// AttrGram.cpp

// A simple parser for the grammar
//
//   <assign> -> <var> = <expr> $
//   <expr> -> <var> + <var>
//   <expr> -> <var>
//   <var> -> A | B | ... | Z

#include <iostream.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>

enum TYPE { REAL, INTEGER };

enum TOKEN { VAR, ASSIGNOP, PLUSOP, ERRTOK, EOFTOK };

struct SYMBOL_TABLE {
	char name;
	TYPE vartype;
};

SYMBOL_TABLE SYMTAB[3] =
	{{'A', INTEGER}, {'B', REAL}, {'C', INTEGER}};

TYPE lookup(char name) {
	int i;
	for (i = 0; i < 3; i++)
		if (SYMTAB[i].name == name)
			return SYMTAB[i].vartype;
}

struct TOK {
	TOKEN tok;
	char name;
};

TOK curtok;

// nexttoken is the scanner
void nexttoken() {
	char ch;
	while (cin >> ch) {
		if (isspace(ch))
			continue;	// eat up white space
		else if (isalpha(ch) && ch >= 'A' && ch <= 'C') {
			curtok.name = ch;
			curtok.tok = VAR;
			break;
		}
		else if (ch == '=') {
		    curtok.tok = ASSIGNOP;
			break;
		}
		else if (ch == '+') {
			curtok.tok = PLUSOP;
			break;
		}
		else if (ch == '$') {
			curtok.tok = EOFTOK;
			break;
		}
		else {
			cout << "Lexical error: " << ch << endl;
			curtok.tok = ERRTOK;
			break;
		}
	}
	
}

// returns a printable version of tok
char* tok2str(TOKEN tok) {
	switch (tok) {
		case VAR : return "variable";
		case ASSIGNOP : return "\"=\"";
		case PLUSOP : return "\"+\"";
		case EOFTOK : return "\"$\"";
	}
}
			


void match(TOKEN tok) {
	if (curtok.tok == tok) {
		nexttoken();
	}
	else {
		cout << "Error: " << tok2str(tok) << " expected\n";
		exit(1);
	}
}

void assign();
void expr(TYPE &var_type);
void var(TYPE &expr_type);

void assign() {
	TYPE lhs_type, expr_type;
	var(lhs_type);
	match(ASSIGNOP);
	expr(expr_type);
	if (lhs_type == expr_type)
		cout << "Type check ok\n";
	else
		cout << "Type mismatch\n";
}

void expr(TYPE &expr_type) {
	TYPE var1type, var2type;
	var(var1type);
	if (curtok.tok == PLUSOP) {
		match(PLUSOP);
	    var(var2type);
		if (var1type == var2type)
			expr_type = var1type;
		else
			expr_type = REAL;
	}
	else
		expr_type = var1type;
}

void var(TYPE &var_type) {
	char name = curtok.name;
	match(VAR);
	var_type = lookup(name);
}

void main() {
	cout << "Assignment statement parser" << endl;
	cout << "Enter a statement: ";	
	nexttoken();
	assign();
	if (curtok.tok == EOFTOK)
	   cout << "Syntax check ok\n";
	else
	   cout << "End of file expected\n";
}