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

using namespace std;

struct elreader
{ string line;
  int pos; };

void start(elreader & er, string s)
{ er.line = s + "_";
  er.pos = 0; }

char getnext(elreader & er)
{ if (er.pos >= er.line.length())
    return '_';
  char c = er.line[er.pos];
  er.pos = er.pos + 1;
  return c; }

char peeknext(const elreader & er)
{ if (er.pos >= er.line.length())
    return '_';
  return er.line[er.pos]; }

string readsym(elreader & er)
{ string ans = "";
  char c1 = getnext(er);
  if (isupper(c1))
  { char c2 = peeknext(er);
    if (islower(c2))
    { getnext(er);
      return ans + c1 + c2; } 
    return ans + c1; }
  cout << "readsym doesn't understand \"" << c1 << "\"\n";
  return "_"; }

int readnum(elreader & er)
{ int value = 0;
  while (isdigit(peeknext(er)))
  { int dig = getnext(er);
    value = value * 10 + dig - '0'; }      // could use digittoint()
  return value; }

struct item
{ char type;
  string s;
  int i; };

item readpart(elreader & E)
{ item result;
  char c = peeknext(E);
  if (isdigit(c))
  { result.type = 'N';
    result.i = readnum(E); }
  else if (isalpha(c))
  { result.type = 'S';
    result.s = readsym(E); }
  else if (c == '_')
    result.type = c;
  else if (c == '(' || c == ')')
  { result.type = getnext(E); }
  else
  { cout << "Unexpected in formula: " << c << "\n";
    result.type = 'X'; }
  return result; }

struct element
{ string name, symbol;
  double atwt;
  int atno; };

element makeel(int an, string sy, string nm, double aw)
{ element x;
  x.atno = an;
  x.atwt = aw;
  x.symbol = sy;
  x.name = nm;
  return x; }

void print(const element & x)
{ cout << x.atno   << " " 
       << x.name   << " " 
       << x.atwt   << " " 
       << x.symbol << "\n"; }

int read_table(element T[], int max)
{ ifstream elfile("elements.txt");
  if (elfile.fail())
  { cerr << "could not open elements.txt\n";
    return 0; }

  int num = 0;

  while (true)
  { string n, s;
    double a;

    elfile >> s >> n >> a;
    if (elfile.fail())
      break;

    num = num + 1;
    if (num >= max)
    { cerr << "too many elements\n";
      exit(1); }

    T[num] = makeel(num, s, n, a); }

  elfile.close();
  cout << num << " elements read\n";
  return num; }

element find_sym(const string & sy, element T[], int Tsize)
{ int i = 1;
  while (i <= Tsize)
  { if (T[i].symbol == sy)
      return T[i];
    i = i + 1; }
  return makeel(0, "Q", "Nosuchthingium", 0.0); }

double process_formula(elreader & E, element ptable[], int num)
{ double total = 0, previous = 0;
  while (true)
  { item it = readpart(E);
    if (it.type == '_' || it.type == ')')
      return total;
    else if (it.type == '(')
    { double subwt = process_formula(E, ptable, num);
      total = total + subwt;
      previous = subwt;
      cout << "   Total now " << total << "\n"; }
    else if (it.type == 'S')
    { string elem = it.s;
      cout << elem << " : ";
      element e = find_sym(elem, ptable, num);
      print(e);
      total = total + e.atwt;
      previous = e.atwt;
      cout << "   Total now " << total << "\n"; }
    else if (it.type == 'N')
    { int n = it.i;
      cout << "number " << n << "\n";
      total = total + previous * (n-1);
      previous = 0;
      cout << "   Total now " << total << "\n"; } } }

int main()
{ element ptable[150];
  int num = read_table(ptable, 150);
  if (num < 80)
  { cerr << "not enough (" << num << ") elements\n";
    exit(1); }

  while (true)
  { cout << "\n? ";
    string s;
    cin >> s;
    elreader E;
    start(E, s);
    double result = process_formula(E, ptable, num);
    cout << "\nTotal molecular weight is " << result << "\n"; } }