Parse chemical expression (ie : (CH4)2HeLi )

Chemical formula parser

#include <iostream>
#include <sstream>


typedef std::map<std::string,int> chemicalComposition;
    
void transferChemical(chemicalComposition& left, chemicalComposition& right)
{   
    for(chemicalComposition::iterator it = right.begin(); it!=right.end(); ++it)
        left[it->first] += it->second;  
    right.clear();
}

chemicalComposition parseChemical(std::string formula)
{
    chemicalComposition total; 
    chemicalComposition remainder;  

    size_t pos = 0;

    while(formula.size() != 0) 
    {
        if (isupper(formula[pos]) && isalpha(formula[pos])) // new element
        {
            // add previous remainder if there is one
            transferChemical(total,remainder);
  
            // read the name
            std::string name;
            name += formula[pos]; pos++;
            while (pos<formula.size() && !isupper(formula[pos]) && isalpha(formula[pos])) { name += formula[pos]; pos++; }
            remainder[name] = 1;    
        }
        else if (isdigit(formula[pos])) // number
        {
            if (remainder.size() == 0)
                throw std::invalid_argument("can't parse expression");

            // read number
            int multiplicity = 0;   
            while(pos<formula.size() && isdigit(formula[pos])) {multiplicity = 10*multiplicity + int(formula[pos] - '0') ; ++pos;}
            
            // muliply remainder    
            for(chemicalComposition::iterator it = remainder.begin(); it!=remainder.end(); ++it) it->second *= multiplicity;

        }
        else if (formula[pos] == '(')   
        {
            // add previous remainder if there is one
            transferChemical(total,remainder);

            // look for the right parenthesis
            int left = pos;
            ++pos;
            int count = 1;
            while(pos<formula.size() && count!=0)
            {
                if (formula[pos] == '(')
                    ++count;
                else if (formula[pos] == ')')
                    --count;
                ++pos;
            }
            int right = pos;
            if (pos==formula.size())
                throw std::invalid_argument("can't parse expression");
            remainder = parseChemical(formula.substr(left+1,right-left-2));
            pos = right;
        }
        formula = formula.substr(pos);
        pos = 0;
    }
    // add previous remainder if there is one
    transferChemical(total,remainder);

    return total;
}