ant.rb

ant.rb

require 'progressbar'

class Ant
  attr_reader :x, :y

  def initialize
    @x, @y = 0, 0
  end

  def move
    case rand(1 .. 5)
    when 1
      @y += 1 if @y < 2
    when 2
      @x += 1 if @x < 2
    when 3
      @y -= 1 if @y > -2
    when 4
      @x -= 1 if @x > -2
    end
  end
end


NUM_TRIALS = (ARGV.shift || 100_000).to_i
NUM_STEPS  = (ARGV.shift || 3_600).to_i

bar = ProgressBar.create title: 'Simulating', total: NUM_TRIALS

# Run the simulation and record ending points
results = {}
NUM_TRIALS.times do
  bar.increment
  ant = Ant.new
  NUM_STEPS.times do
    ant.move
  end
  results[ [ant.x, ant.y] ] ||= 0
  results[ [ant.x, ant.y] ] += 1
end

# Tally up number that ended on an edge
on_edge = 0
results.each do |(x,y), count|
  if x == 2 || x == -2 || y == 2 || y == -2
    on_edge += count
  end
end

def percent n
  ((100.0 * n) / NUM_TRIALS).to_s + '%'
end

puts "#{percent on_edge} on edge"

ranked = results.sort_by { |_, count| count }

sq, count = ranked.first
puts "#{sq}: #{count} (#{percent count})"

sq, count = ranked.last
puts "#{sq}: #{count} (#{percent count})"

count = results[ [0,0] ]
puts "[0, 0]: #{count} (#{percent count})"