/*
 * Expression is a simple math expression interpreter.
 * expression support + - * / ^(pow) -(neg)calculate sign
 * expression support "()" to constrol the calculate order
 * the number in expression can only be the real number,don't support the scientific notation.
 * SetExprStr failure will break down the original expression
 * support 26 letter variable name(case insensitive)
 * use SetVar to set the variable value
 * the default variable value is 1
 * when GetResult ,if the expression is not set or the variable is not set, it will return an error code.
 * GetExprErrorStr get the expression error code string
 * GetResultErrorStr get the result error code string
 */
namespace expInterpreter {
enum AtomType{ATOM_ERROR = 0, ATOM_SYMBOL, ATOM_CONSTANT, ATOM_VARIABLE, ATOM_END};

// the struct of a syntax unit, it can be a symbol, a constant or a variable
typedef struct
{
	AtomType type;
	union
	{
		char symbol;
		double constant;
		char variable;
	} value;
} Atom;

// read a syntax unit from the given stringŁ¬begin at the startŁ¬finish at the end unitŁ¬return -1 if it is null.
int read_atom(Atom *atom, const char *exprStr, int start);

// expression
class Expression
{
private:
	const static int poland_MAX;
	Atom *poland;
	int poland_N;
	double *var;
public:
	Expression();
	~Expression();
	
	const char* GetExprErrorStr(int err);	// return the error string according to the error code	
	const char* GetResultErrorStr(int err);	// return the error string according to the error code
	int SetExprStr(const char *exprStr);	// set the expression,generate the reverse polish notation,return 0 if succeed,error code if fail.
	void SetVar(char x, double value);		// set the variable value
	int GetResult(double *result);			// get the result according to the last set variable and expression,return 0 if succeed,error code if fail.
	const Atom *GetPoland();

};
}