map reduce for Couch statistics

map-reduce.js

var a = {
  place: 'europe',
  animal: 'dog'
},
b = {
  animal: 'dog'
},
c = {
  place: 'africa'
},
d = {
  place: 'africa',
  animal: 'dog'
}
e = {
  place: 'africa',
  animal: 'cat'
};
function reduce(keys, values, rereduce) {
  function setDefault(obj, key, value) {
    if (!(key in obj)) {
      obj[key] = value;
    }
  }
  function setOrSum(obj, key, value) {
    obj[key] = obj[key] ? obj[key] + value : value;
  };
  function doubleSetOrSum(value1, value2, obj, key) {
    setDefault(obj, key, {});
    setDefault(obj[key], value1, {});
    setOrSum(obj[key][value1], value2, 1);
  }
  function add(value, stats) {
    setOrSum(stats, value.animal, 1);
    doubleSetOrSum(value.place, value.animal, stats, 'place');
  }
  function forKeyIn(obj, fun) { Object.getOwnPropertyNames(obj).forEach(fun); }
  function merge(stat, merged) {
    forKeyIn(stat, function(key) {
      if (typeof stat[key] === 'number') {
        setOrSum(merged, key, stat[key]);
      } else {
        setDefault(merged, key, {});
        forKeyIn(stat[key], function(key2) {
          setOrSum(merged[key], key2, stat[key][key2]);
        });
      }
    });
  }
  function accumulate(arr, fun, accumulator) {
    arr.forEach(function(el) {
      fun(el, accumulator);
    });
    return accumulator;
  }
  var fun = rereduce ? merge : add;
  return accumulate(values, fun, {});
}

var test = reduce(null, [reduce(null, [a, b]), reduce(null, [c, d, e])], true);
require('assert').deepEqual(test, {"dog":3,"place":{"europe":{"dog":1},"undefined":{"dog":1},"africa":{"dog":1,"cat":1, "undefined":1}},"cat":1, "undefined":1});