Kebab file names

snippets/caesar-cipher.md

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+---
+title: Caesar cipher
+tags: algorithm,string
+cover: ancient-greek-building
+firstSeen: 2020-12-28T20:10:18+02:00
+lastUpdated: 2020-12-29T12:29:21+02:00
+---
+
+Encrypts or decrypts a given string using the Caesar cipher.
+
+- Use the modulo (`%`) operator and the ternary operator (`?`) to calculate the correct encryption/decryption key.
+- Use the spread operator (`...`) and `Array.prototype.map()` to iterate over the letters of the given string.
+- Use `String.prototype.charCodeAt()` and `String.fromCharCode()` to convert each letter appropriately, ignoring special characters, spaces etc.
+- Use `Array.prototype.join()` to combine all the letters into a string.
+- Pass `true` to the last parameter, `decrypt`, to decrypt an encrypted string.
+
+```js
+const caesarCipher = (str, shift, decrypt = false) => {
+  const s = decrypt ? (26 - shift) % 26 : shift;
+  const n = s > 0 ? s : 26 + (s % 26);
+  return [...str]
+    .map((l, i) => {
+      const c = str.charCodeAt(i);
+      if (c >= 65 && c <= 90)
+        return String.fromCharCode(((c - 65 + n) % 26) + 65);
+      if (c >= 97 && c <= 122)
+        return String.fromCharCode(((c - 97 + n) % 26) + 97);
+      return l;
+    })
+    .join('');
+};
+```
+
+```js
+caesarCipher('Hello World!', -3); // 'Ebiil Tloia!'
+caesarCipher('Ebiil Tloia!', 23, true); // 'Hello World!'
+```