MongoDB Map-Reduce script for aggregating toppers for every group under different categories

a_problem_description.txt

I have a "users" collection with documents like this : 
{ 
        name : "Jack" 
        group: "Group-1" 
        total : 121 
        subScores : { 
                        cat1 : 38 
                        cat2 : 42 
                        cat3 : 41 
                     } 
} 

total is the total user's score 
subScores are different category based scores 

Now I want to list out  the following on a weekly basis: 
1) Top 10 users from each group 
2) Top 10 users from each group in each category 
I am thinking of having the results for both of these operations in a 
single collection in this format : 
{ 
        group : "group-1" 
        sub : total 
        names : ["jack", "john", "foo", "bar".......] 
} 

{ 
        group : "group-1" 
        sub : cat1 
        names : ["jill", "paul", "foo", "bar".......] 
} 

or in an aggregated form like this : 
{ 
        group : "group-1" 
        toppers : { 
                        total : ["jack", "john", "foo", "bar".......] 
                        cat1 : ["jill", "paul", "foo", "bar".......] 
                        ....... 
                        ....... 
                      } 
} 

Note : The final results of the Map-Reduce are different. They are like this :

total_toppers collection has documents like this :
{
  _id : "group-1"
  value : {
            students : [
                         { name : "Jack", score : 157 }
                         { name : "Jill", score : 154 }
                         ...........
                         ...........
                         ...........
                         ...........
                       ]
          }
}

subject_toppers have exactly similar documents, except that the _id field has values of the form "group-1::cat1"

mongo_students_data_generate.js

var schools = [
  "school-1",
  "school-2",
  "school-3",
  "school-4",
  "school-5",
  "school-6",
  "school-7",
  "school-8",
  "school-9",
  "school-0"
];

function getSchool() {
  return schools[Math.floor(Math.random() * 10)]
}

function getName() {
 var chars = "ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz";
 var string_length = 8;
 var randomstring = '';
 for (var i=0; i<string_length; i++) {
  var rnum = Math.floor(Math.random() * chars.length);
  randomstring += chars.substring(rnum,rnum+1);
 }
 return randomstring;
}

function getScores() {
  var s1 = Math.floor(Math.random() * 50 + 1);
  var s2 = Math.floor(Math.random() * 50 + 1);
  var s3 = Math.floor(Math.random() * 50 + 1);
  return new Array(s1, s2, s3, s1+s2+s3);
}

var mydb = db.getSisterDB("map_reduce");
print("Begin : " + Date.now() + "\n");

for(var i = 0; i < 50000; i++) {
  s = getScores();
  a = {
    name : getName(),
    school : getSchool(),
    total : s[3],
    subScores : {
          math : s[0],
          sci : s[1],
          eng : s[2]
        }
  }
  mydb.students.insert(a);
}

print("End : " + Date.now() + "\n");

mongo_students_toppers_map_reduce.js

function totalMap() {
    emit(this.school, {students : [{name : this.name, score : this.total}]});
}

function subjectMap() {
    for(var sub in this.subScores) {
      emit(this.school + "::" + sub, {students : [{name : this.name, score : this.subScores[sub]}]});
    }
}

function reduce(school, values) {
    toppers = [];
    length = values.length;
    for (var i = 0; i < length; i++) {
      stLen = values[i].students.length;
      for(var j = 0; j < stLen; j++) {
        if (toppers.length == 10) {
            if (values[i].students[j].score > toppers[9].score)
                toppers[9] = values[i].students[j];
        } else {
            toppers.push(values[i].students[j]);
        }   
        toppers.sort(function(a, b) { return b.score - a.score; }); 
      }   
    }   
    // Cannot return an array at the top level, so had to make it an object
    return {students : toppers};
}

var mydb = db.getSisterDB("map_reduce");
var start = Date.now();
var end = 0;
print("Begin : " + start);
try {
  res = mydb.runCommand({mapReduce : "students", map : totalMap, reduce : reduce, verbose : true, out : "total_toppers"});
  res = mydb.runCommand({mapReduce : "students", map : subjectMap, reduce : reduce, verbose : true, out : "subject_toppers"});
  printjson(res);
  end = Date.now();
  print("Success : End : " + end);
} catch (ex) {
  printjson(ex);
  end = Date.now();
  print("With exception : End : " + end);
}

print("Time taken in milliseconds  = " + (end - start));