Binary search with Ruby

binary_search.rb

def binary_search(a, needle)
  start = 0
  finish = a.size - 1

  while start <= finish do
    middle = start + (finish - start) / 2

    if a[middle] == needle
      return middle
    elsif a[middle] < needle
      start = middle + 1
    else
      finish = middle - 1
    end    
  end

  - (start + 1)
end