Sparse ruby IO class

sparse_file.rb

class SparseFile < IO
  def initialize(ranges, total_length, fill = "\0")
    @ranges = ranges
    @total_length = total_length
    @position = 0
    @fill = fill
  end

  def rewind
    @position = 0
  end

  # read([length [, outbuf]]) → string, outbuf, or nil
  # Reads length bytes from the I/O stream.
  def read(length=-1, outbuf=nil)
    buf = []
    buf_length = 0

    # rest of file
    length = @total_length-@position if length == -1

    length = length - @position if @position+length > @total_length
    lenght = 0 if length < 0

    # 01234567890123456789
    #      ABCDEF  GHIJK
    # [ ]                  before
    #    [  ]              half
    #        [ ]           full
    #           [   ]      combined
    #                [  ]  half

    @ranges.each do |offset, buffer|
      # to far
      if @position+length < offset
        break
      end

      # enough
      if length == 0
        break
      end

      # skip
      if offset+buffer.length < @position
        next
      end

      # pad before
      if @position < offset
        n = offset-@position
        buf.push(@fill*n)
        length -= n
        @position += n
      end

      o = @position-offset

      n = length > buffer.length-o ? buffer.length-o : length

      buf.push(buffer[o,n])
      length -= n
      @position += n
    end

    # pad after
    buf.push(@fill*(length-buf_length)) if buf_length < length

    @position += length

    buf.join
  end

  # seek(amount, whence=IO::SEEK_SET) -> 0
  # Seeks to a given offset anInteger in the stream according to the value of whence:
  def seek(amount, whence=IO::SEEK_SET)
    @position = case whence
                when IO::SEEK_CUR then @position+amount
                when IO::SEEK_END then @total_length+amount
                when IO::SEEK_SET then amount
                end
    @position = @total_length if @position > @total_length
    @position = 0 if @position < 0
    0
  end

#  def method_missing(m, *args, &block) 
#    puts "missing", m
#  end
end

if false
  r = SparseFile.new([
    [5, "abcdef"],
    [13, "ghijk"]
  ], 20, "-")


  expected = [
    "---",
    "--ab",
    "cde",
    "f--gh",
    "ijk-"
  ]

  actual = [
    r.read(3),
    r.read(4),
    r.read(3),
    r.read(5),
    r.read(4)
  ]

  puts
  puts expected
  puts
  puts actual
  puts
  r.rewind
  puts r.read(20)
end