1 files changed, 41 insertions, 0 deletions
diff --git a/src/x.js b/src/x.js
new file mode 100644
index 0000000..894e748
--- /dev/null
+++ b/src/x.js
@@ -0,0 +1,41 @@
+import constant from "./constant";
+
+export default function(x) {
+  var strength = constant(0.1),
+      nodes,
+      strengths,
+      xz;
+
+  if (typeof x !== "function") x = constant(x == null ? 0 : +x);
+
+  function force(alpha) {
+    for (var i = 0, n = nodes.length, node; i < n; ++i) {
+      node = nodes[i], node.vx += (xz[i] - node.x) * strengths[i] * alpha;
+    }
+  }
+
+  function initialize() {
+    if (!nodes) return;
+    var i, n = nodes.length;
+    strengths = new Array(n);
+    xz = new Array(n);
+    for (i = 0; i < n; ++i) {
+      strengths[i] = isNaN(xz[i] = +x(nodes[i], i, nodes)) ? 0 : +strength(nodes[i], i, nodes);
+    }
+  }
+
+  force.initialize = function(_) {
+    nodes = _;
+    initialize();
+  };
+
+  force.strength = function(_) {
+    return arguments.length ? (strength = typeof _ === "function" ? _ : constant(+_), initialize(), force) : strength;
+  };
+
+  force.x = function(_) {
+    return arguments.length ? (x = typeof _ === "function" ? _ : constant(+_), initialize(), force) : x;
+  };
+
+  return force;
+}