Class: PDF::Reader::Filter::RunLength

Inherits:
Object
  • Object
show all
Defined in:
lib/pdf/reader/filter/run_length.rb

Overview

implementation of the run length stream filter

Instance Method Summary collapse

Constructor Details

#initialize(options = {}) ⇒ RunLength

Returns a new instance of RunLength.



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# File 'lib/pdf/reader/filter/run_length.rb', line 11

def initialize(options = {})
  @options = options
end

Instance Method Details

#filter(data) ⇒ Object

Decode the specified data with the RunLengthDecode compression algorithm



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# File 'lib/pdf/reader/filter/run_length.rb', line 17

def filter(data)
  pos = 0
  out = "".dup

  while pos < data.length
    length = data.getbyte(pos)
    pos += 1

    unless length.nil?
      case
        # nothing
      when length == 128
        break
      when length < 128
        # When the length is < 128, we copy the following length+1 bytes
        # literally.
        out << data[pos, length + 1]
        pos += length
      else
        # When the length is > 128, we copy the next byte (257 - length)
        # times; i.e., "\xFA\x00" ([250, 0]) will expand to
        # "\x00\x00\x00\x00\x00\x00\x00".
        previous_byte = data[pos, 1] || ""
        out << previous_byte * (257 - length)
      end
    end

    pos += 1
  end

  Depredict.new(@options).filter(out)
end