Add Caesar

Author: kaiquekandykoga

ciphers/caesar.rb

@@ -0,0 +1,33 @@
+# Caesar Cipher replaces characters rotating X number of positions to the left or to the right.
+#
+# Alphabet
+# a b c d e f g h i j k l m n o p q r s t u v w x y z
+#
+# shift 4 >> it means to rotate 4 places
+#
+# After shifting
+# e f g h i j k l m n o p q r s t u v w x y z a b c d
+#
+# plaintext -> apple
+# ciphertext -> ettpi
+
+class CaesarCipher
+  ALPHABET = ('a'..'z').to_a
+
+  def self.encrypt(plaintext, shift)
+    plaintext.chars.map do |letter|
+      temp = letter.ord + shift
+      temp -= ALPHABET.length while temp > 'z'.ord
+      temp.chr
+    end.join
+  end
+
+  def self.decrypt(ciphertext, shift)
+    ciphertext.chars.map do |letter|
+      temp = letter.ord - shift
+      temp += ALPHABET.length while temp < 'a'.ord
+      temp.chr
+    end.join
+  end
+end
+

ciphers/caesar_test.rb

@@ -0,0 +1,21 @@
+require 'minitest/autorun'
+require_relative 'caesar'
+
+class CaesarCipherTest < Minitest::Test
+  def test_shift4
+    run_tests('apple', 'ettpi', 4)
+  end
+
+  def test_shift27
+    run_tests('amateur', 'bnbufvs', 27)
+  end
+
+  private
+
+  def run_tests(plaintext, expected_cipher, shift)
+    encrypted = CaesarCipher.encrypt(plaintext, shift)
+    assert_equal encrypted, expected_cipher
+    decrypted = CaesarCipher.decrypt(encrypted, shift)
+    assert_equal decrypted, plaintext
+  end
+end