Write Vigenère cipher exercise

16_vigenere.lua

@@ -0,0 +1,64 @@
+-- The Vigenère cipher is a way to encrypt text, where a key is
+-- repeated for the length of the plaintext and the alphabetical
+-- position of the keyword letter determines the amount the letter of
+-- the plaintext is shifted by.  For example, for plaintext "FOO" and
+-- key "BAR", the first letter of the cipher text would be G, the
+-- result of shifting the letter F (first letter of the plaintext)
+-- over by 1 (the zero-based alphabetical position of B, first letter
+-- of the key).
+--
+-- Implement a function to encode a message with a given keyword.  The
+-- message should be converted to upper-case and any non-alphabetical
+-- characters should be discarded.
+--
+-- Solution --------------------------------------------------------------------
+local a_value = 65
+
+function apply_shift(letter, shift)
+	local shifted = (letter - a_value + shift) % 26
+	return a_value + shifted
+end
+
+function vigenere_encode(plaintext, keyword)
+	local key = {}
+	for i = 1, #keyword do
+		local shift = string.byte(keyword, i) - a_value
+		table.insert(key, shift)
+	end
+
+	plaintext = string.gsub(string.upper(plaintext), "%A+", "")
+	ciphertext = ""
+	for i = 1, #plaintext do
+		local letter = string.byte(plaintext, i)
+		local shift = key[(i - 1) % #key + 1]
+		local shifted = apply_shift(letter, shift)
+		ciphertext = ciphertext .. string.char(shifted)
+	end
+
+	return ciphertext
+end
+
+-- Tests -----------------------------------------------------------------------
+
+local luaunit = require("luaunit.luaunit")
+
+function test_encoding_of_foo_with_key_BAR()
+	luaunit.assertEquals(vigenere_encode("FOO", "BAR"), "GOF")
+end
+
+function test_encoding_of_the_quick_brown_fox_with_key_JUMPSOVER()
+	local plaintext = "The quick brown fox"
+	local key = "JUMPSOVER"
+	local expected = "CBQFMWXOSAIICXCS"
+	luaunit.assertEquals(vigenere_encode(plaintext, key), expected)
+end
+
+function test_encoding_longer_phrase_with_key_LEAR()
+	local plaintext =
+		"They dined on mince and slices of quince, which they ate with a runcible spoon."
+	local key = "LEAR"
+	local expected = "ELEPOMNVOSNDTRCVLRDJWMCVDSFHFMNTPAHZNLTYPCAKPAIKSERLYGISWISGZSN"
+	luaunit.assertEquals(vigenere_encode(plaintext, key), expected)
+end
+
+os.exit(luaunit.LuaUnit.run())