generate_square.rb

rows = [
  [9, 0, 5, 0, 0, 8, 0, 0, 2],
  [0, 0, 8, 0, 0, 0, 5, 0, 0],
  [0, 4, 0, 0, 3, 5, 0, 6, 9],
  [4, 0, 3, 7, 0, 2, 0, 0, 0],
  [0, 0, 7, 0, 4, 0, 2, 0, 0],
  [0, 0, 0, 3, 0, 9, 6, 0, 4],
  [2, 8, 0, 4, 9, 0, 0, 1, 0],
  [0, 0, 9, 0, 0, 0, 4, 0, 0],
  [5, 0, 0, 8, 0, 0, 7, 0, 6]
]

rows.map { |row| row.each_slice(3).to_a }
    .transpose
    .map { |row| row.each_slice(3).map(&:flatten) }
    .transpose
    .flatten(1)

# [
#   [9, 0, 5, 0, 0, 8, 0, 4, 0],
#   [0, 0, 8, 0, 0, 0, 0, 3, 5],
#   [0, 0, 2, 5, 0, 0, 0, 6, 9],
#   [4, 0, 3, 0, 0, 7, 0, 0, 0],
#   [7, 0, 2, 0, 4, 0, 3, 0, 9],
#   [0, 0, 0, 2, 0, 0, 6, 0, 4],
#   [2, 8, 0, 0, 0, 9, 5, 0, 0],
#   [4, 9, 0, 0, 0, 0, 8, 0, 0],
#   [0, 1, 0, 4, 0, 0, 7, 0, 6]
# ]