OC-KittenOS/inst/bdivide/core.lua

-- Copyright (C) 2018-2021 by KittenOS NEO contributors
--
-- Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted.
--
-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF
-- THIS SOFTWARE.

-- BDIVIDE r5 edition
-- Algorithm simplified for smaller implementation and potentially better compression
-- format:
-- 0-127 for constants
-- <128 + (length - 4)>, <position high>, <position low>
-- Position is where in the window it was found, minus 1.
-- windowSize must be the same between the encoder and decoder,
--  and is the amount of data preserved after cropping.

local bdivCore = {}

function bdivCore.bdivide(blk, p)
 local out = ""

 local windowSize = 0x10000
 local windowData = ("\x00"):rep(windowSize)

 while blk ~= "" do
  p(blk)
  local bestData = blk:sub(1, 1)
  assert(blk:byte() < 128, "BDIVIDE does not handle 8-bit data")
  local bestRes = bestData
  for lm = 0, 127 do
   local al = lm + 4
   local pfx = blk:sub(1, al)
   if #pfx ~= al then
    break
   end
   local p = windowData:find(pfx, 1, true)
   if not p then
    break
   end
   local pm = p - 1
   local thirdByte = pm % 256
   bestData = string.char(128 + lm, math.floor(pm / 256), thirdByte)
   bestRes = pfx
  end
  -- ok, encode!
  out = out .. bestData
  -- crop window
  windowData = (windowData .. bestRes):sub(-windowSize)
  blk = blk:sub(#bestRes + 1)
 end
 return out
end

-- Adds padding if required
function bdivCore.bdividePad(data)
 local i = 1
 -- Basically, if it ends on a literal,
 --  then the literal won't get read without two padding bytes.
 -- Otherwise (including if no data) it's fine.
 local needsPadding = false
 while i <= #data do
  if data:byte(i) > 127 then
   i = i + 3
   needsPadding = false
  else
   i = i + 1
   needsPadding = true
  end
 end
 if needsPadding then
  return data .. "\x00\x00"
 end
 return data
end

return bdivCore