// Code derived from Stroustrup's PPP2 book
// § 7.4 Negative numbers
//  -and beginning on p 230

#include <iostream>
#include <stdexcept>
#include <string>

using namespace std;

void error(const char* s) { throw runtime_error(s); }

//------------------------------------------------------------------------------
class Token {
 public:
  Token(char ch) : kind{ch} {}

  Token(char ch, double val) : kind{ch}, value{val} {}

  char   kind  = '0';
  double value = 0.0;
};

//------------------------------------------------------------------------------
class Token_stream {
 public:
  Token_stream();

  Token get();
  void  putback(Token t);

 private:
  bool  full;
  Token buffer;
};

//------------------------------------------------------------------------------
Token_stream::Token_stream() : full{false}, buffer{0} {}

//------------------------------------------------------------------------------
// read a token from cin
Token Token_stream::get()
{
  if (full) {  // check if we already have a Token ready
    full = false;
    return buffer;
  }

  char ch;
  cin >> ch;

  // clang-format off
  switch (ch) {
    case ';':
    case 'q':
    case '(': case ')': case '+': case '-': case '*': case '/':
      return Token{ch};
      break;
    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;
      return Token{'8', val};  // let '8' represent "a number"
      break;
    }
    default:
      error("Bad token");
      return Token{'K'};  // invalid, shouldn't reach here
  }
  // clang-format on
}

//------------------------------------------------------------------------------
// put token back into stream
void Token_stream::putback(Token t)
{
  if (full)
    error("putback() into a full buffer");

  buffer = t;
  full   = true;
}

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

Token_stream ts;

double expression();

//------------------------------------------------------------------------------
// deal with numbers, parentheses, unary -, and unary +
double primary()
{
  Token t = ts.get();

  switch (t.kind) {
    case '(': {
      double d = expression();

      t = ts.get();
      if (t.kind != ')')
        error("')' expected");

      return d;
      break;
    }
    case '8':  // we use '8' to represent a number
      return t.value;
      break;
    case '-': return -primary(); break;
    case '+': return primary(); break;
    default:
      error("primary expected");
      // shouldn't reach here
      return 0.0;
  }
}

//------------------------------------------------------------------------------
// deal with *, /
double term()
{
  double left = primary();

  Token t = ts.get();

  while (true) {
    switch (t.kind) {
      case '*': left *= primary(); break;
      case '/': {
        double d = primary();
        if (d == 0)
          error("divide by zero");

        left /= d;
        break;
      }
      default: ts.putback(t); return left;
    }

    t = ts.get();
  }
}

//------------------------------------------------------------------------------
// deal with + and -
double expression()
{
  double left = term();

  Token t = ts.get();

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

    t = ts.get();
  }
}

//------------------------------------------------------------------------------
int main()
try {
  while (cin) {
    cout << "> ";

    Token t = ts.get();

    while (t.kind == ';')
      t = ts.get();  // eat ';'

    if (t.kind == 'q')
      cout << "\b\b";  // hack to rm final '>'

    ts.putback(t);

    cout << "= " << expression() << '\n';
  }

} catch (exception& e) {
  cerr << e.what() << '\n';
  return 1;

} catch (...) {
  cerr << "exception \n";
  return 2;
}