C++/PCRE

#include <string>
#include <iostream>
#include <map>
#include <algorithm>

#include <pcrecpp.h>

int main(int, char** )
{
    const pcrecpp::RE re("\"GET (?:https?://.+?/)?(.+?\\.mp3) "
                     "HTTP/1\\.[01]\" \\d{3} (\\d+)");

    std::map<std::string, std::pair<long, long>> stat;
    for( std::string line; std::getline(std::cin, line); )
    {
        std::string fname;
        long size;

        if( re.PartialMatch( line, &fname, &size ) )
        {
            auto it = stat.find( fname );
            if( it == stat.end() )
                stat.emplace( fname, std::make_pair(size, size) );
            else
            {
                const auto sum = it->second.first + size;
                const auto max = std::max( it->second.second, size );
                it->second = std::make_pair( sum, max );
            }
        }
    }

    for( const auto& info: stat )
    {
        const double sum = info.second.first;
        const auto max = std::max(info.second.second, 1L); // avoid division by zero
        const auto downloads = sum / max;
        std::cout << "Key: " << info.first << " downloads: " << downloads
              << " (max size: " << max << ")" << std::endl;
    }
}