//
// This is example code from Chapter 7.2 "Input and output" of
// "Programming -- Principles and Practice Using C++" by Bjarne Stroustrup
//

#include "std_lib_facilities.h"

const string prompt = "> ";
//------------------------------------------------------------------------------

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) error("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 '/': 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':
    {
        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:
        error("Bad token");
    }
}

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

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

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

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

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

double factorial(double n) {
    if(n-int(n)!=0 || n < 0) error("bad factorial");
    if(n==0 || n==1) return 1;
    else return n*factorial(n-1);
}
// 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 != ')') error("')' expected");
        t = ts.get();
        if(t.kind == '!')
            return factorial(d);
        cin.putback(t.kind);
        return d;
    }case '-':
        return -primary();
    case '+':
        return primary();
    case '8': {           // we use '8' to represent a number
        double v = t.value;
        t = ts.get();
        if(t.kind == '!')
            return factorial(v);
        cin.putback(t.kind);
        return v;  // return the number's value
    }

    default:
        error("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) error("divide by zero");
            left /= d;
            t = ts.get();
            break;
        }
        case '%': {
            double d = primary();
            if (d == 0) error("divide by zero");
            while(left-d>0)
                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() {
    double val = 0;
    while (cin) {
        cout << prompt;          // print prompt
        Token t = ts.get();
        if (t.kind == 'q') break;
        while(t.kind == ';') t = ts.get();
        if (t.kind == ';')
            cout << "= " << val << '\n'; // print result
        if(t.kind == 'q') return 0;
        ts.putback(t);
        cout << "= " << expression() << endl;;
    }
}

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