source: Dev/trunk/d3/src/layout/treemap.js @ 76

// Squarified Treemaps by Mark Bruls, Kees Huizing, and Jarke J. van Wijk
// Modified to support a target aspect ratio by Jeff Heer
d3.layout.treemap = function() {
  var hierarchy = d3.layout.hierarchy(),
      round = Math.round,
      size = [1, 1], // width, height
      padding = null,
      pad = d3_layout_treemapPadNull,
      sticky = false,
      stickies,
      ratio = 0.5 * (1 + Math.sqrt(5)); // golden ratio

  // Compute the area for each child based on value & scale.
  function scale(children, k) {
    var i = -1,
        n = children.length,
        child,
        area;
    while (++i < n) {
      area = (child = children[i]).value * (k < 0 ? 0 : k);
      child.area = isNaN(area) || area <= 0 ? 0 : area;
    }
  }

  // Recursively arranges the specified node's children into squarified rows.
  function squarify(node) {
    if (!node.children) return;
    var rect = pad(node),
        row = [],
        children = node.children.slice(), // copy-on-write
        child,
        best = Infinity, // the best row score so far
        score, // the current row score
        u = Math.min(rect.dx, rect.dy), // initial orientation
        n;
    scale(children, rect.dx * rect.dy / node.value);
    row.area = 0;
    while ((n = children.length) > 0) {
      row.push(child = children[n - 1]);
      row.area += child.area;
      if ((score = worst(row, u)) <= best) { // continue with this orientation
        children.pop();
        best = score;
      } else { // abort, and try a different orientation
        row.area -= row.pop().area;
        position(row, u, rect, false);
        u = Math.min(rect.dx, rect.dy);
        row.length = row.area = 0;
        best = Infinity;
      }
    }
    if (row.length) {
      position(row, u, rect, true);
      row.length = row.area = 0;
    }
    node.children.forEach(squarify);
  }

  // Recursively resizes the specified node's children into existing rows.
  // Preserves the existing layout!
  function stickify(node) {
    if (!node.children) return;
    var rect = pad(node),
        children = node.children.slice(), // copy-on-write
        child,
        row = [];
    scale(children, rect.dx * rect.dy / node.value);
    row.area = 0;
    while (child = children.pop()) {
      row.push(child);
      row.area += child.area;
      if (child.z != null) {
        position(row, child.z ? rect.dx : rect.dy, rect, !children.length);
        row.length = row.area = 0;
      }
    }
    node.children.forEach(stickify);
  }

  // Computes the score for the specified row, as the worst aspect ratio.
  function worst(row, u) {
    var s = row.area,
        r,
        rmax = 0,
        rmin = Infinity,
        i = -1,
        n = row.length;
    while (++i < n) {
      if (!(r = row[i].area)) continue;
      if (r < rmin) rmin = r;
      if (r > rmax) rmax = r;
    }
    s *= s;
    u *= u;
    return s
        ? Math.max((u * rmax * ratio) / s, s / (u * rmin * ratio))
        : Infinity;
  }

  // Positions the specified row of nodes. Modifies `rect`.
  function position(row, u, rect, flush) {
    var i = -1,
        n = row.length,
        x = rect.x,
        y = rect.y,
        v = u ? round(row.area / u) : 0,
        o;
    if (u == rect.dx) { // horizontal subdivision
      if (flush || v > rect.dy) v = rect.dy; // over+underflow
      while (++i < n) {
        o = row[i];
        o.x = x;
        o.y = y;
        o.dy = v;
        x += o.dx = v ? round(o.area / v) : 0;
      }
      o.z = true;
      o.dx += rect.x + rect.dx - x; // rounding error
      rect.y += v;
      rect.dy -= v;
    } else { // vertical subdivision
      if (flush || v > rect.dx) v = rect.dx; // over+underflow
      while (++i < n) {
        o = row[i];
        o.x = x;
        o.y = y;
        o.dx = v;
        y += o.dy = v ? round(o.area / v) : 0;
      }
      o.z = false;
      o.dy += rect.y + rect.dy - y; // rounding error
      rect.x += v;
      rect.dx -= v;
    }
  }

  function treemap(d) {
    var nodes = stickies || hierarchy(d),
        root = nodes[0];
    root.x = 0;
    root.y = 0;
    root.dx = size[0];
    root.dy = size[1];
    if (stickies) hierarchy.revalue(root);
    scale([root], root.dx * root.dy / root.value);
    (stickies ? stickify : squarify)(root);
    if (sticky) stickies = nodes;
    return nodes;
  }

  treemap.size = function(x) {
    if (!arguments.length) return size;
    size = x;
    return treemap;
  };

  treemap.padding = function(x) {
    if (!arguments.length) return padding;

    function padFunction(node) {
      var p = x.call(treemap, node, node.depth);
      return p == null
          ? d3_layout_treemapPadNull(node)
          : d3_layout_treemapPad(node, typeof p === "number" ? [p, p, p, p] : p);
    }

    function padConstant(node) {
      return d3_layout_treemapPad(node, x);
    }

    var type;
    pad = (padding = x) == null ? d3_layout_treemapPadNull
        : (type = typeof x) === "function" ? padFunction
        : type === "number" ? (x = [x, x, x, x], padConstant)
        : padConstant;
    return treemap;
  };

  treemap.round = function(x) {
    if (!arguments.length) return round != Number;
    round = x ? Math.round : Number;
    return treemap;
  };

  treemap.sticky = function(x) {
    if (!arguments.length) return sticky;
    sticky = x;
    stickies = null;
    return treemap;
  };

  treemap.ratio = function(x) {
    if (!arguments.length) return ratio;
    ratio = x;
    return treemap;
  };

  return d3_layout_hierarchyRebind(treemap, hierarchy);
};

function d3_layout_treemapPadNull(node) {
  return {x: node.x, y: node.y, dx: node.dx, dy: node.dy};
}

function d3_layout_treemapPad(node, padding) {
  var x = node.x + padding[3],
      y = node.y + padding[0],
      dx = node.dx - padding[1] - padding[3],
      dy = node.dy - padding[0] - padding[2];
  if (dx < 0) { x += dx / 2; dx = 0; }
  if (dy < 0) { y += dy / 2; dy = 0; }
  return {x: x, y: y, dx: dx, dy: dy};
}