heapselect.js

var heapselect = crossfilter.heapselect = heapselect_by(crossfilter_identity);

heapselect.by = heapselect_by;

function heapselect_by(f) {
  var heap = heap_by(f);

  // Returns a new array containing the top k elements in the array a[lo:hi].
  // The returned array is not sorted, but maintains the heap property. If k is
  // greater than hi - lo, then fewer than k elements will be returned. The
  // order of elements in a is unchanged by this operation.
  function heapselect(a, lo, hi, k) {
    var queue = new Array(k = Math.min(hi - lo, k)),
        min,
        i,
        x,
        d;

    for (i = 0; i < k; ++i) queue[i] = a[lo++];
    heap(queue, 0, k);

    if (lo < hi) {
      min = f(queue[0]);
      do {
        if (x = f(d = a[lo]) > min) {
          queue[0] = d;
          min = f(heap(queue, 0, k)[0]);
        }
      } while (++lo < hi);
    }

    return queue;
  }

  return heapselect;
}