module HexaPDF::Filter::LZWDecode
Implements the LZW filter.
Since LZW uses a tightly packed bit stream in which codes are of varying bit lengths and are not aligned to byte boundaries, this filter is not as fast as the other filters. If speed is a concern, the FlateDecode
filter should be used instead.
See: HexaPDF::Filter
, PDF1.7 s7.4.4
Public Class Methods
decoder(source, options = nil)
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See HexaPDF::Filter
# File lib/hexapdf/filter/lzw_decode.rb, line 67 def self.decoder(source, options = nil) fib = Fiber.new do # initialize decoder state code_length = 9 table = INITIAL_DECODER_TABLE.dup stream = HexaPDF::Utils::BitStreamReader.new result = ''.b finished = false last_code = CLEAR_TABLE while !finished && source.alive? && (data = source.resume) stream.append_data(data) while (code = stream.read(code_length)) # Decoder is one step behind => subtract 1! # We check the table size before entering the next code into it => subtract 1, but # there is one exception: After table entry 4095 is written, the clear table code # also gets written with code length 12, case table.size when 510, 1022, 2046 code_length += 1 when 4096 if code != CLEAR_TABLE raise FilterError, "Maximum of 12bit for codes in LZW stream exceeded" end end if code == EOD finished = true break elsif code == CLEAR_TABLE # reset decoder state code_length = 9 table = INITIAL_DECODER_TABLE.dup elsif last_code == CLEAR_TABLE unless table.key?(code) raise FilterError, "Unknown code in LZW encoded stream found" end result << table[code] else unless table.key?(last_code) raise FilterError, "Unknown code in LZW encoded stream found" end last_str = table[last_code] str = if table.key?(code) table[code] else last_str + last_str[0] end result << str table[table.size] = last_str + str[0] end last_code = code end Fiber.yield(result) result = ''.b end end if options && options[:Predictor] Predictor.decoder(fib, options) else fib end end
encoder(source, options = nil)
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See HexaPDF::Filter
# File lib/hexapdf/filter/lzw_decode.rb, line 139 def self.encoder(source, options = nil) if options && options[:Predictor] source = Predictor.encoder(source, options) end Fiber.new do # initialize encoder state code_length = 9 table = INITIAL_ENCODER_TABLE.dup # initialize the bit stream with the clear-table marker stream = HexaPDF::Utils::BitStreamWriter.new result = stream.write(CLEAR_TABLE, 9) str = ''.b while source.alive? && (data = source.resume) data.each_char do |char| newstr = str + char if table.key?(newstr) str = newstr else result << stream.write(table[str], code_length) table[newstr.freeze] = table.size str = char end case table.size when 512 then code_length = 10 when 1024 then code_length = 11 when 2048 then code_length = 12 when 4096 result << stream.write(CLEAR_TABLE, code_length) # reset encoder state code_length = 9 table = INITIAL_ENCODER_TABLE.dup end end Fiber.yield(result) result = ''.b end result = stream.write(table[str], code_length) result << stream.write(EOD, code_length) result << stream.finalize result end end