Map reduce example in dart

mapreduce.dart

String sortedKey(List couple) {
  couple.sort();
  return Strings.concatAll(couple);
}

List mapper(Map m) {
  var res = [];
  for (var person in m.keys) {
    var friends = m[person];
    for (var friend in friends) {
      String key = sortedKey([person, friend]);
      res.add({'$key': friends});
    }
  }

  return res;
}

List intersect(List lists) {
  var intersections = [];
  if (lists.length > 1) {
    for (var f in lists[1]) {
      if (lists[0].contains(f)) {
        intersections.add(f);
      }
    }
  }

  return intersections;
}

Map reducer(String intermediate_key, List intermediate_value_list) {
  Map intersection = {};
  intersection[intermediate_key] = intersect(intermediate_value_list);
  return intersection;
}

void main() {
  Map m = {'A': ['B','C','D'],
           'B': ['A','C','D','E'],
           'C': ['A','B','D','E'],
           'D': ['A','B','C','E'],
           'E': ['B','C','D']};

  var intermediate = mapper(m);
  var groups = {};
  var group = new Set.from(intermediate.map((v)=>v.keys[0]));

  for (var g in group) {
    var items = intermediate.filter((v)=> v.keys[0] == g);
    groups[g] = [];
    for (Map i in items) {
      groups[g].addAll(i.values);
    }
  }

  List reducer_res = [];
  for (var intermediate_key in group) {
    reducer_res.add(reducer(intermediate_key, groups[intermediate_key]));
  }

  // sort the results;
  reducer_res.sort((a, b) => a.keys[0].compareTo(b.keys[0]));

  for (var friendship in reducer_res) {
    print("${friendship.keys[0][0]} & ${friendship.keys[0][1]} have ${friendship[friendship.keys[0]]} as common friends ");
  }
}

http://www.stevekrenzel.com/articles/finding-friends

https://gist.github.com/184137