Rangefinder

Helps you find ranges of IDs, like when you're scraping a website and you need to guess IDs.

You tell it what a valid ID is and it looks for ranges of consecutive valid IDs. It assumes that each probe is expensive.

Installation

Add this line to your application's Gemfile:

gem 'rangefinder'

And then execute:

$ bundle

Or install it yourself as:

$ gem install rangefinder

Usage

Let's say you're rainbow tabling a website but you have to guess the IDs. What you don't know is that all valid ids are in the ranges 100..11_000 and 100_000..110_000. You pass a "probe" block that returns true if an ID is valid:

ranges = Rangefinder.new.probe do |possible_id|
  # your probe code here. for example:
  response = http.get "http://example.com/items", id: possible_id
  response.status == 200
end

You get back ranges where we think there are valid IDs. In this case, pretty good! (See Goals above)

>> ranges
=> [ 0..12_200, 99_455..111_600 ]

Now you can scrape them one by one:

ranges.each do |range|
  range.each do |id|
    # scrape this ID
  end
end

Please do cache

It's nice when your probe block makes a call that is cached somehow. That way when you go back and use the ranges, you're not hitting all those URLs over again.

Goals

By default

Detect at least 90% of valid IDs in 1000-long ranges with up to 90% intra-range sparsity
Tolerate gaps of 100,000
Probe no more than 5% of the range

Maybe

Don't overestimate valid ranges more than X

Wishlist

Accept a known ID as the basis for smarter probing
Internally, calculate density and use that to choose min_range and samp

Contributing

Fork it ( http://github.com//rangefinder/fork )
Create your feature branch (git checkout -b my-new-feature)
Commit your changes (git commit -am 'Add some feature')
Push to the branch (git push origin my-new-feature)
Create new Pull Request