Generate a matrix with a diagonal pattern

main.rb

[
  [[0, 0], [1, 0], [2, 0], [3, 0]],
  [[0, 1], [1, 1], [2, 1], [3, 1]],
  [[0, 2], [1, 2], [2, 2], [3, 2]],
  [[0, 3], [1, 3], [2, 3], [3, 3]],
]

$rows = 4
$columns = 4

def get_next_index(index)
  x, y = index
  if (y+1) == $rows # bottom row, jump to right column
    [y, x + 1]
  elsif x == 0 # most left column, jump to top line
    [y + 1, 0]
  else
    [x - 1, y + 1]
  end
end

def assert(input, output)
  raise [input, output, get_next_index(input)].inspect unless get_next_index(input) == output
end

class IndexGenerator
  include Enumerable

  def initialize
    @current = [0, 0]
  end

  def each
    loop do
      yield @current
      @current = get_next_index(@current)
    end
  end
end

# left column jumps
assert([0, 0], [1, 0])
assert([0, 1], [2, 0])
assert([0, 2], [3, 0])

# bottom row jumps
assert([0, 3], [3, 1])
assert([1, 3], [3, 2])
assert([2, 3], [3, 3])

# normal jump
assert([1, 0], [0, 1])
assert([2, 0], [1, 1])
assert([1, 1], [0, 2])

elements_to_generate = $rows * $columns
p values = (1..elements_to_generate).to_a
p indexes = IndexGenerator.new.first(elements_to_generate)

matrix = Array.new($rows).map { |row| Array.new($columns) }

p matrix
indexes.each_with_index do |val, index|
  x,y = val
  value = values[index]
  matrix[y][x] = value
end
p matrix