01-rotation-index.rb

infile = ARGV[ 0 ]
words = File.readlines( infile )

# O(log n)

def rotation_index( array, lowest_so_far = nil, from = 0, to = nil )
  lowest_so_far ||= array.first.downcase
  to ||= array.size - 1

  if from > to
    return array.size == from ? 0 : from
  end

  mid = ( from + to ) / 2
  current = array[ mid ].downcase

  if current > lowest_so_far
    rotation_index array, lowest_so_far, mid + 1, to
  elsif current < lowest_so_far
    rotation_index array, current, 0, mid - 1
  end
end

puts rotation_index( words )