/* Code of Figure 7.11, pages 294-297 from
   Kenneth C. Louden, Programming Languages
   Principles and Practice 2nd Edition
   Copyright (C) Brooks-Cole/ITP, 2003
*/

#include <iostream>
#include <string>

using namespace std;

int token;

struct InputError {} inputError;
struct UnexpectedChar { char got; };
struct NumberExpected { char got; };
struct ExtraChars { string chars; };
struct Unwind {} unwind;

void command() throw (Unwind,InputError);
int expr() throw (Unwind, InputError);
int term() throw (Unwind, InputError);
int factor() throw (Unwind, InputError);
int number() throw (NumberExpected,InputError);
int digit() throw (NumberExpected,InputError);

void getToken() throw (InputError)
{ token = cin.get();
  if (cin.fail()) throw inputError;
}

void match(char c) throw (InputError,UnexpectedChar)
{ if (token == c) getToken();
  else
  { UnexpectedChar u; // struct not required in C++
    u.got = token;
    throw u;
  }
}

void command() throw (Unwind,InputError)
{ try
  { int result = expr();
    if (token != '\n')
    { ExtraChars e; // struct not reqired
      while (token != '\n')
      { e.chars += token;
        getToken();
      }
      throw e;
    }
    else
      cout << "The result is: " << result;
  }
  catch (ExtraChars e)
  { cout << "Extra characters found in command: " 
         << e.chars << endl
         << "(missing operator or extra right parenthesis?)\n";
    throw unwind;
  }
  catch (Unwind u)
  { cout << "Unwinding command\n";
    throw;
  }
}

int expr() throw (Unwind,InputError)
{ try
  { int result = term();
    while (token == '+')
    { match('+');
      result += term();
    }
    return result;
  }
  catch (Unwind u)
  { cout << "Unwinding expression\n";
    throw;
  }
}

int term() throw (Unwind,InputError)
{ try
  { int result = factor();
    while (token == '*')
    { match('*');
      result *= factor();
    }
    return result;
  }
  catch (Unwind u)
  { cout << "Unwinding term\n";
    throw;
  }
}

int factor() throw (Unwind,InputError)
{ try
  { int result;
    if (token == '(')
    { match('(');
      result = expr();
      match(')');
    }
    else
      result = number();
    return result;
  }
  catch (UnexpectedChar u)
  { cout << "Expected right parenthesis in factor\n"
          << "found: " << u.got << endl;
    throw unwind;
  }
  catch (Unwind u)
  { cout << "Unwinding factor\n";
    throw;
  }
  catch (NumberExpected n)
  { cout << "Number expected in factor\n"
         << "found: " << n.got << endl;
    throw unwind;
  }
}

int number() throw (NumberExpected,InputError)
{ int result = digit();
  while (isdigit(token))
    result = 10 * result + digit();
  return result;
}

int digit() throw (NumberExpected,InputError)
{ if (isdigit(token))
  { int result = token - '0';
    match(token);
    return result;
  } 
  else
  { NumberExpected n; // struct not required
    n.got = token;
    throw n;
  }
}

void parse() throw (InputError)
{ try
  { getToken();
    command();
  }
  catch (Unwind u)
  { cout << "Unwinding parse\n";
  }
}

int main()
{ try
  { parse();
  }
  catch (InputError i)
  { cout << "Input error in parse\n";
  }
  return 0;
}