Exercise implementing multidimensional arrays in Ruby

arrayn.rb

# Exercise for implementing multi-dimesional arrays in Ruby

# TODO:

# Level 0: Implement `row_major_index` and `to_a`.
# Level 1: How could tests be improved?
# Level 2: How could the interface of ArrayN be extended? Made more idiomatic?

# Multi-dimensional array
class ArrayN
  def initialize (dimension, *dimensions)
    @dimensions = dimension, *dimensions
    @dimensions.freeze
    @contents = Array.new total_size
  end
  attr_reader :dimensions

  def total_size
    @total_size ||= @dimensions.reduce(:*)
  end

  def rank
    @rank ||= dimensions.size
  end

  def in_bounds? (subscript, *subscripts)
    all = subscript, *subscripts
    return false unless all.size == rank
    rank.times do |idx|
      one = all.fetch(idx)
      return false if one < 0
      return false unless one < dimensions.fetch(idx)
    end
    true
  end

  def fetch (subscript, *subscripts)
    @contents.fetch row_major_index(subscript, *subscripts)
  end

  def store (item, subscript, *subscripts)
    @contents[row_major_index(subscript, *subscripts)] = item
  end

  def row_major_index (subscript, *subscripts)
    raise NotImplementedError, "Left as exercise to student."
  end
  
  def to_a
    raise NotImplementedError, "Left as exercise to student."
  end
end

require 'minitest/autorun'

class ArrayNTest < Minitest::Test
  def setup
    @subject = ArrayN.new 2, 3, 5
  end
  
  def test_total_size
    assert_equal 30, @subject.total_size
  end

  def test_rank
    assert_equal 3, @subject.rank
  end

  def test_in_bounds?
    assert @subject.in_bounds?(1, 2, 4)
    refute @subject.in_bounds?(2, 2, 4)
  end

  def test_store_fetch
    @subject.store 0, 0, 0, 0
    assert_equal 0, @subject.fetch(0, 0, 0)
  end

  def test_row_major_index
    assert_equal  1, @subject.row_major_index(0, 0, 1)
    assert_equal  5, @subject.row_major_index(0, 1, 0)
    assert_equal 15, @subject.row_major_index(1, 0, 0)
  end

  def test_to_a
    assert_equal [[[nil, nil, nil, nil, nil],
                   [nil, nil, nil, nil, nil],
                   [nil, nil, nil, nil, nil]],
                  [[nil, nil, nil, nil, nil],
                   [nil, nil, nil, nil, nil],
                   [nil, nil, nil, nil, nil]]], @subject.to_a
  end

  end
end