gistfile1.txt

12:37 <Baz___> Specifically, I have ratings ({ accountID, itemID, rating }) and am trying to find out how many times pairs of users rated the same items: [{ accountID1, accountID2, numTimesRatedSameItem }]

map:

emit([doc.itemID, doc.accountID]);

reduce:

var permute = function(arr) {
  // return an array of every pair of values in arr,
  // each array item is sorted.
};

if (!rereduce) {
  var result = [];
  var items = unique values from all keys[0];
  for (var item in items) {
    var accountsForItem = sorted array of accountID's for this item
    for (var permutation in permute(accountsForItem)) {
      emit(permutation, 1);
    }
  }
  return result;
} else {
  return sum(values);  
}