Simple semaphore implementation

semaphore.rb

require 'monitor'

# Simple semaphore implementation using Monitor and ConditionVariable
class Semaphore
  def initialize(initial_value = 0)
     @val = initial_value
     @lock = Monitor.new
     @available = ConditionVariable.new
  end

  def wait(take = 1)
    take = permit_check(take)

    @lock.synchronize do
      while @val - take < 0
        @available.wait(@lock)
      end

      @val -= take
    end
  end

  def signal(put = 1)
    put = permit_check(put)

    @lock.synchronize do
      @val += put
      @available.broadcast
    end
  end

  def synchronize(permits = 1)
    permits = permit_check(permits)

    wait(permits)
    begin
      yield
    ensure
      signal(permits)
    end
  end

private
  # Check the permits and throw if illegal.  Otherwise return the value.
  def permit_check(permits)
    x = permits.to_int
    raise ArgumentError, "Illegal no. of permits: #{x.inspect}" if x <= 0
    x
  end
end

# test
if __FILE__ == $0
  c = 0
  sem = Semaphore.new 1

  4.times.map do |i|
    Thread.new i do |ii|
      3.times { sem.synchronize { c += 1; printf "%2d: %3d\n", ii, c } }
    end
  end.each(&:join)
end