new tasks

Task/Caesar_cipher/Lua/caesar_cipher.lua

@@ -0,0 +1,30 @@
+local function encrypt(text, key)
+	return text:gsub("%a", function(t)
+			local base = (t:lower() == t and string.byte('a') or string.byte('A'))
+
+			local r = t:byte() - base
+			r = r + key
+			r = r%26 -- works correctly even if r is negative
+			r = r + base
+			return string.char(r)
+		end)
+end
+
+local function decrypt(text, key)
+	return encrypt(text, -key)
+end
+
+caesar = {
+	encrypt = encrypt,
+	decrypt = decrypt,
+}
+
+-- test
+do
+	local text = "ABCDEFGHIJKLMNOPQRSTUVWXYZ abcdefghijklmnopqrstuvwxyz"
+	local encrypted = caesar.encrypt(text, 7)
+	local decrypted = caesar.decrypt(encrypted, 7)
+	print("Original text:  ", text)
+	print("Encrypted text: ", encrypted)
+	print("Decrypted text: ", decrypted)
+end