Making CouchDB map/reduce a little less nutty...

mapreduce.js

// This is the map function to give to CouchDB:
function(doc) {
  function xemit(key, val) {
    emit(key, {key: key, val: val});
  }

  // Your code goes here, working with the doc provided.
  // Use xemit(key, value) instead of CouchDB's emit

  // ...
}

// This is the reduce function to give to CouchDB:
function (keys, docs) {
  function xreduce(key, values) {
    // Your code goes here, working with a single key and list of values,
    // as we discussed in class.  Note you can rename the args 'key' and
    // 'values' of this xreduce function if you'd like.  Remember to
    // return a single value of the same type as you passed as value to xemit().

    // ...

  }

  // This stuff is just to make reduce behave sensibly:
  var vals = [];
  for(var i = 0; i < docs.length; ++i) {
    vals.push(docs[i].val);
  }
  return {key: docs[0].key, val: xreduce(docs[0].key, vals)};
}

// Note that the results of this will have an extra level of object in the output.
// For example, if you do something like xemit('Smith', 25), the resulting
// couchdb view will actually be more like:
// { Smith: { key: 'Smith', val: 25 } }


// Here's an example of using the template above to compute the
// average year each author's books were published.  The parts
// marked between BEGIN and END should look like the map/reduce
// for average that we looked at in class -- the only difference
// is that this code is avg_year for the book collection instead
// of avg_age for a person collection:

// map
function(doc) {
  function xemit(key, val) {
    emit(key, {key: key, val: val});
  }

  // BEGIN Code to do the average:  
  for(var i = 0; i < doc.books.length; ++i) {
    xemit(doc.author.last_name,
          {count: 1, year_sum: doc.books[i].year});
  }
  // END
}

// reduce
function (keys, docs) {
  function xreduce(key, values) {
    // BEGIN Code to do the average:
    var result = {count: 0, year_sum: 0};
    for(var i = 0; i < values.length; ++i) {
      result.count += values[i].count;
      result.year_sum += values[i].year_sum;
    }
    return result;
    // END
  }

  var vals = [];
  for(var i = 0; i < docs.length; ++i) {
    vals.push(docs[i].val);
  }
  return {key: docs[0].key, val: xreduce(docs[0].key, vals)};
}