duplexer-redux.js

module.exports = function(counter) {
  var duplexer = require('duplexer')
    , through = require('through')    
    , thr = through(write)
    , countries = {}
    ;

  return duplexer(thr, counter);

  function write(buf) {
    if (countries.hasOwnProperty(buf.country)) {
      countries[buf.country] += 1;      
    }
    else {
      countries[buf.country] = 1;
    }
  }
  function end(buf) {
    counter.setCounts(countries);
  }
};