Dining witches with Mutex locks

witch-wands.rb

require 'thread'

class Witch
  def initialize(name, left_wand, right_wand)
    @name = name
    @left_wand = left_wand
    @right_wand = right_wand
    while true
      conjure
      dine
    end
  end

  def conjure
    puts "Witch #@name is conjuring..."
    sleep rand()
    puts "Witch #@name needs food badly..."
  end

  def dine
    while true
      @left_wand.lock
      puts "Witch #@name has one wand to summon food..."
      if @right_wand.try_lock
        break
      else
        puts "Witch #@name cannot grab a second wand"
        @left_wand.unlock
      end
    end

    puts "Witch #@name has the second wand!"

    puts "Witch #@name summons food and eats..."
    sleep rand()
    puts "Witch #@name finishes eating..."
    @left_wand.unlock
    @right_wand.unlock
  end
end

n = 5

wands = (1..n).reduce([]) do |acc, i|
  acc << Mutex.new
end

threads = (1..n).reduce([]) do |acc, i|
  acc << Thread.new do
    if i < n
      left_wand = wands[i]
      right_wand = wands[i + 1]
    else
      # last witch wraps to use wand 5 and 1
      left_wand = wands[0]
      right_wand = wands[n]
    end
    Witch.new(i, left_wand, right_wand)
  end
end

threads.each { |thread| thread.join }