require "table2"

enum chao begin
    encrypt,
    decrypt
end

local function exec(text, mode, show_steps)
    local len = #text
    local left  = "HXUCZVAMDSLKPEFJRIGTWOBNYQ"
    local right = "PTLNBQDEOYSFAVZKGJRIHWXUMC"
    local etext = table.rep(len, 0)
    local temp  = table.rep(26, "")
    for i = 1, len do
        if show_steps then print($"{left}  {right}") end
        local index
        if mode == chao.encrypt then
            index = right:find(text[i], 1, true)
            etext[i] = left[index]
        else
            index = left:find(text[i], 1, true)
            etext[i] = right[index]
        end
        if i == len then break end

        -- Permute left.
        for j = index, 26 do temp[j - index + 1] = left[j] end
        for j = 1, index - 1 do temp[27 - index + j] = left[j] end
        local store = temp[2]
        for j = 3, 14 do temp[j - 1] = temp[j] end
        temp[14] = store
        left = temp:concat("")

        -- Permute right.
        for j = index, 26 do temp[j - index + 1] = right[j] end
        for j = 1, index - 1 do temp[27 - index + j] = right[j] end
        store = temp[1]
        for j = 2, 26 do temp[j - 1] = temp[j] end
        temp[26] = store
        store = temp[3]
        for j = 4, 14 do temp[j - 1] = temp[j] end
        temp[14] = store
        right = temp:concat("")
    end
    return etext:concat("")
end

local plain_text = "WELLDONEISBETTERTHANWELLSAID"
print($"The original plaintext is : {plain_text}")
io.write("\nThe left and right alphabets after each permutation ")
print("during encryption are :\n")
local cipher_text = exec(plain_text, chao.encrypt, true)
print($"\nThe ciphertext is : {cipher_text}")
local plain_text2 = exec(cipher_text, chao.decrypt, false)
print($"\nThe recovered plaintext is : {plain_text2}")