Read and Write Morse Code

marcy.lua

-- a lua program that encodes and decodes morse code messages. run it with the
-- command `lua marcy.lua decode`, followed by the morse code in quotes. it
-- translates the dots and dashes into plain text. translate plain text into
-- morse code using `lua marcy.lua encode`.

-- an example of the morse code it reads:  -. . ...- . .-. / .-. ..- -.
-- use `lua marcy.lua decode "-. . ...- . .-. / .-. ..- -."` to decode it.

local morse_code_dict = {
  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 = "--..",

  ["1"] = ".----", ["2"] = "..---", ["3"] = "...--", ["4"] = "....-",
  ["5"] = ".....", ["6"] = "-....", ["7"] = "--...", ["8"] = "---..",
  ["9"] = "----.", ["0"] = "-----",

  ["."] = ".-.-.-", [","] = "--..--", ["?"] = "..--..", ["'"] = ".----.",
  ["!"] = "-.-.--", ["/"] = "-..-.", ["("] = "-.--.", [")"] = "-.--.-",
  ["&"] = ".-...",  [":"] = "---...", [";"] = "-.-.-.", ["="] = "-...-",
  ["+"] = ".-.-.",  ["-"] = "-....-", ["_"] = "..--.-", ['"'] = ".-..-.",
  ["$"] = "...-..-", ["@"] = ".--.-."
}

morse_code_decode_dict = {}

function encode(text_str)
  encoded_text = ""
  for i = 1, #text_str do
    chr = text_str:sub(i , i)
    if chr ~= " " then
	   if i ~= 1 then
         encoded_text = encoded_text .. " "
        end
        encoded_text = encoded_text .. (morse_code_dict[chr])
    else
       encoded_text = encoded_text .. " /"
    end
  end
  return encoded_text
end

function decode(text_str)
  -- init decode table
  if next(morse_code_decode_dict) == nil then
    for letter, morse in pairs(morse_code_dict) do
      morse_code_decode_dict[morse] = letter
    end
  end
  
  decoded_text = ""
  morse_buf = ""
  for i = 1, #text_str do
      chr = text_str:sub(i , i)
      if chr ~= " " then
        if chr == "/" then
          if morse_buf ~= "" then
            decoded_letter = morse_code_decode_dict[morse_buf]
           	if decoded_letter == nil then
           	  return "error"
            end
            decoded_text = decoded_text .. decoded_letter
            morse_buf = ""
          end
          decoded_text = decoded_text .. " "
        else
          morse_buf = morse_buf .. chr
        end
      else
        if morse_buf ~= "" then
          decoded_letter = morse_code_decode_dict[morse_buf]
          if decoded_letter == nil then
            return "error"
          end
          decoded_text = decoded_text .. decoded_letter
          morse_buf = ""
        end
      end
  end
  if morse_buf ~= "" then
  	decoded_letter = morse_code_decode_dict[morse_buf]
  	if decoded_letter == nil then
      return "error"
    end
  	decoded_text = decoded_text .. decoded_letter
  end
  return decoded_text
end

if arg[1] == "encode" and arg[2] ~= nil then
  print("Encoded: " .. encode(arg[2]))
end

if arg[1] == "decode" and arg[2] ~= nil then
  print("Decoded: " .. decode(arg[2]))
end