// Chapter 06, drill
//
// This is example code from Chapter 6.7 "Trying the second version" of
// "Software - Principles and Practice using C++" by Bjarne Stroustrup
//
/*
This file is known as calculator02buggy.cpp
I have inserted 5 errors that should cause this not to compile
I have inserted 3 logic errors that should cause the program to give wrong results
First try to find an remove the bugs without looking in the book.
If that gets tedious, compare the code to that in the book (or posted source code)
Happy hunting!
*/

#include <iostream>
using namespace std;
//------------------------------------------------------------------------------

class Token {
public:
	char kind;        // what kind of token
	double value;     // for numbers: a value
	Token(char ch)    // make a Token from a char
		:kind(ch), value(0) { }
	Token(char ch, double val)     // make a Token from a char and a double
		:kind(ch), value(val) { }
};

//------------------------------------------------------------------------------

class Token_stream {
public:
	Token_stream();   // make a Token_stream that reads from cin
	Token get();      // get a Token (get() is defined elsewhere)
	void putback(Token t);    // put a Token back
private:
	bool full;        // is there a Token in the buffer?
	Token buffer;     // here is where we keep a Token put back using putback()
};

//------------------------------------------------------------------------------

// The constructor just sets full to indicate that the buffer is empty:
Token_stream::Token_stream()
	:full(false), buffer(0)    // no Token in buffer
{ }

//------------------------------------------------------------------------------

// The putback() member function puts its argument back into the Token_stream's buffer:
void Token_stream::putback(Token t) {
	if (full) cerr << "putback() into a full buffer";
	buffer = t;       // copy t to buffer
	full = true;      // buffer is now full
}

//------------------------------------------------------------------------------

Token Token_stream::get() {
	if (full) {       // do we already have a Token ready?
		   // remove token from buffer
		full = false;
		return buffer;
	}

	char ch;
	cin >> ch;    // note that >> skips whitespace (space, newline, tab, etc.)

	switch (ch) {
	case ';':    // for "print"
	case 'q':    // for "quit"
	case '(': case ')': case '+': case '-': case '*': case '/':
		return Token(ch);        // let each character represent itself
	case '.':
	case '0': case '1': case '2': case '3': case '4':
	case '5': case '6': case '7': case '8': case '9': // FIX add '8'
	{
		cin.putback(ch);         // put digit back into the input stream
		double val;
		cin >> val;              // read a floating-point number
		return Token('8', val);   // let '8' represent "a number"
	}
	default:
		cerr << "Bad token";
	}
}

//------------------------------------------------------------------------------

Token_stream ts;        // provides get() and putback()

//------------------------------------------------------------------------------

double expression();    // declaration so that primary() can call expression()

//------------------------------------------------------------------------------

// deal with numbers and parentheses

double primary() {
	Token t = ts.get();
	switch (t.kind) {
	case '(':    // handle '(' expression ')'
	{
		double d = expression();
		t = ts.get();
		if (t.kind != ')') cerr << "')' expected";
		return d;
	}
	case '8':            // we use '8' to represent a number
		return t.value;  // return the number's value
	default:
		cerr << "primary expected";
	}
}

//------------------------------------------------------------------------------

// deal with *, /, and %
double term() {
	double left = primary();
	Token t = ts.get();        // get the next token from token stream

	while (true) {
		switch (t.kind) {
		case '*':
			left *= primary();
			t = ts.get();
			break;
		case '/': {
			double d = primary();
			if (d == 0) cerr << "divide by zero";
			left /= d;
			t = ts.get();
			break;
		}
		default:
			ts.putback(t);     // put t back into the token stream
			return left;
		}
	}
}

//------------------------------------------------------------------------------

// deal with + and -
double expression() {
	double left = term();      // read and evaluate a Term
	Token t = ts.get();        // get the next token from token stream

	while (true) {
		switch (t.kind) {
		case '+':
			left += term();    // evaluate Term and add
			t = ts.get();
			break;
		case '-':
			left -= term();    // evaluate Term and subtract
			t = ts.get();
			break;
		default:
			ts.putback(t);     // put t back into the token stream
			return left;       // finally: no more + or -: return the answer
		}
	}
}

//------------------------------------------------------------------------------

int main() {
	cout << "Welcome to our simple calculator.\n";
	cout << "Please enter expressions using floating-point numbers.\n";
	cout << "Operators available: +, -, *, /, ().\n";
	cout << "To print the result, enter ';' and to quit, enter 'q'.\n";
	while (cin) {
		cout << "> ";   // print prompt
		Token t = ts.get();
		while (t.kind == ';') t = ts.get();   // eat ';'
		if (t.kind == 'q') {
			return 0;
		}
		ts.putback(t);
		cout << "= " << expression() << endl;
	}
	
	return 0;
}