LightSwitch.cs

using System;

namespace ConcurrencyUtilities
{
	/// <summary>
	/// A lightswitch can be used to give multiple threads shared access to a single permisson. It works like a light switch in a room -- the first person to enter the room turns the light on, and the last person to leave turns the light off. Here, the light is a metaphor for a permisson. When the light is on, any thread using this light switch has (the) permission. When the light switch is off, that permission is free to be acquired by another thread
	/// </summary>
	// Status: complete, test complete, marked off
	public class LightSwitch
	{
		Semaphore _managedPermisson; // The semaphore that the lightswitch will control permisson with
		int _numThreads = 0;
		Mutex _accessToNumThreads;

		/// <summary>
		/// Initialise the light switch -- you need to supply it with a semaphore
		/// </summary>
		/// <param name="permissonToManage">The permission semaphore that the light switch is to manage.</param>
		public LightSwitch(Semaphore permissonToManage) {
			_managedPermisson = permissonToManage;
			_accessToNumThreads = new Mutex();
		}

		/// <summary>
		/// Start using the lightswitch's permisson in the current thread. If there was no threads using the lightswitch-managed permisson, take the permisson (acquire a token from the semaphore). If there were already threads, we already have permission.
		/// In other words:
		/// Enter the room. If there was no one in the room, turn on the light.
		/// </summary>
		public void Acquire() {
			_accessToNumThreads.Acquire();
				if (_numThreads == 0) // Check if there was no one in the room
					_managedPermisson.Acquire();
				_numThreads++;
			_accessToNumThreads.Release();
		}

		/// <summary>
		/// Stop using the lightswitch's permisson in the current thread. If there are no other threads using the lightswitch-managed permisson, release the permisson (release a token back to the semaphore). If there are still other threads using the permisson, leave the permission still acquired.
		/// In other words:
		/// Leave the room. If there is now no one left in the room, turn off the light.
		/// </summary>
		public void Release() {
			_accessToNumThreads.Acquire();
				_numThreads--;
				if (_numThreads == 0) // Check if there is now no one in the room
					_managedPermisson.Release();
			_accessToNumThreads.Release();
		}
	}
}