unique key count reduce

reduce.js

function(ks, vs, rr) {
  // values are discarded by reduce
  // todo: configurable key function, for complex queries
  var fk, lk;
  function uniqCount(keys) {
    var count = 0, obj = {};
    for (var i=0; i < keys.length; i++) {
      if (!obj[keys[i]]) {
        count++;
        obj[keys[i]] = true;
      }
    };
    return count;
  };
  if (rr) {
    var fv = vs.shift()
      , lv = vs.pop()
      , count = fv[1] + lv[1]
      , leftover_keys = [fv[2], lv[0]]
      , v, i
      ;
    fk = fv[0];
    lk = lv[2];
    for (i=0; i < vs.length; i++) {
      v = vs[i];
      count = count + v[1];
      leftover_keys.push(v[0]);
      leftover_keys.push(v[2]);
    };
    return [fk, uniqCount(leftover_keys) + count, lk];
  } else {
    fk = ks.shift();
    lk = ks.pop();
    return [fk, uniqCount(ks), lk];
  }
};