#ifndef EXPRESSION_H
#define EXPRESSION_H

#include <vector>
#include <utility>
#include <string>
#include <boost/shared_ptr.hpp>

// an enumeration for the supported operators
enum Operator { PLUS, MINUS, MULTIPLY, DIVIDE };

// converts a symbol +, -, * or / into an Operator
Operator to_operator(const std::string& s);

// thrown by to_operator for a symbol that is not recognised
class BadOperatorSymbol { };

// base class for expression types representing rules in the grammar
class Expression
{
	public:
		virtual int eval() const = 0;
		virtual ~Expression();
};

typedef boost::shared_ptr<Expression> ExpressionPtr;
typedef std::vector<std::pair<boost::shared_ptr<Expression>, Operator> > ExpressionVector;

// represents <AddExpression> in the grammar
class AddExpression : public Expression
{
	private:
		ExpressionPtr first;
		ExpressionVector expr;
		
	public:
		AddExpression(ExpressionPtr f, ExpressionVector e);
		virtual int eval() const;
};

// represents <MultExpression> in the grammar
class MultExpression : public Expression
{
	private:
		ExpressionPtr first;
		ExpressionVector expr;
		
	public:
		MultExpression(ExpressionPtr f, ExpressionVector e);
		virtual int eval() const;
};

// represents <Integer> in the grammar
class IntegerExpression : public Expression
{
	private:
		int value;
		
	public:
		IntegerExpression(int v);
		virtual int eval() const;
};

#endif