# frozen_string_literal: true

# Copyright, 2017, by Samuel G. D. Williams. <http://www.codeotaku.com>
# 
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
# 
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
# 
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.

module Async
	# A semaphore is used to control access to a common resource in a concurrent system. A useful way to think of a semaphore as used in the real-world systems is as a record of how many units of a particular resource are available, coupled with operations to adjust that record safely (i.e. to avoid race conditions) as units are required or become free, and, if necessary, wait until a unit of the resource becomes available.
	class Semaphore
		def initialize(limit = 1, parent: nil)
			@count = 0
			@limit = limit
			@waiting = []
			
			@parent = parent
		end
		
		# The current number of tasks that have acquired the semaphore.
		attr :count
		
		# The maximum number of tasks that can acquire the semaphore.
		attr :limit
		
		# The tasks waiting on this semaphore.
		attr :waiting
		
		# Allow setting the limit. This is useful for cases where the semaphore is used to limit the number of concurrent tasks, but the number of tasks is not known in advance or needs to be modified.
		#
		# On increasing the limit, some tasks may be immediately resumed. On decreasing the limit, some tasks may execute until the count is < than the limit. 
		#
		# @parameter limit [Integer] The new limit.
		def limit= limit
			difference = limit - @limit
			@limit = limit
			
			# We can't suspend 
			if difference > 0
				difference.times do
					break unless fiber = @waiting.shift
					
					fiber.resume
				end
			end
		end
		
		# Is the semaphore currently acquired?
		def empty?
			@count.zero?
		end
		
		# Whether trying to acquire this semaphore would block.
		def blocking?
			@count >= @limit
		end
		
		# Run an async task. Will wait until the semaphore is ready until spawning and running the task.
		def async(*arguments, parent: (@parent or Task.current), **options)
			wait
			
			parent.async(**options) do |task|
				@count += 1
				
				begin
					yield task, *arguments
				ensure
					self.release
				end
			end
		end
		
		# Acquire the semaphore, block if we are at the limit.
		# If no block is provided, you must call release manually.
		# @yield when the semaphore can be acquired
		# @return the result of the block if invoked
		def acquire
			wait
			
			@count += 1
			
			return unless block_given?
			
			begin
				return yield
			ensure
				self.release
			end
		end
		
		# Release the semaphore. Must match up with a corresponding call to `acquire`. Will release waiting fibers in FIFO order.
		def release
			@count -= 1
			
			while (@limit - @count) > 0 and fiber = @waiting.shift
				if fiber.alive?
					fiber.resume
				end
			end
		end
		
		private
		
		# Wait until the semaphore becomes available.
		def wait
			fiber = Fiber.current
			
			if blocking?
				@waiting << fiber
				Task.yield while blocking?
			end
		rescue Exception
			@waiting.delete(fiber)
			raise
		end
	end
end