#include <iostream>
#include <fstream>
#include <string>

using namespace std;

struct input
{ string line;
  int pos, len;
          // function inside struct
          //          no return type
          //          same name as struct
          // is an automatic constructor
  input()
  { line = "";
    pos = 0;
    len = 0; }

  void fill(string s)
  { line = s;
    pos = 0;
    len = s.length(); }

  void print()
  { cout << "line is '" << line << "'\n";
    cout << "pos is " << pos << "\n";
    cout << "len is " << len << "\n"; }

  char next()
  { if (pos >= len)
    { pos += 1;
      return '\n'; }
    char result = line[pos];
    pos += 1;
    return result; }

  void un_next()
  { if (pos <= 0)
      return;
    pos -= 1; }

  char sneak()
  { char x = next();
    un_next();
    return x; }
 };

char whats_next(input & x)
{ char s = x.sneak();
  if (s >= 'A' && s <='Z')
    return 'A';      // an Atom is coming next
  if (s >= '0' && s <='9')
    return 'N';      // a Number is coming next
  if (s == '\n')
    return 'E';      // End of formula is next
  return s; }

string read_atom(input & x)
{ string s = "";
  char c1 = x.next();
  if (c1 < 'A' || c1 > 'Z')
  { cout << "Capital expected at beginning of atom\n";
    return "XXXXXX"; }
  s += c1;
  char c2 = x.next();
  if (c2 >= 'a' && c2 <= 'z')
    return s + c2;
  x.un_next();
  return s; } 

int read_number(input & x)
{ int value = 0;
  while (true)
  { char c = x.next();
    if (c < '0' || c > '9')
      break;
    value = value * 10 + c - '0'; }
  x.un_next();
  return value; }

struct element
{ string symbol, name;
  int at_no;
  double at_wt; };

void print(element e)
{ cout << e.at_no << ": " << e.symbol << ", "
       << e.name << ", at wt = " << e.at_wt << "\n"; }

const int max_num_els = 110;

element table[max_num_els];

int num_els = 0;

void read_table(ifstream & f)
{ while (num_els < max_num_els)
  { f >> table[num_els+1].symbol 
      >> table[num_els+1].name 
      >> table[num_els+1].at_wt; 
    table[num_els+1].at_no = num_els + 1;
    if (f.fail())
      return; 
    num_els += 1; } }

int find(string sy)
{ for (int i = 1; i <= num_els; i += 1)
    if (table[i].symbol == sy)
      return i;
  return 0; }

double compute_formula(input & I)
{ double prev_wt = 0.0, total = 0.0;
  while (true)
  { char nx = whats_next(I);
    if (nx == 'A')   // an atom
    { string at = read_atom(I);
      int epos = find(at);
      if (epos < 1)
      { cout << "element " << at << " not known\n";
        break; }
      print(table[epos]);
      prev_wt = table[epos].at_wt; 
      total += prev_wt; }
    else if (nx == 'N')   // a number
    { int n = read_number(I);
      total += (n - 1) * prev_wt;
      cout << "  correcting " << prev_wt << " to " << n * prev_wt << "\n"; }
    else if (nx == '(')
    { I.next();
      double subtotal = compute_formula(I);
      cout << "subtotal is " << subtotal << "\n";
      prev_wt = subtotal;
      total += prev_wt; }
    else if (nx == ')')   // end of sub-formula
    { I.next();
      break; }
    else if (nx == 'E')   // end of input
      break;
    else
    { cout << "Bad input: '" << nx << "'\n";
      break; } }
  return total; }

int main()
{ ifstream file("table.txt");
  if (file.fail())
  { cout << "Can't read table file\n";
    exit(1); }
  read_table(file);
  file.close();
  cout << num_els << " elements read\n";
  input I;
  string s;
  cout << "Enter a line: ";
  getline(cin, s);
  I.fill(s);
  double total = compute_formula(I);
  cout << "--------------------------------------\n";
  cout << "Molecular weight = " << total << "\n";
  cout << "--------------------------------------\n"; }