gistfile1.rb

require 'thread'

class ReadWriteLock
  def initialize
    @critical_section = Mutex.new
    @are_writers_finished = ConditionVariable.new
    @are_readers_finished = ConditionVariable.new
    @readers = 0
    @writers = 0
    @writer_locked = false
  end
  
  def read
    begin
      start_read
      yield
    ensure
      end_read
    end
  end
  
  def start_read
    @critical_section.lock
    while (@writers != 0 || @writer_locked)
      @are_writers_finished.wait(@critical_section)
    end
    @readers += 1
    @critical_section.unlock
  end
  
  def end_read
    @critical_section.lock
    if (@readers -= 1) == 0
      @are_readers_finished.broadcast
    end
    @critical_section.unlock
  end
  
  def write
    begin
      start_write
      yield
    ensure
      end_write
    end
  end
  
  def start_write
    @critical_section.lock
    @writers += 1
    while @readers > 0
      @are_readers_finished.wait(@critical_section)
    end
    while @writer_locked
      @are_writers_finished.wait(@critical_section)
    end
    @writers -= 1
    @writer_locked = true
    @critical_section.unlock
  end
  
  def end_write
    @critical_section.lock
    @writer_locked = false
    @are_writers_finished.broadcast
    @critical_section.unlock
  end
end