I found this P5*.js sketch on openprocessing https://www.openprocessing.org/user/23616/ by Manoylov AC which was in need of some serious refactoring, here is my JRubyArt version (javascript is just horrible) which is much shorter and a bit drier.

attr_reader :cols, :coloured

PALETTE = %w[#152A3B #158ca7 #F5C03E #D63826 #0F4155 #7ec873 #4B3331].freeze

def settings
  size(600, 600)
end

def setup
  background 0
  sketch_title 'Arc Pattern'
  # create a java primitive array of signed int
  @cols = web_to_color_array(PALETTE)
  stroke_weight 1.5
  stroke_cap SQUARE
  stroke(0, 200)
  @coloured = true
end

def draw
  arc_pattern
end

def sep_index(idx, length)
  (idx - (length - 1) * 0.5).abs.floor
end

def sep_color(idx, number)
  cols[sep_index(idx - 1, number + 1)]
end

def arc_pattern
  circ_number = rand(4..10)
  block_size = rand(30..70)
  back_color = coloured ? cols.last : 255
  fill(back_color)
  rect(0, 0, width, height)
  half_block = block_size / 2
  two_block = 2 * block_size
  grid(width + two_block, height + two_block, block_size, block_size) do |x, y|
    push_matrix
    translate x, y
    rotate HALF_PI * rand(0..4)
    circ_number.downto(0) do |i|
      diam = two_block * i / (circ_number + 1)
      ccolor = i < 2 || !coloured ? back_color : sep_color(i, circ_number)
      fill ccolor
      arc(-half_block, -half_block, diam, diam, 0, HALF_PI)
      arc(half_block, half_block, diam, diam, PI, PI + HALF_PI)
    end
    pop_matrix
  end
  no_loop
end

def mouse_pressed
  @cols = shuffle_array(cols) if coloured
  loop
end

def key_typed
  case key
  when 'c', 'C'
    @coloured = !coloured
  when 's', 'S'
    save(data_path('arc_pattern.png'))
  end
end

# do a ruby shuffle! on a primitive java array
def shuffle_array(ar)
  cols = ar.to_a
  cols.shuffle!
  cols.to_java(Java::int)
end

arc_pattern.png