From: Robert Feldt <robert.feldt@...>
Date: 2006-11-01T14:50:48+09:00
Subject: Probabilistic BDD?

Hi,

I'm playing around with BDD � la test/spec and foudn that I need to
specify properties probabilistically ie saying that they are
likely/unlikely. Has there been any previous work along these lines?

Should we add something like this to test/spec (Christian are you listening? :))

diff -rN old-testspec/lib/test/spec.rb new-testspec/lib/test/spec.rb
286a287,300
>     def unlikely(specname, probability = 0.01, preName = "unlikely", &block)
>       specify(preName + " " + specname) do
>         count = 0
>         num_repetitions = 100
>         num_repetitions.times {count += 1 if (block.call == true)}
>         actual_probability = count.to_f / num_repetitions
>         assert actual_probability <= probability, "Expected probability of #{probability} but was #{actual_probability} (#{count} in #{num_repetitions} repetitions)"
>       end
>     end
>
>     def likely(specname, probability = 0.99, &block)
>       unlikely(specname, 1.0 - probability, "likely") {!block.call}
>     end
>

so that one can write specs like

> # Example of probabilistic specifications
>
> context "random generation" do
>   unlikely "that two consecutive calls to rand gives the same value" do
>     rand(1000) == rand(1000)
>   end
>
>   likely "that two consecutive calls to rand gives different values" do
>     rand(1000) != rand(1000)
>   end
> end

?

This should probably be generalized so that the number of repetitions
to run depends on the probability of the event but I think you get the
idea.

Comments?

/Robert Feldt