From 260bae963f04766a19fcaa61085e9699dd194910 Mon Sep 17 00:00:00 2001
From: "HyunWoo Lee (Nunu Lee)" <54518925+l2hyunwoo@users.noreply.github.com>
Date: Mon, 17 Jun 2024 18:39:42 +0900
Subject: [PATCH] feat(maxBy): Add maxBy function that select element that have
 max value by given condition in array (#64)

* feat(maxBy): Add maxBy function

* feat(maxBy): Add benchamark test

* feat(maxBy): Add unit test of maxBy

* docs(maxBy): Add reference of maxBy function

* feat(maxBy): use for-each loop instead of index-increasing fashion

* fix(maxBy): Change code by code review

* fix(maxBy): Changed by code review, all tests are fine

* fix(maxBy): remove explicit undefined return

* fix(maxBy): Fix reference due to signature modification

* Update src/math/maxBy.ts

---------

Co-authored-by: Sojin Park <raon0211@gmail.com>
---
 benchmarks/maxBy.bench.ts       | 23 +++++++++++++++++++
 docs/ko/reference/math/maxBy.md | 27 +++++++++++++++++++++++
 docs/reference/math/maxBy.md    | 27 +++++++++++++++++++++++
 src/math/index.ts               |  1 +
 src/math/maxBy.spec.ts          | 39 +++++++++++++++++++++++++++++++++
 src/math/maxBy.ts               | 32 +++++++++++++++++++++++++++
 6 files changed, 149 insertions(+)
 create mode 100755 benchmarks/maxBy.bench.ts
 create mode 100644 docs/ko/reference/math/maxBy.md
 create mode 100644 docs/reference/math/maxBy.md
 create mode 100644 src/math/maxBy.spec.ts
 create mode 100644 src/math/maxBy.ts

diff --git a/benchmarks/maxBy.bench.ts b/benchmarks/maxBy.bench.ts
new file mode 100755
index 00000000..c4621ba7
--- /dev/null
+++ b/benchmarks/maxBy.bench.ts
@@ -0,0 +1,23 @@
+import { bench, describe } from 'vitest';
+import { maxBy as maxByToolkit } from 'es-toolkit';
+import { maxBy as maxByLodash } from 'lodash';
+
+describe('maxBy', () => {
+  bench('es-toolkit', () => {
+    const people = [
+      { name: 'Mark', age: 25 },
+      { name: 'Nunu', age: 30 },
+      { name: 'Overmars', age: 20 },
+    ];
+    maxByToolkit(people, person => person.age);
+  });
+
+  bench('lodash', () => {
+    const people = [
+      { name: 'Mark', age: 25 },
+      { name: 'Nunu', age: 30 },
+      { name: 'Overmars', age: 20 },
+    ];
+    maxByLodash(people, person => person.age);
+  });
+});
diff --git a/docs/ko/reference/math/maxBy.md b/docs/ko/reference/math/maxBy.md
new file mode 100644
index 00000000..862f70cc
--- /dev/null
+++ b/docs/ko/reference/math/maxBy.md
@@ -0,0 +1,27 @@
+# maxBy
+
+주어진 배열 내의 요소들 중에서 조건에 따라 최대값을 가지는 첫 번째 요소를 선택하는 함수에요.
+
+배열이 비어있지 않다면 조건에 따라 최대값을 가지는 첫 번째 요소를 반환하고, 비어있다면 `undefined`를 반환해요.
+
+## 인터페이스
+
+```typescript
+function maxBy<T>(elements: T[], selector: (element: T) => number): T
+```
+
+### 파라미터
+
+- `elements`: 검색할 요소들의 배열
+- `selector`: 요소를 받아서 객체의 속성을 반환하는 함수
+
+### 반환값
+
+함수의 최대값을 가지는 배열의 첫 번째 요소. 만약 배열이 비어있다면 `undefined`를 반환해요.
+
+### 예시
+
+```typescript
+maxBy([{ a: 1 }, { a: 2 }, { a: 3 }], x => x.a) // 결과: { a: 3 }
+maxBy([], x => x.a) // 결과: undefined
+```
diff --git a/docs/reference/math/maxBy.md b/docs/reference/math/maxBy.md
new file mode 100644
index 00000000..18e64114
--- /dev/null
+++ b/docs/reference/math/maxBy.md
@@ -0,0 +1,27 @@
+# maxBy
+
+Selects the first element of a list that has the maximum value of a function.
+
+If the list is empty, returns `undefined`.
+
+## Signature
+
+```typescript
+function maxBy<T>(elements: T[], selector: (element: T) => number): T
+```
+
+### Parameters
+
+- `elements`: an array of elements to search through.
+- `selector`: a function that takes an element and returns a number that the property of the object.
+
+### Returns
+
+The first element of the list that has the maximum value of the function. If the list is empty, returns `undefined`.
+
+### Example
+
+```typescript
+maxBy([{ a: 1 }, { a: 2 }, { a: 3 }], x => x.a) // Returns: { a: 3 }
+maxBy([], x => x.a) // Returns: undefined
+```
diff --git a/src/math/index.ts b/src/math/index.ts
index c8d91443..c8081c48 100644
--- a/src/math/index.ts
+++ b/src/math/index.ts
@@ -3,3 +3,4 @@ export { random } from './random';
 export { randomInt } from './randomInt';
 export { round } from './round';
 export { sum } from './sum';
+export { maxBy } from './maxBy';
diff --git a/src/math/maxBy.spec.ts b/src/math/maxBy.spec.ts
new file mode 100644
index 00000000..514dc142
--- /dev/null
+++ b/src/math/maxBy.spec.ts
@@ -0,0 +1,39 @@
+import { describe, expect, it } from "vitest";
+import { maxBy } from './maxBy';
+
+describe('maxBy', () => {
+  it('maxBy selects one max value in array', () => {
+    const people = [
+      { name: 'Mark', age: 25 },
+      { name: 'Nunu', age: 30 },
+      { name: 'Overmars', age: 20 },
+    ];
+    const result = maxBy(people, person => person.age);
+    expect(result).toEqual({ name: 'Nunu', age: 30 });
+  });
+
+  it('if there are two max values, first one is selected', () => {
+    const people = [
+      { name: 'Mark', age: 25 },
+      { name: 'Nunu', age: 30 },
+      { name: 'Overmars', age: 30 },
+    ];
+    const result = maxBy(people, person => person.age);
+    expect(result).toEqual({ name: 'Nunu', age: 30 });
+  });
+
+  it('if array is single-element, return unique element of array', () => {
+    const people = [
+      { name: 'Mark', age: 25 },
+    ];
+    const result = maxBy(people, person => person.age);
+    expect(result).toEqual({ name: 'Mark', age: 25 });
+  });
+
+  it('if array is empty, return undefined', () => {
+    type Person = { name: string, age: number };
+    const people: Person[] = [];
+    const result = maxBy(people, person => person.age);
+    expect(result).toBeUndefined();
+  });
+});
diff --git a/src/math/maxBy.ts b/src/math/maxBy.ts
new file mode 100644
index 00000000..2618c9a1
--- /dev/null
+++ b/src/math/maxBy.ts
@@ -0,0 +1,32 @@
+/**
+ * Returns the element of the specified array that has the maximum value, according to the specified selector.
+ *
+ * This function takes an array of elements and a selector function, and returns the element with the maximum value according to the selector.
+ *
+ * @param {T[]} elements - An array of elements to be compared.
+ * @param {(element: T) => number} selector - A function that takes an element and returns a number.
+ * @returns {T} The element with the maximum value according to the selector.
+ *
+ * @example
+ * const people = [
+ *  { name: 'Mark', age: 25 },
+ *  { name: 'Nunu', age: 30 },
+ *  { name: 'Overmars', age: 20 }
+ *  ];
+ *  const result = maxBy(people, person => person.age);
+ *  // result will be { name: 'Nunu', age: 30 }
+ */
+export function maxBy<T>(elements: T[], selector: (element: T) => number): T {
+  let maxElement = elements[0];
+  let max = -Infinity;
+
+  for (const element of elements) {
+    const value = selector(element);
+    if (value > max) {
+      max = value;
+      maxElement = element;
+    }
+  }
+
+  return maxElement;
+}