'use strict';

var a = require('./a');
var _map = require('./_map');

module.exports = function filter (concurrency) {
  return _map(concurrency, then);
  function then (collection, done) {
    return function filter (err, results) {
      function exists (item, key) {
        return !!results[key];
      }
      function ofilter () {
        var filtered = {};
        Object.keys(collection).forEach(function omapper (key) {
          if (exists(null, key)) { filtered[key] = collection[key]; }
        });
        return filtered;
      }
      if (err) { done(err); return; }
      done(null, a(results) ? collection.filter(exists) : ofilter());
    };
  }
};