From 2716b04477bde05a154057d92368ee86f96eaabd Mon Sep 17 00:00:00 2001
From: Andrej Dyck <43913051+andrej-dyck@users.noreply.github.com>
Date: Sat, 30 Dec 2023 16:11:35 +0100
Subject: [PATCH] add memoize function

---
 memoize/index.ts        | 14 ++++++++++++++
 memoize/memoize.test.ts | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+)
 create mode 100644 memoize/index.ts
 create mode 100644 memoize/memoize.test.ts

diff --git a/memoize/index.ts b/memoize/index.ts
new file mode 100644
index 0000000..e9cfb16
--- /dev/null
+++ b/memoize/index.ts
@@ -0,0 +1,14 @@
+/**
+ * Returns a function that will memoize the return values for arguments of (pure) function f and return those on subsequent calls.
+ * Note that it only supports functions with exactly 1 argument and the values must be equatable (e.g., objects won't work)
+ */
+export const memoize = <T, R>(f: (arg: T) => R): (arg: T) => R => {
+  const values = new Map<T, R>()
+  return (arg: T) => {
+    if(!values.has(arg)) {
+      values.set(arg, f(arg))
+    }
+    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+    return values.get(arg)!
+  }
+}
diff --git a/memoize/memoize.test.ts b/memoize/memoize.test.ts
new file mode 100644
index 0000000..f9d6816
--- /dev/null
+++ b/memoize/memoize.test.ts
@@ -0,0 +1,38 @@
+import { describe, expect, test } from 'vitest'
+import { memoize } from './index.js'
+
+describe('memoize', () => {
+
+  test('delegates to function of first call', () => {
+    const mf = memoize((n: number) => n * 2)
+    expect(mf(2)).toEqual(4)
+  })
+
+  test('uses memoized return value on subsequent calls', () => {
+    let calls = 0
+    const mf = memoize((n: number) => {
+      calls++
+      return n * 2
+    })
+
+    expect(mf(2)).toEqual(4)
+    expect(mf(2)).toEqual(4)
+    expect(calls).toEqual(1)
+  })
+
+  test('memorizes return value pre argument', () => {
+    let calls = 0
+    const mf = memoize((n: number) => {
+      calls++
+      return n * 2
+    })
+
+    expect(mf(2)).toEqual(4)
+    expect(mf(2)).toEqual(4)
+
+    expect(mf(1)).toEqual(2)
+    expect(mf(1)).toEqual(2)
+
+    expect(calls).toEqual(2)
+  })
+})