aPLib decruncher for 68000

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r57shell
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Re: aPLib decruncher for 68000

Post by r57shell » Fri Sep 01, 2017 9:45 am

flamewing wrote:
Fri Sep 01, 2017 9:19 am
Can you give more details on this format? It sounds like the kind of thing that could be brought to perfect compression level using my generic LZSS backend.
Pseudocode (mix of python, c++, and pascal :lol: ):

Code: Select all

copy first byte.
last_offset := uninitialized // some trash that is happened to be in this variable
LWM := 2  // Last Was Match = false
while (true) do:
  if (read_bit() == 0): // 0 literal
    LWM := 2
    write_byte(read_byte)
    continue
  endif
  if (read_bit() == 0): // 10 copy
    offset := read_gamma() - LWM;

    if (offset != 0): // full copy encode ( !=  means not equal)
      offset := (offset - 1)*256 + read_byte()
      last_offset := offset
      len := read_gamma()
      if (offset >= 32000):
        len := len + 2
      else if (offset >= 1280):
        len := len + 1
      else if (offset >= 128):
        do_nothing // no operation
      else:
        len := len + 2
      endif
    else: // copy with omitted offset
      len := read_gamma()
      offset := last_offset
    endif
    for (i=0; i<len; ++i):
      write_byte(get_byte_from_output_at_offset_from_end(offset))
    LWM := 1 // Last Was Match = true
    continue
  endif // end if copy encode
  if (read_bit() == 0) // 110 short copy
    offset := read_byte()
    len := (offset % 2) + 2 // % = remainder
    offset := floor(offset / 2) // logical shift right
    last_offset := offset
    if (offset == 0):
      break // end of data
    endif
    for (i=0; i<len; ++i):
      write_byte(get_byte_from_output_at_offset_from_end(offset))
    LWM := 1 // Last Was Match = true
    continue
  endif
  // 111 single byte copy or null
  LWM := 2 // Last Was Match = true
  offset := 0;
  for (i=0; i<4; ++i)
    offset := offset*2 + read_bit()
  if (offset == 0):
    write_byte(0) // null
  else
    write_byte(get_byte_from_output_at_offset_from_end(offset))
  endif
end
read_bit() is tricky. It has buffer 8 bits long. All input is done by reading whole bytes.
So, when read_bit() has empty buffer of bits, it will read whole byte from input, and save all 8 bits into its buffer.

Code: Select all

byte read_gamma():
  result := 1
  while(true):
    result = result*2 + read_bit()
    if (read_bit() == 0):
      break
    endif
  end
  return result
end
so this returns always number greater or equal to 2. And using only read_bit() routine.
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flamewing
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Re: aPLib decruncher for 68000

Post by flamewing » Fri Sep 01, 2017 3:34 pm

Yeah, it can definitely achieve perfect compression with my generic LZSS backend; this format is a LZ variant. It won't be fast, though, because the sliding window is limited only by file size, as is the lenght of the lookahead buffer; because of this, match finding will dominate, and worst case performance will be awful — currently, this is O(m*n*p) (where m is file lenght, n is sliding window size and p is lead buffer size), which is cubic because n and p are asymptotically equal to m for this format.

Even improving match finding on my algorithm (which I plan to do anyway), the best that can because hoped for in this format is a quadratic time in file size.

I can still try implementing the format and see if the reductions in file size compensate the added compression time; who knows, maybe the other features of the format may make it more efficient than I am expecting...

r57shell
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Re: aPLib decruncher for 68000

Post by r57shell » Fri Sep 01, 2017 4:18 pm

flamewing wrote:
Fri Sep 01, 2017 3:34 pm
Yeah, it can definitely achieve perfect compression with my generic LZSS backend; this format is a LZ variant.
Nope, Trouble is last_offset & LWM.
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r57shell
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Re: aPLib decruncher for 68000

Post by r57shell » Tue Oct 17, 2017 3:58 pm

Yet another update of my packer.
https://www.mediafire.com/file/22a2f2jz ... _pack2.exe
It had stupid bug with if () .. if () ... instead of if () ... else if () ...
Now it's fixed. It was hitting only copy comands when their offset >= 32000.
Thus, if your file is less than 32000, it was never happen.

Also, max limit of input increased up to 2^19 by request.
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