added test of shuffle

Author: taisukef

shuffle.js

@@ -1,7 +1,7 @@
 import { rnd } from "./rnd.js";
 
 export const shuffle = (array) => {
-  for (let i = array.length - 1; i >= 1; i--) {
+  for (let i = array.length - 1; i > 0; i--) {
     const n = rnd(i + 1);
   	const tmp = array[i];
 		array[i] = array[n];

test/shuffle.test.js

@@ -0,0 +1,69 @@
+import * as t from "https://deno.land/std/testing/asserts.ts";
+import { shuffle } from "../shuffle.js";
+import { rnd } from "../rnd.js";
+
+// 偏りあり
+const shuffle0 = (array) => {
+  for (let i = 0; i < array.length; i++) {
+    const n = rnd(array.length);
+    const tmp = array[i];
+    array[i] = array[n];
+    array[n] = tmp;
+  }
+};
+
+const make = n => {
+  const res = new Array(n);
+  for (let i = 0; i < n; i++) res[i] = i;
+  return res;
+};
+
+/*
+const chk = (a) => {
+  return a.reduce((pre, n, idx) => n == idx ? pre + 1 : pre, 0);
+};
+
+const chk1000 = (func) => {
+  let cnt = 0;
+  const n = 1000;
+  for (let i = 0; i < n; i++) {
+    const a = make(100);
+    func(a);
+    cnt += chk(a);
+  }
+  return cnt / n;
+};
+*/
+
+const chk2 = (test, func) => {
+  const count = {};
+  const trials = 600000;
+  const arr = [0, 1, 2];
+
+  for (let i = 0; i < trials; i++) {
+      const a = [...arr];
+      func(a);
+      const key = a.join('');
+      count[key] = (count[key] || 0) + 1;
+  }
+
+  console.log("output:", test);
+  let min = 100000;
+  let max = 0;
+  Object.keys(count).sort().forEach(key => {
+    const v = count[key];
+    if (v < min) min = v;
+    if (v > max) max = v;
+    console.log(`${key}: ${v}`);
+  });
+  const w = max - min;
+  console.log("width", w);
+  return w;
+};
+
+Deno.test("simple", () => {
+  //console.log(chk1000(shuffle));
+  //console.log(chk1000(shuffle0));
+  t.assert(chk2("new", shuffle) < 2000); // 偏り少ない
+  t.assert(chk2("old", shuffle0) > 2000); // 偏り大きい
+});