Huffman Coding

local function getL(tebr)
  local teb = {}
  for i,v in pairs(tebr) do
    teb[i] = v
  end

  local lowm
  local low2m
  local low
  local low2
  for i,v in pairs(teb) do
    if not low then
      low = i
      lowm = v[1]
    elseif v[1] < lowm then
      low = i
      lowm = v[1]
    end
  end

  teb[low] = nil

  for i,v in pairs(teb) do
    if not low2 then
      low2 = i
      low2m = v[1]
    elseif v[1] < low2m then
      low2 = i
      low2m = v[1]
    end
  end


  if low2 < low then
    local buf = low2
    low2 = low
    low = buf
  end

  return low, low2
end

local function it(com, cur, tab, hur)
  for i,v in pairs(tab) do
    if type(v[2]) == "table" then
      it(com .. i - 1, cur, v[2], hur + 1)
    else
      cur[v[2]] = com .. i - 1
      -- print(string.rep(" ", hur) .. v[2] .. " = " .. cur[v[2]])
    end
  end
  return com, cur
end

local function encode(text)
  local newt = {}

  local freq = {}
  for i = 1, #text do
    local char = string.byte(text:sub(i, i))
    if not freq[char] then
      freq[char] = 0
    end
    freq[char] = freq[char] + 1
  end

  for i,v in pairs(freq) do
    table.insert(newt, {v, i})
  end

  while #newt > 1 do
    local l1, l2 = getL(newt)

    local l1m = newt[l1]
    local l2m = newt[l2]

    table.remove(newt, l1)
    table.remove(newt, l2 - 1)
    table.insert(newt, {l1m[1] + l2m[1], {l1m, l2m}})
  end

  local d ,t = it("", {}, newt[1][2], 0)

  local new = ""
  for i = 1, #text do
    new = new .. t[text:sub(i, i):byte()]
  end

  return new, t
end

local function decode(text, dic)
  local new = ""

  for i = 1, #text do
    local got = false
    for m,v in pairs(dic) do
      if text:sub(1, #v) == v then
        new = new .. string.char(m)
        text = text:sub(#v + 1)
        break
      end
    end
  end
  return new
end

local tArgs = {...}

if tArgs[1] == "e" then
  local file = fs.open(tArgs[2], "r")
  local dat = file.readAll()
  file.close()

  local t, d = encode(dat)

  local ard = textutils.serialize(d)

  local bw = ""
  local gof = {}
  for i = 1, #t do
    table.insert(gof, 1, tonumber(t:sub(i, i)))
    if #gof == 8 then
      local nby = 0
      for m,s in pairs(gof) do
        nby = nby + s * (2^(m - 1))
      end
      bw = bw .. string.char(nby)
      gof = {}
    end
  end

  local nby = 0
  for i = 1, 8-#gof do
    table.insert(gof, 1, 0)
  end
  for m,s in pairs(gof) do
    nby = nby + s * (2^(m - 1))
  end
  if nby ~= 0 then
    bw = bw .. string.char(nby)
  end

  file = fs.open(tArgs[3], "wb")
  local tow = #ard .. "a" .. ard .. #t .. "b" .. bw
  for i = 1, #tow do
    file.write(tow:sub(i, i):byte())
  end
  file.close()
elseif tArgs[1] == "d" then
  local file = fs.open(tArgs[2], "rb")
  local newc = ""
  while true do
    local by = file.read()
    if by == nil then
      break
    else
      newc = newc .. string.char(by)
    end
  end

  local num = ""
  for i = 1, #newc do
    if newc:sub(i, i) == "a" then
      break
    else
      num = num .. newc:sub(i, i)
    end
  end

  num = tonumber(num)
  local nom = #tostring(num) + 1
  local toc = ""
  for i = 1 + nom, num + nom do
    toc = toc .. newc:sub(i, i)
  end

  nom = nom + #toc

  local num2 = ""

  for i = nom + 1, #newc do
    if newc:sub(i, i) == "b" then
      break
    else
      num2 = num2 .. newc:sub(i, i)
    end
  end
  num2 = tonumber(num2)
  nom = nom + #tostring(num2) + 1

  local newl = ""

  for i = 1 + nom, #newc do
    local by = newc:sub(i, i):byte()
    local newr = ""
    for i = 8, 1, -1 do
      if by >= 2^(i - 1) then
        newr = newr .. "1"
        by = by - 2^(i - 1)
      else
        newr = newr .. "0"
      end
    end
    newl = newl .. newr
  end

  newl = newl:sub(1, num2)
  local d = decode(newl, textutils.unserialize(toc))
  print(d)

  file.close()
end