From 8bfa6b9cf4d2d3ac6aec2ab56a8a2d8cb440ea9e Mon Sep 17 00:00:00 2001
From: sarthakgupta1907-cell <sarthakgupta1907@gmail.com>
Date: Mon, 2 Nov 2026 18:18:35 +0530
Subject: [PATCH] feat: add Caesar cipher algorithm

---
 Ciphers/CaesarCipher.js           | 62 +++++++++++++++++--------------
 Ciphers/test/CaesarCipher.test.js | 62 +++++++++++++++++++++----------
 2 files changed, 77 insertions(+), 47 deletions(-)

diff --git a/Ciphers/CaesarCipher.js b/Ciphers/CaesarCipher.js
index 8a942564e9..4b9ad6d9a7 100644
--- a/Ciphers/CaesarCipher.js
+++ b/Ciphers/CaesarCipher.js
@@ -1,32 +1,40 @@
 /**
- * @function caesarsCipher
- * @description - In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence.
- * @see - [wiki](https://en.wikipedia.org/wiki/Caesar_cipher)
- * @param  {string} str - string to be encrypted
- * @param {number} rotation - the number of rotation, expect real number ( > 0)
- * @return {string} - decrypted string
+ * @function caesarCipher
+ * @description Caesar cipher is one of the simplest encryption techniques.
+ *   It works by shifting each letter in the plaintext by a fixed number
+ *   of positions in the alphabet.
+ * @param {string} str - The input string to encrypt/decrypt
+ * @param {number} shift - The number of positions to shift (can be negative)
+ * @returns {string} - The encrypted/decrypted string
+ * @see https://en.wikipedia.org/wiki/Caesar_cipher
+ * @example caesarCipher("ABC", 1) => "BCD"
+ * @example caesarCipher("XYZ", 3) => "ABC"
+ * @example caesarCipher("Hello World", 13) => "Uryyb Jbeyq"
  */
-const caesarCipher = (str, rotation) => {
-  if (typeof str !== 'string' || !Number.isInteger(rotation) || rotation < 0) {
-    throw new TypeError('Arguments are invalid')
+const caesarCipher = (str, shift) => {
+  if (typeof str !== 'string') {
+    throw new TypeError('Input must be a string')
   }
-
-  const alphabets = new Array(26)
-    .fill()
-    .map((_, index) => String.fromCharCode(97 + index)) // generate all lower alphabets array a-z
-
-  const cipherMap = alphabets.reduce(
-    (map, char, index) => map.set(char, alphabets[(rotation + index) % 26]),
-    new Map()
-  )
-
-  return str.replace(/[a-z]/gi, (char) => {
-    if (/[A-Z]/.test(char)) {
-      return cipherMap.get(char.toLowerCase()).toUpperCase()
-    }
-
-    return cipherMap.get(char)
-  })
+  if (!Number.isInteger(shift)) {
+    throw new TypeError('Shift must be an integer')
+  }
+  const normalizedShift = ((shift % 26) + 26) % 26
+  return str
+    .split('')
+    .map((char) => {
+      if (char.match(/[a-z]/)) {
+        return String.fromCharCode(
+          ((char.charCodeAt(0) - 97 + normalizedShift) % 26) + 97
+        )
+      }
+      if (char.match(/[A-Z]/)) {
+        return String.fromCharCode(
+          ((char.charCodeAt(0) - 65 + normalizedShift) % 26) + 65
+        )
+      }
+      return char
+    })
+    .join('')
 }
 
-export default caesarCipher
+export { caesarCipher }
diff --git a/Ciphers/test/CaesarCipher.test.js b/Ciphers/test/CaesarCipher.test.js
index c39c09d061..5fa67f6a9e 100644
--- a/Ciphers/test/CaesarCipher.test.js
+++ b/Ciphers/test/CaesarCipher.test.js
@@ -1,22 +1,44 @@
-import caesarCipher from '../CaesarCipher'
-
-describe('Testing the caesarsCipher function', () => {
-  it('Test - 1, Testing for invalid types', () => {
-    expect(() => caesarCipher(false, 3)).toThrow()
-    expect(() => caesarCipher('false', -1)).toThrow()
-    expect(() => caesarCipher('true', null)).toThrow()
-  })
-
-  it('Test - 2, Testing for valid string and rotation', () => {
-    expect(caesarCipher('middle-Outz', 2)).toBe('okffng-Qwvb')
-    expect(caesarCipher('abcdefghijklmnopqrstuvwxyz', 3)).toBe(
-      'defghijklmnopqrstuvwxyzabc'
-    )
-    expect(caesarCipher('Always-Look-on-the-Bright-Side-of-Life', 5)).toBe(
-      'Fqbfdx-Qttp-ts-ymj-Gwnlmy-Xnij-tk-Qnkj'
-    )
-    expect(
-      caesarCipher('THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG', 23)
-    ).toBe('QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD')
+import { caesarCipher } from '../CaesarCipher.js'
+
+describe('caesarCipher', () => {
+  test('shifts letters by 1', () => {
+    expect(caesarCipher('ABC', 1)).toBe('BCD')
+  })
+
+  test('wraps around Z to A', () => {
+    expect(caesarCipher('XYZ', 3)).toBe('ABC')
+  })
+
+  test('preserves case', () => {
+    expect(caesarCipher('Hello', 13)).toBe('Uryyb')
+  })
+
+  test('preserves spaces and punctuation', () => {
+    expect(caesarCipher('Hello World!', 13)).toBe('Uryyb Jbeyq!')
+  })
+
+  test('shift of 0 returns same string', () => {
+    expect(caesarCipher('ABC', 0)).toBe('ABC')
+  })
+
+  test('shift of 26 returns same string', () => {
+    expect(caesarCipher('ABC', 26)).toBe('ABC')
+  })
+
+  test('negative shift works', () => {
+    expect(caesarCipher('BCD', -1)).toBe('ABC')
+  })
+
+  test('decryption reverses encryption', () => {
+    const original = 'Hello World'
+    expect(caesarCipher(caesarCipher(original, 13), -13)).toBe(original)
+  })
+
+  test('throws TypeError for non-string input', () => {
+    expect(() => caesarCipher(123, 1)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for non-integer shift', () => {
+    expect(() => caesarCipher('ABC', 1.5)).toThrow(TypeError)
   })
 })