risk_battle.rb

def die_roll
  rand(6) + 1
end

def attack(attacking_count, defending_count)
  attacking_dice = [attacking_count, 3].min
  defending_dice = [defending_count, 2].min

  attacking_rolls = attacking_dice.times.map { die_roll }.sort.reverse
  defending_rolls = defending_dice.times.map { die_roll }.sort.reverse

  attackers_lost = 0
  defenders_lost = 0
  if attacking_rolls[0] > defending_rolls[0]
    defenders_lost += 1
  else
    attackers_lost += 1
  end

  if attacking_rolls[1] && defending_rolls[1]
    if attacking_rolls[1] > defending_rolls[1]
      defenders_lost += 1
    else
      attackers_lost += 1
    end
  end

  [attackers_lost, defenders_lost]
end

def battle(attacking_count, defending_count)
  while attacking_count > 0 && defending_count > 0
    attackers_lost, defenders_lost = attack(attacking_count, defending_count)
    attacking_count -= attackers_lost
    defending_count -= defenders_lost
  end

  success = defending_count.zero?
  [attacking_count, defending_count, success]
end

attacking_count = ARGV[0].to_i
defending_count = ARGV[1].to_i

simulations = 100_000
success_count = 0
attackers_left_array = []
simulations.times do
  result = battle(attacking_count, defending_count)
  success_count += 1 if result.last
  attackers_left_array << result.first
  puts result.join(',')
end

puts "Change of success: #{100 * success_count.fdiv(simulations)}%"
puts "Average number of attackers left: #{attackers_left_array.sum.fdiv(simulations)}"