Chapter 6: excercise 2-3

//
// Stroustrup - Programming Principles & Practice
//
// Chapter 6 - Calculator
//
// упражнение 2 - Добавить в использование {} {(4+5)*6}/(3+4)};
//
// упражнение 3 - Добавить факториал '!'.
//

#include "std_lib_facilities.h"

class Token {
public:
    char kind;
    double value;

    Token(char k) : kind{ k }, value{ 0 } { }
    Token(char k, double v) : kind{ k }, value{ v } { }
};

class Token_stream {
public:
    Token get();                // get a Token
    void putback(Token t);      // put a token back
private:
    bool full { false };        // is there a Token in the buffer?
    Token buffer{ '0' };         // here is where putback() stores a Token
};

void Token_stream::putback(Token t)
{
    buffer = t;                 // copy t to buffer
    full = true;                // buffer is now full
};

Token Token_stream::get()
// ex 2 - added '{' & '}'
// ex 3 - added '!'
{
    if (full) {             // do we already have a Token?
        full = false;       // remove Token from buffer
        return buffer;
    }

    char ch;
    cin >> ch;              // note that >> skips whitespace

    switch (ch) {
    case ';':           // for "print"
    case 'q':           // for "quit"
    case '(': case ')': case '{': case '}': case '!':
    case '+': case '-': case '*': case '/':
        return Token{ ch };
    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 input stream
        double val;
        cin >> val;
        return Token{ '8', val };  // let '8' represent a number
    }
    default:
        error("Bad token");
    }
};

Token_stream ts;            // provides get() and putback()
double expression();        // forward declaration for primary to call

double primary()            // deal with numbers and parenthesis
// ex 2 - added '{' case
{
    Token t = ts.get();
    switch (t.kind) {
    case '(':                   // handle '(' expression ')'
    {
        double d = expression();
        t = ts.get();
        if (t.kind != ')')
            error("')' expected");
        return d;
    }
    case '{':
    {
        double d = expression();
        t = ts.get();
        if (t.kind != '}')
            error("'}' excepted");
        return d;
    }
    case '8':                   // we use '8' to represent a number
        return t.value;         // return the number's value
    default:
        error("primary expected");
    }
}

double secondary(){
    double left = primary();
    Token t = ts.get();

    while (true) {
        if (t.kind == '!') {
            if (left == 0)
                return 1;

            for (int i = left - 1; i > 0; --i)
                left *= i;

            t = ts.get();
        }
        else {
            ts.putback(t);
            return left;
        }
    }
}

double term()
{
    double left = secondary();
    Token t = ts.get();

    while (true) {
        switch (t.kind) {
        case '*':
            left *= secondary();
            t = ts.get();
            break;
        case '/':
        {
            double d = secondary();
            if (d == 0) error("divide by zero");
            left /= d;
            t = ts.get();
            break;
        }
        default:
            ts.putback(t);
            return left;
        }
    }
}

double expression()
{
    double left = term();
    Token t = ts.get();

    while (true) {
        switch (t.kind) {
        case '+':
            left += term();
            t = ts.get();
            break;
        case '-':
            left -= term();
            t = ts.get();
            break;
        default:
            ts.putback(t);
            return left;
        }
    }
}

int main()
try {
    double val = 0;
    while (cin) {
        Token t = ts.get();

        if (t.kind == 'q') break;       // 'q' for "quit"
        if (t.kind == ';')              // ';' for "print now"
            cout << "=" << val << '\n';
        else
            ts.putback(t);
        val = expression();
    }
}
catch (exception& e) {
    cerr << "Exception: " << e.what() << '\n';
    return 1;
}
catch (...) {
    cerr << "Unknown exception\n";
    return 2;
}