Tuenti Contest 2 (Pablo Moreno)

/*
    Author: Pablo Moreno Olalla
    Email address: darthbrevu@yahoo.es
*/
#include "BigNumber.h"

#include <list>
#include <cstdio>
#include <string>
#include <iostream>
#include <algorithm>

using namespace std;

inline bool add(const list<Number> &in,Number &out) {
    if (in.empty()) return false;
    list<Number>::const_iterator it=in.begin();
    out=*it;
    list<Number>::const_iterator end=in.end();
    for (++it;it!=end;++it) out+=*it;
    return true;
}

inline bool mul(const list<Number> &in,Number &out) {
    if (in.empty()) return false;
    list<Number>::const_iterator it=in.begin();
    out=*it;
    list<Number>::const_iterator end=in.end();
    for (++it;it!=end;++it) out*=*it;
    return true;
}

inline bool sub(const list<Number> &in,Number &out) {
    if (in.empty()) return false;
    list<Number>::const_reverse_iterator it=in.rbegin();
    out=*it;
    out.flipSign();
    list<Number>::const_reverse_iterator rend=in.rend();
    for (++it;it!=rend;++it) out+=*it;
    return true;
}

string numberChrs="+-0123456789";

string validChars="=#@";

bool parse(const string &in,Number &out,size_t &pos);

bool parseNumber(const string &in,Number &out,size_t &pos)  {
    size_t pos2=in.find_first_not_of(numberChrs,pos);
    if (pos2==string::npos) pos2=in.size();
    try {
        out=Number(in.substr(pos,pos2-pos));
    }   catch (const runtime_error &)   {
        return false;
    }
    pos=pos2;
    return true;
}

bool parseExpr(const string &in,Number &out,size_t &pos)    {
    //Whenever this method is called, we're sure that the first character is "^". So, skip it.
    ++pos;
    while (pos<in.size()&&in[pos]==' ') ++pos;
    if (pos==in.size()) return false;
    char op=in[pos];
    if (validChars.find(op)==string::npos) return false;
    ++pos;
    list<Number> parsed;
    Number tmpN;
    for (;;)    {
        while (pos<in.size()&&in[pos]==' ') ++pos;
        if (pos==in.size()) return false;
        if (in[pos]=='$') break;
        if (!parse(in,tmpN,pos)) return false;
        parsed.push_back(tmpN);
    }
    ++pos;  //So we point just AFTER the '$'.
    if (op=='=') return add(parsed,out);
    else if (op=='#') return mul(parsed,out);
    else if (op=='@') return sub(parsed,out);
}

bool parse(const string &in,Number &out,size_t &pos)    {
    if (pos>=in.size()) return false;
    while (in[pos]==' ')    {
        ++pos;
        if (pos>=in.size()) return false;
    }
    //Not sure if this is correct.
    if (in[pos]=='^') return parseExpr(in,out,pos);
    else if (numberChrs.find(in[pos])) return parseNumber(in,out,pos);
    else return false;
}

int main(int argc,char **argv)  {
    //The first two lines are not strictly necessary, but make the code a little more handy.
    if (argc>=2) freopen(argv[1],"r",stdin);
    if (argc>=3) freopen(argv[2],"w",stdout);
    string tmp;
    Number n;
    size_t pos;
    do  {
        pos=0;
        getline(cin,tmp);
        if (parse(tmp,n,pos)) cout<<static_cast<string>(n)<<endl;
    }   while (!cin.eof());
    return 0;
}