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diff --git a/ruby_2_2/lib/thwait.rb b/ruby_2_2/lib/thwait.rb
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+#
+#   thwait.rb - thread synchronization class
+#       $Release Version: 0.9 $
+#       $Revision: 1.3 $
+#       by Keiju ISHITSUKA(Nihon Rational Software Co.,Ltd.)
+
+require "thread.rb"
+require "e2mmap.rb"
+
+#
+# This class watches for termination of multiple threads.  Basic functionality
+# (wait until specified threads have terminated) can be accessed through the
+# class method ThreadsWait::all_waits.  Finer control can be gained using
+# instance methods.
+#
+# Example:
+#
+#   ThreadsWait.all_waits(thr1, thr2, ...) do |t|
+#     STDERR.puts "Thread #{t} has terminated."
+#   end
+#
+#
+#   th = ThreadsWait.new(thread1,...)
+#   th.next_wait # next one to be done
+#
+#
+class ThreadsWait
+  extend Exception2MessageMapper
+  def_exception("ErrNoWaitingThread", "No threads for waiting.")
+  def_exception("ErrNoFinishedThread", "No finished threads.")
+
+  #
+  # Waits until all specified threads have terminated.  If a block is provided,
+  # it is executed for each thread as they terminate.
+  #
+  def ThreadsWait.all_waits(*threads) # :yield: thread
+    tw = ThreadsWait.new(*threads)
+    if block_given?
+      tw.all_waits do |th|
+        yield th
+      end
+    else
+      tw.all_waits
+    end
+  end
+
+  #
+  # Creates a ThreadsWait object, specifying the threads to wait on.
+  # Non-blocking.
+  #
+  def initialize(*threads)
+    @threads = []
+    @wait_queue = Queue.new
+    join_nowait(*threads) unless threads.empty?
+  end
+
+  # Returns the array of threads that have not terminated yet.
+  attr_reader :threads
+
+  #
+  # Returns +true+ if there are no threads in the pool still running.
+  #
+  def empty?
+    @threads.empty?
+  end
+
+  #
+  # Returns +true+ if any thread has terminated and is ready to be collected.
+  #
+  def finished?
+    !@wait_queue.empty?
+  end
+
+  #
+  # Waits for specified threads to terminate, and returns when one of
+  # the threads terminated.
+  #
+  def join(*threads)
+    join_nowait(*threads)
+    next_wait
+  end
+
+  #
+  # Specifies the threads that this object will wait for, but does not actually
+  # wait.
+  #
+  def join_nowait(*threads)
+    threads.flatten!
+    @threads.concat threads
+    for th in threads
+      Thread.start(th) do |t|
+        begin
+          t.join
+        ensure
+          @wait_queue.push t
+        end
+      end
+    end
+  end
+
+  #
+  # Waits until any of the specified threads has terminated, and returns the one
+  # that does.
+  #
+  # If there is no thread to wait, raises +ErrNoWaitingThread+.  If +nonblock+
+  # is true, and there is no terminated thread, raises +ErrNoFinishedThread+.
+  #
+  def next_wait(nonblock = nil)
+    ThreadsWait.fail ErrNoWaitingThread if @threads.empty?
+    begin
+      @threads.delete(th = @wait_queue.pop(nonblock))
+      th
+    rescue ThreadError
+      ThreadsWait.fail ErrNoFinishedThread
+    end
+  end
+
+  #
+  # Waits until all of the specified threads are terminated.  If a block is
+  # supplied for the method, it is executed for each thread termination.
+  #
+  # Raises exceptions in the same manner as +next_wait+.
+  #
+  def all_waits
+    until @threads.empty?
+      th = next_wait
+      yield th if block_given?
+    end
+  end
+end
+
+##
+# An alias for ThreadsWait from thwait.rb
+
+ThWait = ThreadsWait
+
+# Documentation comments:
+#  - Source of documentation is evenly split between Nutshell, existing
+#    comments, and my own rephrasing.
+#  - I'm not particularly confident that the comments are all exactly correct.