google interview question

simulation.rb

# Simulation for question #27 from https://www.inc.com/business-insider/google-hardest-interview-questions.html

class Simulation
    def initialize(drop_rate, sidewalk_size)
        @drop_rate      = drop_rate         # drops per second
        @sidewalk_size  = sidewalk_size     # size of the sidewalk in drops(100 for 1m sidewalk with 1cm drop size)
        @sidewalk       = Hash.new          # use a hash for much quicker lookup compared to a 2-dimentional array
    end
    
    def run
        i = 0
    
        while !covered?
            x = rand(0...@sidewalk_size)
            y = rand(0...@sidewalk_size)
            @sidewalk[x] ||= Hash.new
            @sidewalk[x][y] = true
            i +=1
        end
        
        i / @drop_rate
    end
    
    def covered?
        !@sidewalk.empty? && @sidewalk.all? { |k, v| v.size == @sidewalk_size }
    end
end

simulation_runs = 10

simulation_runs.times do
    seconds = Simulation.new(1, 100).run
    puts "#{seconds} seconds needed to cover the sidewalk"  
end