#include <vector>
#include <map>
#include <string>
#include <list>
#include <charconv>
#include <cstring>

template <typename T>
class resizing_vector : public std::vector<T> {
public:
    using std::vector<T>::vector;
    T& operator[](std::size_t index) {
        if (index >= std::vector<T>::size()) {
            std::vector<T>::resize(index + 1); 
        }   
        return std::vector<T>::operator[](index);
    }   
};

std::list<std::string> g_symbols;
std::map<std::string_view, resizing_vector<std::string> > g_cache;
int
main()
{
    const char *msg = "|5=AAPL|270=some_value";
    std::string_view key(msg+3,4);
    auto iter = g_cache.find(key);
    if( iter == g_cache.end())
    {
        g_symbols.emplace_back(key);
        iter = g_cache.insert( std::make_pair(g_symbols.back(), 0) ).first;
    }
    auto &tag_values = iter->second;
    
    // do this for each tag in message
    int tag = 0;
    std::from_chars(msg+8,msg+11, tag);
    auto &value = tag_values[tag];
    value.resize(10);
    memcpy(value.data(),msg+12,10);

    // dump the cache
    for(auto &[key,tag_values] : g_cache)
    {
        printf("key: %.*s\n", (int)key.size(), key.data());
        for(int i=0;i<tag_values.size(); i++)
            if(tag_values[i] != "")
                printf("\ttag: %d, value: %s\n", i, tag_values[i].c_str());
    }
    return 0;
}