class MArray # for working with multidimensional data
  attr_reader :sizes

  def initialize(*args) # args = [size for each dimension]
    @sizes = args
    @data = Array.new(@sizes.product)
    @data.each_with_index { |x, i| @data[i] = yield i } if block_given? # to support block initialisation
  end

  def [](*args)
    raise ArgumentError if args.size != self.dimension
    @data[self.get_offset(*args)]
  end

  def []=(*args)
    raise ArgumentError if args.size != self.dimension + 1
    value = args.pop
    @data[self.get_offset(*args)] = value
  end

  def dimension
    @dimension ||= @sizes.size
  end

  def get_offset(*args)
    offset = 0
    args.each_with_index do |a, i|
      raise IndexError unless a < sizes[i]
      offset += a * self.sizes.first(i).product # product of an empty array is 1
    end
    offset
  end

  def each
    @data.each { |v| yield v } # avoiding Proc, conversion is slow!
  end

  include Enumerable

  def flatten
    @data.dup
  end

  # TODO: methods to reduce dimensions (by 1 or by n - 1)
  def reduce(d, r) # d is the dimension (index in self.sizes)
    # r is which 'hyperrow' is selected in the dimension being reduced
  end
  # TODO: unwind: returns an array of arrays of arrays of arrays of arrays ... of arrays.
end