Class: WR::LZSS

Inherits:
Object
  • Object
show all
Defined in:
lib/wrb/lzss.rb

Constant Summary collapse

PatMin = 
3
PatMax =

16 pattern (3 to 18)

18

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(str) ⇒ LZSS

Returns a new instance of LZSS.



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# File 'lib/wrb/lzss.rb', line 13

def initialize(str)
  @src = str.b
end

Class Method Details

.compress(str) ⇒ Object 



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# File 'lib/wrb/lzss.rb', line 10

def self.compress(str) self.new(str).compress; end

.expand(str) ⇒ Object 



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# File 'lib/wrb/lzss.rb', line 11

def self.expand(str) self.new(str).expand; end

Instance Method Details

#compress ⇒ Object 



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# File 'lib/wrb/lzss.rb', line 17

def compress()
  rp = 0
  ht = {}
  dst = "".b
  def dst.concat_bits(bint, blen)
    raise ArgumentError, "Argument is too big." if bint > 0xffff || blen > 16
    @bit_offset ||= 0
    len = @bit_offset + blen
    ival = bint << (32 - len) 
    n, new_offset = len.divmod(8)
    if @bit_offset==0
      self.concat([ival].pack('N')[0, new_offset==0 ? n : n + 1])
    else
      s = [self[-1].ord << 24 | ival].pack('N')
      self[-1] = s[0]
      self.concat(s[1, new_offset==0 ? n - 1 : n]) if len > 8
    end
    @bit_offset = new_offset
    self
  end
  
  while rp < @src.bytesize
    found = len = nil
    max = @src.size - rp > PatMax ? PatMax : @src.size - rp
    max.downto(PatMin) {|i|  # search pattern.

      s = @src[rp, i]
      if (found = ht[s])
        if rp - found > 4095 # length of offset is 12 bits.

          found = nil        # only replace address of pattern.

        elsif found + i < rp 
          len = i
          break
        else
          found = nil
          break
        end
      end
      ht[s] = rp
    }
    if found
      offset = rp - found
      dst.concat_bits(1, 1)
      dst.concat_bits((offset << 4) + len - 3, 16) # length of pattern is 4bits(3 to 18).

      rp += len
    else
      dst.concat_bits(0, 1)
      dst.concat_bits(@src[rp, 1].ord, 8)
      rp += 1
    end
  end
  dst
end

#expand ⇒ Object 



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# File 'lib/wrb/lzss.rb', line 70

def expand()
  dst = "".b
  bp = 0
  while true
    q, r = bp.divmod(8)
    s = @src[q]        # get 1 byte

    break unless s
    data = s.ord       # convert to 8 bits integer value

    break unless @src[q+1]
    data = (data << 8) + @src[q + 1].ord # join more 1 byte

    if data[15-r] == 0                   # read flag bit

      bp += 9
      dst.concat ((data >> 8 - r - 1) & 0xff).chr # return 1 byte to char

    else
      data = (data << 8) + @src[q + 2].ord # join more 1 byte

      x = (data >> (8 - r - 1)) & 0xffff   # get 16 bits which containes length and offset.

      dic_len = (x & 0xf) + PatMin
      dic_off = x >> 4                     # length of pattern is 4bits(3 to 18).

      bp += 17
      dst.concat dst[dst.size - dic_off, dic_len] # get from dictionary

    end
  end
  dst
end