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import constant from "./constant";
export default function(y) {
var strength = constant(0.1),
nodes,
strengths,
yz;
if (typeof y !== "function") y = constant(y == null ? 0 : +y);
function force(alpha) {
for (var i = 0, n = nodes.length, node; i < n; ++i) {
node = nodes[i], node.vy += (yz[i] - node.y) * strengths[i] * alpha;
}
}
function initialize() {
if (!nodes) return;
var i, n = nodes.length;
strengths = new Array(n);
yz = new Array(n);
for (i = 0; i < n; ++i) {
strengths[i] = isNaN(yz[i] = +y(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.y = function(_) {
return arguments.length ? (y = typeof _ === "function" ? _ : constant(+_), initialize(), force) : y;
};
return force;
}