pile_of_sand.rb

pile_of_sand.rb

# This script implements a simple cellular automaton to simulate sand,
# inspired by the game "Noita". View introduce a grid which holds
# boolean values (true means there is sand). Then, we define three
# simple rules how those cells evolve. Finally, we advance this grid
# over and over to get the simulation.
#
class PileOfSand
  attr_reader :width, :height

  def initialize(width:, height:)
    @width, @height = width, height

    # The grid (col, row) contains boolean values and a cell gets set
    # to true if it is sand.
    @grid = Array.new(width) { Array.new(height) }
  end

  # Sets the spawner position to generate a grain of sand
  def start_spawn(x, y)
    @spawner = { x: x, y: y }
  end

  # Removes the spawner position
  def stop_spawn
    @spawner = nil
  end

  # Advances the simulation of the cellular automaton.
  # Each cell gets iterated and decides which cells to set/unset.
  def advance

    # Spawn new grain of sand if spawner is set
    @grid[@spawner[:x]][@spawner[:y]] = true if @spawner

    # Iterate blocks (col, row) and compute the next state of the grid.
    # Please note, that for rows we are iterating from the bottom up to
    # achieve a more realistic simulation.
    @grid.size.times do |x|
      @grid[x].size.downto(0) do |y|

        # Do nothing for non-sand and bottom row
        next if !@grid[x][y] || y == @grid[x].size - 1

        # If nothing is below, move there
        if !@grid[x][y+1]
          @grid[x][y+1] = true
          @grid[x][y] = false

        # If nothing is below/left, move there
        elsif x > 0 && !@grid[x-1][y+1]
          @grid[x-1][y+1] = true
          @grid[x][y] = false

        # If nothing is below/right, move there
        elsif x < @grid.size - 1 && !@grid[x+1][y+1]
          @grid[x+1][y+1] = true
          @grid[x][y] = false
        end
      end
    end

    # Return advanced grid
    @grid
  end
end

Use it like that:

pile = PileOfSand.new(width: 60, height: 40)
grid = pile.advance

Or use it with ruby2d:

Screen.Recording.2022-05-14.at.16.48.25.mov

# Use ruby2d library for drawing
require "ruby2d"

# Set the window size
# Recommended to set it in ratio to the grid
set width: 600, height: 400

# Compute block size to convert between pixels and grid
BLOCK_SIZE_X = (get :width) / pile.width
BLOCK_SIZE_Y = (get :height) / pile.height

# Register event handler to set sand spawner
Ruby2D::Window::on :mouse_down do |event|
  pile.start_spawn(event.x / BLOCK_SIZE_X, event.y / BLOCK_SIZE_Y)
end

# Register event handler to remove sand spawner
Ruby2D::Window::on :mouse_up do |event|
  pile.stop_spawn
end

Ruby2D::Window::update do

  # Clear window
  clear

  # Advance simulation
  grid = pile.advance

  # Draw grid. Note, that this does not necessarily have to use rectangles
  grid.size.times do |x|
    grid[x].size.times do |y|
      next unless grid[x][y]
      Rectangle.new(
        x: x * BLOCK_SIZE_X, y: y * BLOCK_SIZE_Y,
        width: BLOCK_SIZE_X, height: BLOCK_SIZE_Y,
        color: "#C2B280"
      )
    end
  end
end

# Render window
Ruby2D::Window::show

Very cool 👍 ⏳