deterministic shuffling (#190)

fixes #189
note: shuffling is here to increase performance (#83)

src/array.js

@@ -1,16 +1,19 @@
+import lcg from "./lcg.js";
+
 export default function(x) {
   return typeof x === "object" && "length" in x
     ? x // Array, TypedArray, NodeList, array-like
     : Array.from(x); // Map, Set, iterable, string, or anything else
 }
 
 export function shuffle(array) {
-  var m = array.length,
+  const random = lcg();
+  let m = array.length,
       t,
       i;
 
   while (m) {
-    i = Math.random() * m-- | 0;
+    i = random() * m-- | 0;
     t = array[m];
     array[m] = array[i];
     array[i] = t;

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;
+}