Add deterministic LCG.

README.md

@@ -130,7 +130,7 @@ Returns the node closest to the position ⟨*x*,*y*⟩ with the given search *ra
 
 <a name="simulation_randomSource" href="#simulation_randomSource">#</a> <i>simulation</i>.<b>randomSource</b>([<i>source</i>]) [<>](https://github.com/d3/d3-force/blob/master/src/simulation.js "Source"))
 
-If *source* is specified, sets the function used to generate random numbers; this should be a function that returns a number between 0 (inclusive) and 1 (exclusive). If *source* is not specified, returns this simulation’s current random source which defaults to Math.random. A custom random source can be used to guarantee deterministic behavior. See also [*random*.source](https://github.com/d3/d3-random/blob/master/README.md#random_source).
+If *source* is specified, sets the function used to generate random numbers; this should be a function that returns a number between 0 (inclusive) and 1 (exclusive). If *source* is not specified, returns this simulation’s current random source which defaults to a fixed-seed [linear congruential generator](https://en.wikipedia.org/wiki/Linear_congruential_generator). See also [*random*.source](https://github.com/d3/d3-random/blob/master/README.md#random_source).
 
 <a name="simulation_on" href="#simulation_on">#</a> <i>simulation</i>.<b>on</b>(<i>typenames</i>, [<i>listener</i>]) [<>](https://github.com/d3/d3-force/blob/master/src/simulation.js#L145 "Source")
 

src/lcg.js

@@ -0,0 +1,9 @@
+// https://en.wikipedia.org/wiki/Linear_congruential_generator#Parameters_in_common_use
+const a = 1664525;
+const c = 1013904223;
+const m = 4294967296; // 2^32
+
+export default function() {
+  let s = 1;
+  return () => (s = (a * s + c) % m) / m;
+}

src/simulation.js

@@ -1,5 +1,6 @@
 import {dispatch} from "d3-dispatch";
 import {timer} from "d3-timer";
+import lcg from "./lcg.js";
 
 export function x(d) {
   return d.x;
@@ -22,7 +23,7 @@ export default function(nodes) {
       forces = new Map(),
       stepper = timer(step),
       event = dispatch("tick", "end"),
-      random = Math.random;
+      random = lcg();
 
   if (nodes == null) nodes = [];