Add d3.forceContain.

index.js

@@ -1,6 +1,7 @@
 export {version} from "./build/package";
 export {default as forceCenter} from "./src/center";
 export {default as forceCollide} from "./src/collide";
+export {default as forceContain} from "./src/contain";
 export {default as forceLink} from "./src/link";
 export {default as forceManyBody} from "./src/manyBody";
 export {default as forcePosition} from "./src/position";

src/contain.js

@@ -0,0 +1,57 @@
+import constant from "./constant";
+
+export default function(radius, x, y) {
+  var nodes,
+      rz,
+      xz,
+      yz;
+
+  if (typeof radius !== "function") radius = constant(radius == null ? 100 : +radius);
+  if (typeof x !== "function") x = constant(x == null ? 0 : +x);
+  if (typeof y !== "function") y = constant(y == null ? 0 : +y);
+
+  function force() {
+    for (var i = 0, n = nodes.length, node, x, y, l, r; i < n; ++i) {
+      node = nodes[i];
+      x = node.x - xz[i];
+      y = node.y - yz[i];
+      if ((l = x * x + y * y) > (r = rz[i]) * r) {
+        l = Math.sqrt(l), l = (l - r) / l;
+        node.vx -= x * l;
+        node.vy -= y * l;
+      }
+    }
+  }
+
+  function initialize() {
+    if (!nodes) return;
+    var i, n = nodes.length;
+    rz = new Array(n);
+    xz = new Array(n);
+    yz = new Array(n);
+    for (i = 0; i < n; ++i) {
+      rz[i] = +radius(nodes[i], i, nodes);
+      xz[i] = +x(nodes[i], i, nodes);
+      yz[i] = +y(nodes[i], i, nodes);
+    }
+  }
+
+  force.initialize = function(simulation) {
+    nodes = simulation.nodes();
+    initialize();
+  };
+
+  force.radius = function(_) {
+    return arguments.length ? (radius = typeof _ === "function" ? _ : constant(+_), initialize(), force) : radius;
+  };
+
+  force.x = function(_) {
+    return arguments.length ? (x = typeof _ === "function" ? _ : constant(+_), initialize(), force) : x;
+  };
+
+  force.y = function(_) {
+    return arguments.length ? (y = typeof _ === "function" ? _ : constant(+_), initialize(), force) : y;
+  };
+
+  return force;
+}