lualzw.lua

--[[
MIT License

Copyright (c) 2016 Rochet2

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
]]

local char = string.char
local type = type
local select = select
local sub = string.sub
local tconcat = table.concat

local basedictcompress = {}
local basedictdecompress = {}
for i = 0, 255 do
  local ic, iic = char(i), char(i, 0)
  basedictcompress[ic] = iic
  basedictdecompress[iic] = ic
end

local function dictAddA(str, dict, a, b)
  if a >= 256 then
    a, b = 0, b+1
    if b >= 256 then
      dict = {}
      b = 1
    end
  end
  dict[str] = char(a,b)
  a = a+1
  return dict, a, b
end

local function compress(input)
  if type(input) ~= "string" then
    return nil, "string expected, got "..type(input)
  end
  local len = #input
  if len <= 1 then
    return "u"..input
  end
  
  local dict = {}
  local a, b = 0, 1
  
  local result = {"c"}
  local resultlen = 1
  local n = 2
  local word = ""
  for i = 1, len do
    local c = sub(input, i, i)
    local wc = word..c
    if not (basedictcompress[wc] or dict[wc]) then
      local write = basedictcompress[word] or dict[word]
      if not write then
        return nil, "algorithm error, could not fetch word"
      end
      result[n] = write
      resultlen = resultlen + #write
      n = n+1
      if  len <= resultlen then
        return "u"..input
      end
      dict, a, b = dictAddA(wc, dict, a, b)
      word = c
    else
      word = wc
    end
  end
  result[n] = basedictcompress[word] or dict[word]
  resultlen = resultlen+#result[n]
  n = n+1
  if  len <= resultlen then
    return "u"..input
  end
  return tconcat(result)
end

local function dictAddB(str, dict, a, b)
  if a >= 256 then
    a, b = 0, b+1
    if b >= 256 then
      dict = {}
      b = 1
    end
  end
  dict[char(a,b)] = str
  a = a+1
  return dict, a, b
end

local function decompress(input)
  if type(input) ~= "string" then
    return nil, "string expected, got "..type(input)
  end
  
  if #input < 1 then
    return nil, "invalid input - not a compressed string"
  end
  
  local control = sub(input, 1, 1)
  if control == "u" then
    return sub(input, 2)
  elseif control ~= "c" then
    return nil, "invalid input - not a compressed string"
  end
  input = sub(input, 2)
  local len = #input
  
  if len < 2 then
    return nil, "invalid input - not a compressed string"
  end
  
  local dict = {}
  local a, b = 0, 1
  
  local result = {}
  local n = 1
  local last = sub(input, 1, 2)
  result[n] = basedictdecompress[last] or dict[last]
  n = n+1
  for i = 3, len, 2 do
    local code = sub(input, i, i+1)
    local lastStr = basedictdecompress[last] or dict[last]
    if not lastStr then
      return nil, "could not find last from dict. Invalid input?"
    end
    local toAdd = basedictdecompress[code] or dict[code]
    if toAdd then
      result[n] = toAdd
      n = n+1
      dict, a, b = dictAddB(lastStr..sub(toAdd, 1, 1), dict, a, b)
    else
      local tmp = lastStr..sub(lastStr, 1, 1)
      result[n] = tmp
      n = n+1
      dict, a, b = dictAddB(tmp, dict, a, b)
    end
    last = code
  end
  return tconcat(result)
end

return {
  compress = compress,
  decompress = decompress,
}