words.rb

class Array
  def prefixes
    result = []
    (0..size).each {|n| result << take(n) }
    result
  end

  def indices value
    map {|v| v == value ? 0 : 1 }
  end

  def negate
    map {|v| v == 0 ? 1 : 0 }
  end

  def times ary
    result = []
    each_with_index { |v,n| result << v * ary[n] }
    result
  end

  def plus n
    map {|v| v + n }
  end

  def group_on mask
    return [] if size != mask.size
    groups = {}
    mask.each_with_index do |v,n|
      (groups[v] ||= []) << self[n]
    end
    groups.values
  end

end

class String
  def to_a; chars.to_a; end
end


def words text
  rep = ' ' + text
  letters_mask = rep.to_a.indices(' ')
  spaces_mask = letters_mask.negate
  keys_mask = letters_mask.times(spaces_mask.prefixes.map {|v| v.reduce(0,:+)  }.plus(1))
  rep.to_a.group_on(keys_mask).drop(1).map(&:join)
end