First working version on binary chop. May try an OO version next time

chop2.rb

require 'rspec-given'

module Chop
  def chop(coll, search, i = 0)
    return nil if bad_search?(coll, search)
    middle = coll.count / 2
    return i if coll.first == search
    if coll[middle] <= search
      chop(coll[middle..-1], search, i + middle)
    else
      chop(coll[0..middle], search, i)
    end
  end

  def bad_search?(coll, search)
    coll.empty? || coll.size == 1 && coll.first != search
  end
end

describe "binary chop" do
  include Chop
  Then { chop([], :x).nil? }
  Then { chop([:a], :a) == 0 }
  Then { chop([:a,:b], :b) == 1 }
  Then { chop([:a,:b,:c], :c) == 2 }
  Then { chop([:a,:b,:c,:d], :d) == 3 }
  Then { chop([:a,:b,:c,:d,:e], :e) == 4 }
  Then { chop([:a,:b,:c,:d,:e,:f], :f) == 5 }
  Then { chop([:a,:b,:c,:d,:e,:f], :e) == 4 }
  Then { chop([:a,:b,:c,:d,:e,:f], :d) == 3 }
  Then { chop([:a,:b,:c,:d,:e,:f], :c) == 2 }
  Then { chop([:a,:b,:c,:d,:e,:f], :b) == 1 }
  Then { chop([:a,:b,:c,:d,:e,:f], :a) == 0 }
  Then { chop([:a,:b,:c,:d,:e,:f], :x).nil? }
end