crossfilter/src/heap.js

Fast multidimensional filtering for coordinated views.

var heap = crossfilter.heap = heap_by(crossfilter_identity);

heap.by = heap_by;

function heap_by(f) {

  // Builds a binary heap within the specified array a[lo:hi]. The heap has the
  // property such that the parent a[lo+i] is always less than or equal to its
  // two children: a[lo+2*i+1] and a[lo+2*i+2].
  function heap(a, lo, hi) {
    var n = hi - lo,
        i = (n >>> 1) + 1;
    while (--i > 0) sift(a, i, n, lo);
    return a;
  }

  // Sorts the specified array a[lo:hi] in descending order, assuming it is
  // already a heap.
  function sort(a, lo, hi) {
    var n = hi - lo,
        t;
    while (--n > 0) t = a[lo], a[lo] = a[lo + n], a[lo + n] = t, sift(a, 1, n, lo);
    return a;
  }

  // Sifts the element a[lo+i-1] down the heap, where the heap is the contiguous
  // slice of array a[lo:lo+n]. This method can also be used to update the heap
  // incrementally, without incurring the full cost of reconstructing the heap.
  function sift(a, i, n, lo) {
    var d = a[--lo + i],
        x = f(d),
        child;
    while ((child = i << 1) <= n) {
      if (child < n && f(a[lo + child]) > f(a[lo + child + 1])) child++;
      if (x <= f(a[lo + child])) break;
      a[lo + i] = a[lo + child];
      i = child;
    }
    a[lo + i] = d;
  }

  heap.sort = sort;
  return heap;
}