '=' expr
			{	regs[$1] = $3; }
	;
expr	:	expr '|' fctr
			{	$$ = $1 || $3; }
	|	fctr
	;
fctr	:	fctr '&' uexp
			{	$$ = $1 && $3; }
        |	uexp
	;
uexp	:	'~' term
			{	$$ = ! $2; }
	|	term
	;
term	:	'(' expr ')'
			{	$$ = $2; }
	|	LETTER
			{	$$ = regs[$1]; }
	|	number
	;
number	:	DIGIT
			{	$$ = $1; }
	|	number DIGIT {	$$ = 2 * $1 + $2; } /* bad */
	;

%%  /* start of program */

static int yylex()
{
  int c;

/*
  int i;
  if(line_index==0){
    printf("in yylex, line=%s\n", line);
    printf("regs a b c d e f g h i j k l m n o p q r s t u v w x\n");
    printf("     ");
    for(i=0; i<24; i++) printf("%d ",regs[i]);
    printf("\n");
  }
*/

  c=line[line_index++];
  /* printf("yylex read c= %c \n",c); */
  if(islower(c)){
     yylval=c-'a';
     return (LETTER) ;
  }
  if(isdigit(c)){
     yylval=c-'0';
     return (DIGIT) ;
  }
  return (c);
} /* end yylex */

static void yyerror(char *message)
{
  printf("\n yyerror reports %s \n", message); /* can be commented out */
}