Less confusing intialization of node positions.

Fixes #1561.

d3.js

@@ -5751,7 +5751,7 @@ d3 = function() {
       return force;
     };
     force.start = function() {
-      var i, j, n = nodes.length, m = links.length, w = size[0], h = size[1], neighbors, o;
+      var i, n = nodes.length, m = links.length, w = size[0], h = size[1], neighbors, o;
       for (i = 0; i < n; ++i) {
         (o = nodes[i]).index = i;
         o.weight = 0;
@@ -5777,13 +5777,8 @@ d3 = function() {
       charges = [];
       if (typeof charge === "function") for (i = 0; i < n; ++i) charges[i] = +charge.call(this, nodes[i], i); else for (i = 0; i < n; ++i) charges[i] = charge;
       function position(dimension, size) {
-        var neighbors = neighbor(i), j = -1, m = neighbors.length, x;
-        while (++j < m) if (!isNaN(x = neighbors[j][dimension])) return x;
-        return Math.random() * size;
-      }
-      function neighbor() {
         if (!neighbors) {
-          neighbors = [];
+          neighbors = new Array(n);
           for (j = 0; j < n; ++j) {
             neighbors[j] = [];
           }
@@ -5793,7 +5788,9 @@ d3 = function() {
             neighbors[o.target.index].push(o.source);
           }
         }
-        return neighbors[i];
+        var candidates = neighbors[i], j = -1, m = candidates.length, x;
+        while (++j < m) if (!isNaN(x = candidates[j][dimension])) return x;
+        return Math.random() * size;
       }
       return force.resume();
     };

src/layout/force.js

@@ -198,7 +198,6 @@ d3.layout.force = function() {
 
   force.start = function() {
     var i,
-        j,
         n = nodes.length,
         m = links.length,
         w = size[0],
@@ -239,20 +238,12 @@ d3.layout.force = function() {
     if (typeof charge === "function") for (i = 0; i < n; ++i) charges[i] = +charge.call(this, nodes[i], i);
     else for (i = 0; i < n; ++i) charges[i] = charge;
 
-    // initialize node position based on first neighbor
+    // inherit node position from first neighbor with defined position
+    // or if no such neighbors, initialize node position randomly
+    // initialize neighbors lazily to avoid overhead when not needed
     function position(dimension, size) {
-      var neighbors = neighbor(i),
-          j = -1,
-          m = neighbors.length,
-          x;
-      while (++j < m) if (!isNaN(x = neighbors[j][dimension])) return x;
-      return Math.random() * size;
-    }
-
-    // initialize neighbors lazily
-    function neighbor() {
       if (!neighbors) {
-        neighbors = [];
+        neighbors = new Array(n);
         for (j = 0; j < n; ++j) {
           neighbors[j] = [];
         }
@@ -262,7 +253,12 @@ d3.layout.force = function() {
           neighbors[o.target.index].push(o.source);
         }
       }
-      return neighbors[i];
+      var candidates = neighbors[i],
+          j = -1,
+          m = candidates.length,
+          x;
+      while (++j < m) if (!isNaN(x = candidates[j][dimension])) return x;
+      return Math.random() * size;
     }
 
     return force.resume();