From 27b17106bc78e339ff987f86837b237b09303d93 Mon Sep 17 00:00:00 2001
From: Euan MacKenzie <82283260+eumkz@users.noreply.github.com>
Date: Fri, 5 Jul 2024 00:40:19 -0400
Subject: [PATCH] perf(shuffle): use the Fisher-Yates algorithm (#76)

---
 src/random/shuffle.ts        | 19 ++++++++++++++-----
 tests/random/shuffle.test.ts | 10 ++++++++++
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/src/random/shuffle.ts b/src/random/shuffle.ts
index 8d4faeed..de385598 100644
--- a/src/random/shuffle.ts
+++ b/src/random/shuffle.ts
@@ -1,6 +1,15 @@
-export function shuffle<T>(array: readonly T[]): T[] {
-  return array
-    .map(a => ({ rand: Math.random(), value: a }))
-    .sort((a, b) => a.rand - b.rand)
-    .map(a => a.value)
+import * as _ from 'radashi'
+
+export function shuffle<T>(
+  array: readonly T[],
+  random: (min: number, max: number) => number = _.random,
+): T[] {
+  const newArray = array.slice()
+  for (let idx = 0, randomIdx: number, item: T; idx < array.length; idx++) {
+    randomIdx = random(0, array.length - 1)
+    item = newArray[randomIdx]
+    newArray[randomIdx] = newArray[idx]
+    newArray[idx] = item
+  }
+  return newArray
 }
diff --git a/tests/random/shuffle.test.ts b/tests/random/shuffle.test.ts
index 7d10d32c..c89f00e1 100644
--- a/tests/random/shuffle.test.ts
+++ b/tests/random/shuffle.test.ts
@@ -19,4 +19,14 @@ describe('shuffle', () => {
     expect(list).not.toBe(result)
     expect(list).toEqual([1, 2, 3, 4, 5])
   })
+  test('uses custom random function when provided', () => {
+    const list = [1, 2, 3, 4, 5]
+    const mockRandom = vi.fn(() => 1)
+    const result = _.shuffle(list, mockRandom)
+
+    expect(mockRandom).toHaveBeenCalled()
+    expect(result).not.toEqual(list)
+    expect(result.length).toBe(list.length)
+    expect(new Set(result)).toEqual(new Set(list))
+  })
 })