-- 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())