Skip to content

Filter IO streams with a block. Ruby's FilterInputStream.


Notifications You must be signed in to change notification settings


Folders and files

Last commit message
Last commit date

Latest commit


Repository files navigation


Filter IO streams with a block. Ruby's FilterInputStream.

filter_io is analogous to Java's FilterIOStream in that it allows you to intercept and process data in an IO stream. This is particularly useful when cleaning up bad input data for a CSV or XML parser.

filter_io provides a one-pass approach to filtering data which can be much faster and memory efficient than doing two passes (cleaning the source file into a buffer and then calling the original parser).

filter_io has been tested against Ruby 1.8.7 and Ruby 1.9.2.


You can install the gem by running:

gem install filter_io

Example Usage

A Simple Example: ROT-13

io = io do |data| "A-Za-z", "N-ZA-Mn-za-m"

A Useful Example: Line Ending Normalisation

A common usage of filter_io is to normalise line endings before parsing CSV data:

require 'csv'

# open source stream, external_encoding: 'UTF-8') do |io|
  # apply filter to stream
  io = do |data, state|
    # grab another chunk if the last character is a delimiter
    raise FilterIO::NeedMoreData if data =~ /[\r\n]\z/ && !state.eof?
    # normalise line endings to LF
    data.gsub /\r\n|\r|\n/, "\n"

  # process resulting stream normally
  CSV.parse(io, row_sep: "\n") do |row|
    p row


Call with the original IO stream, any options and the filtering block. The returned filter_io object acts like a normal read-only forward-only IO stream.

Block state parameter

An optional second parameter to the block is the state parameter which contains stream metadata which may be useful when processing the chuck. The methods currently available are:

  • bof?: Returns true if this is the first chuck of the stream.
  • eof?: Returns true if this is the last chunk of the stream.

Requesting Additional Data

If the filtering block needs more data to be able to return anything, you can raise a FilterIO::NeedMoreData exception and filter_io will read another block and pass the additional data to you. This can be repeated as necessary until enough data is retrieved.

For example usage of NeedMoreData, see the line ending normalisation example above.

Re-buffering Unprocessed Data

If your block is unable to process the whole chunk of data immediately, it can return both the processed chuck and the remainder to be processed later. This is done by returning a 2 element array: [processed, unprocessed]. If processed is empty and there is unprocessed data, filter_io will grab another block of data from the source stream and call the block again.

Here's an example which processes whole lines and prepends the line length to the beginning of each line.

io = io do |data, state|
  output = ''
  # grab complete lines until we hit EOF
  while data =~ /(.*)\n/ || (state.eof? && data =~ /(.+)/)
    output << "#{$1.size} #{$1}\n"
    data = $'
  # `output` contains the processed lines, `data` contains any left over partial line
  [output, data]

Block Size

When either readline, gets or read(nil) is called, filter_io will process the input stream in 1,024 byte chucks. You can adjust this by passing a :block_size option to new.

Character Encodings

Ruby 1.9 has character encoding support can convert between UTF-8, ISO-8859-1, ASCII-8BIT, etc. This is triggered in IO by using :external_encoding and :internal_encoding when opening the stream. filter_io will use the underlying stream's encoding settings when reading and filtering data. The processing block will be passed data in the internal encoding. As per the core IO object, if read is called with a length (in bytes), the data will be returned in ASCII-8BIT. In summary, everything should Just Work™

Notes on Patches/Pull Requests

  1. Fork the project.
  2. Make your feature addition or bug fix.
  3. Add tests for it. This is important so I don't break it in a future version unintentionally.
  4. Commit, do not mess with Rakefile, VERSION, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
  5. Send me a pull request. Bonus points for topic branches.


Copyright (c) 2010 Jason Weathered. See LICENSE for details.


Filter IO streams with a block. Ruby's FilterInputStream.







No packages published