top N active stocks #bbg#cache,VO

Requirement: implement top_n(int N): top N most active stocks

Requirement 2: add a top_n_last_x_minute(int N, int X): top N that are last traded in the last X millisec

This program also demonstrates how to avoid storing the same string twice, as map key and also as field of the map value

//
#include <unordered_map> //requires -std=c++0x
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <chrono>
#include <assert.h>
using namespace std;

struct Rec{
  string const * strPtr; //a best-effort to avoid storing duplicate strings
  int cumvol; //cum vol
  long lastUpd; //millsec since epoch

  Rec(int vol): strPtr(NULL), cumvol(vol),
    lastUpd( (chrono::system_clock::now().time_since_epoch()).count()) { }

  bool operator<(Rec const & other) const{
        return this->cumvol < other.cumvol;
  }
  friend ostream & operator<<(ostream & os, Rec const & r){
        os<<r.lastUpd<<"-"<<*(r.strPtr)<<"  : "<<r.cumvol;
        return os;
  }
};

typedef unordered_map<string, Rec>::iterator ITR;

class App{
  unordered_map<string, Rec> lookup;
  vector<Rec> v;
  bool dirty;
public:
  App(): dirty(true){}
  void update(string const & symbol, int vol){
    this->dirty = true;
    ITR it = lookup.find(symbol);
    if (it == lookup.end()){
        pair<ITR, bool> pair = lookup.insert(make_pair(symbol, Rec(vol)));
        assert(pair.first->second.strPtr == NULL);
        pair.first->second.strPtr = //second is the valye in the pair
          & pair.first->first; //the key of the new pair
    }else{
        assert(it->second.strPtr != NULL);
        it->second.cumvol += vol;
        it->second.lastUpd = (std::chrono::system_clock::now().time_since_epoch()).count();
    }
  }
  void top_n(int N, unsigned long X=-1){ //unsigned -1 gives the largest positive value!
    size_t sz=lookup.size();
    if (dirty){
      cout<<"resorting due to updates to database ..."<<endl;
      v.clear();
      for(ITR i=lookup.begin(); i!= lookup.end(); ++i){
        v.push_back(i->second);
      }
      sort(v.begin(), v.end());
      dirty = false;
    }
    long now = (std::chrono::system_clock::now().time_since_epoch()).count();
    //cout<<now<<" is now"<<endl;
    for(int k=sz-1, count=0; k>=0; --k){
        if (now - v[k].lastUpd > X) continue;
        cout<<v[k]<<endl;
        if (++count >= N) break;
    }
  }
};
int main(){
  App app;
  app.update("ibm", 1000);
  app.update("gs", 700);
  app.update("gs", 500);
  app.update("goog", 600);
  app.update("msft", 800);
  app.update("ge", 500);
  app.update("ibm", 160);
  app.top_n(6,29);
  app.update("c", 400);
  app.top_n(3,39);
}
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