Create all the combinations of 0 to n with e positions

index.rb

# Array.new(2**5) do |i|
#   [i / 2**4, i / 2**3 % 2, i / 2**2 % 2, i / 2 % 2, i % 2]
# end

# integer_combinations(2, 5)

# => [[0, 0, 0, 0, 0], [0, 0, 0, 0, 1], [0, 0, 0, 1, 0], [0, 0, 0, 1, 1], [0, 0, 1, 0, 0], [0, 0, 1, 0, 1], [0, 0, 1, 1, 0], [0, 0, 1, 1, 1], [0, 1, 0, 0, 0], [0, 1, 0, 0, 1], [0, 1, 0, 1, 0], [0, 1, 0, 1, 1], [0, 1, 1, 0, 0], [0, 1, 1, 0, 1], [0, 1, 1, 1, 0], [0, 1, 1, 1, 1], [1, 0, 0, 0, 0], [1, 0, 0, 0, 1], [1, 0, 0, 1, 0], [1, 0, 0, 1, 1], [1, 0, 1, 0, 0], [1, 0, 1, 0, 1], [1, 0, 1, 1, 0], [1, 0, 1, 1, 1], [1, 1, 0, 0, 0], [1, 1, 0, 0, 1], [1, 1, 0, 1, 0], [1, 1, 0, 1, 1], [1, 1, 1, 0, 0], [1, 1, 1, 0, 1], [1, 1, 1, 1, 0], [1, 1, 1, 1, 1]]

def integer_combinations_of(n, e)
  Array.new(n**e) do |i|
    a = []
    if e > 0
      a << i / n**(e - 1) if e > 1
      if e > 2
        j = e - 2
        while j > 0
          a << i / n**j % n
          j -= 1
        end
      end
    end
    if n > 0
      a << i % n
    else
      a << i
    end
    a
  end
end