Drawing Sandpiles

drawing_sandpiles.rb

# Video: https://t.co/FCdLtWceez
# Based on: https://www.youtube.com/watch?v=1MtEUErz7Gg
# This one is better than mine: http://people.reed.edu/~davidp/web_sandpiles/
require 'graphics'

class Sandpile < Graphics::Simulation
  AVERAGE_HEIGHT = 2.1 # he mentioned this in the video

  def initialize(n)
    super 800, 800
    register_color :sand_zero,   20,  79, 245 # blue
    register_color :sand_one,   151, 198, 250 # sky
    register_color :sand_two,   250, 225,  76 # yellow
    register_color :sand_three, 112,  20,  12 # red
    # color.default_proc = lambda { |rgb| rgb }

    @n             = n
    ideal_diameter = 2 * Math.sqrt(n / AVERAGE_HEIGHT / Math::PI)
    @pile_side     = ideal_diameter.ceil.succ
    @cell_side     = w / @pile_side
    @offset        = (w - @pile_side*@cell_side)/2
    @sandpile      = Array.new(@pile_side) { Array.new @pile_side, 0 }
    @sandpile[@pile_side/2][@pile_side/2] = n
    # nil while increment! @sandpile
    draw_pile @sandpile
  end

  def draw(i)
    return if @n.zero?
    @n -= 1
    draw_pile @sandpile
    increment! @sandpile
  end

  def draw_pile(sandpile)
    sandpile.each_with_index do |row, y|
      row.each_with_index { |cell, x| draw_cell x, y, sand_color(cell) }
    end
  end

  def draw_cell(x, y, color)
    rect @offset+x*@cell_side, @offset+y*@cell_side, @cell_side, @cell_side, color, :fill
  end

  def increment!(sandpile)
    pending = Array.new
    sandpile.each_with_index do |row, y|
      row.each_with_index do |cell, x|
        next if cell < 4
        pending << [y, x, -4]
        pending << [y-1, x, 1] if 0 < y
        pending << [y+1, x, 1] if y.succ < sandpile.length
        pending << [y, x-1, 1] if 0 < x
        pending << [y, x+1, 1] if x.succ < row.length
      end
    end
    pending.each { |y, x, delta| sandpile[y][x] += delta }
    pending.any?
  end

  def sand_color(num_grains)
    case num_grains
    when 0 then :sand_zero
    when 1 then :sand_one
    when 2 then :sand_two
    else        :sand_three
    end
  end
end


Sandpile.new(2**15).run