add Caesar cipher

realDuYuanChao · realDuYuanChao · commit 2c26c13eeea5 · 2020-10-19T20:39:26.000+08:00
diff --git a/src/main/java/com/examplehub/ciphers/CaesarCipher.java b/src/main/java/com/examplehub/ciphers/CaesarCipher.java
@@ -0,0 +1,45 @@
+package com.examplehub.ciphers;
+
+/**
+ * https://en.wikipedia.org/wiki/Caesar_cipher
+ */
+public class CaesarCipher {
+
+    /**
+     * Encrypt a string by caesar cipher algorithm.
+     *
+     * @param origin the origin text.
+     * @param steps  the steps to be moved.
+     * @return the string encrypted.
+     */
+    public static String encrypt(String origin, int steps) {
+        char[] password = origin.toCharArray();
+        for (int i = 0; i < password.length; ++i) {
+            if (Character.isUpperCase(password[i])) {
+                password[i] = (char) ('A' + (password[i] - 'A' + steps) % 26);
+            } else if (Character.isLowerCase(password[i])) {
+                password[i] = (char) ('a' + (password[i] - 'a' + steps) % 26);
+            }
+        }
+        return String.valueOf(password);
+    }
+
+    /**
+     * Decrypt a string.
+     *
+     * @param password the password to be decrypted.
+     * @param steps    the steps to be moved.
+     * @return the string decrypted.
+     */
+    public static String decrypt(String password, int steps) {
+        char[] origin = password.toCharArray();
+        for (int i = 0; i < password.length(); ++i) {
+            if (Character.isUpperCase(origin[i])) {
+                origin[i] = (char) ('A' + (origin[i] - 'A' - steps + 26) % 26);
+            } else if (Character.isLowerCase(origin[i])) {
+                origin[i] = (char) ('a' + (origin[i] - 'a' - steps + 26) % 26);
+            }
+        }
+        return String.valueOf(origin);
+    }
+}
diff --git a/src/test/java/com/examplehub/ciphers/CaesarCipherTest.java b/src/test/java/com/examplehub/ciphers/CaesarCipherTest.java
@@ -0,0 +1,23 @@
+package com.examplehub.ciphers;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class CaesarCipherTest {
+    @Test
+    void testEncrypt() {
+        assertEquals("DEFGHIJKLMNOPQRSTUVWXYZABC",
+                CaesarCipher.encrypt("ABCDEFGHIJKLMNOPQRSTUVWXYZ", 3));
+        assertEquals("defghijklmnopqrstuvwxyzabc",
+                CaesarCipher.encrypt("abcdefghijklmnopqrstuvwxyz", 3));
+    }
+
+    @Test
+    void testDecrypt() {
+        assertEquals("ABCDEFGHIJKLMNOPQRSTUVWXYZ",
+                CaesarCipher.decrypt("DEFGHIJKLMNOPQRSTUVWXYZABC", 3));
+        assertEquals("abcdefghijklmnopqrstuvwxyz",
+                CaesarCipher.decrypt("defghijklmnopqrstuvwxyzabc", 3));
+    }
+}
