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 7

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 13

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

  while pos < data.length
    if data.respond_to?(:getbyte)
      length = data.getbyte(pos)
    else
      length = data[pos]
    end
    pos += 1

    case
    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".
      out << data[pos, 1] * (257 - length)
    end

    pos += 1
  end

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