Class: Pdfrb::Filter::RunLengthDecode

Inherits:
Object
  • Object
show all
Includes:
Base
Defined in:
lib/pdfrb/filter/run_length_decode.rb

Overview

RunLengthDecode (s7.4.5). Length byte 0..127 = N+1 literal bytes; 129..255 = repeat next byte (257 - N) times; 128 = EOD.

Class Method Summary collapse

Class Method Details

.decoder(bytes, _parms, _document) ⇒ Object 



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# File 'lib/pdfrb/filter/run_length_decode.rb', line 13

def decoder(bytes, _parms, _document)
  out = +""
  i = 0
  bs = bytes.bytes
  while i < bs.length
    n = bs[i]
    i += 1
    case n
    when 128 then break
    when 0..127
      out << bs[i, n + 1].pack("C*")
      i += n + 1
    when 129..255
      count = 257 - n
      out << (bs[i] || 0).chr * count if count.positive? && bs[i]
      i += 1
    end
  end
  out.force_encoding(Encoding::BINARY)
end

.encoder(bytes, _parms, _document) ⇒ Object 



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# File 'lib/pdfrb/filter/run_length_decode.rb', line 34

def encoder(bytes, _parms, _document)
  out = +""
  bs = bytes.bytes
  i = 0
  while i < bs.length
    # Try to find a run of identical bytes (3+).
    run_len = 1
    while i + run_len < bs.length && bs[i + run_len] == bs[i] && run_len < 128
      run_len += 1
    end
    if run_len >= 3
      out << (257 - run_len).chr
      out << bs[i].chr
      i += run_len
    else
      # Accumulate literals until next run or 128 bytes.
      literal = []
      while i < bs.length && literal.length < 128
        # Stop if a run of >= 3 starts.
        ahead_run = 0
        while i + ahead_run < bs.length && bs[i + ahead_run] == bs[i] && ahead_run < 3
          ahead_run += 1
        end
        break if ahead_run >= 3

        literal << bs[i]
        i += 1
      end
      out << (literal.length - 1).chr
      out << literal.pack("C*")
    end
  end
  out << 128.chr # EOD
  out.force_encoding(Encoding::BINARY)
end