"Semaphore.jist"

package: jist.process; "includes Process, Processor, NativeMethodFailure"

imports: jist.language.*; "includes Object, Boolean, SmallInteger, ..."
imports: jist.utility.OrderedCollection; "include a specific class"

public class Semaphore extends: Object
[
	"instance variables"
	protected SmallInteger signalCount := 0.
	protected OrderedCollection waitingProcesses := OrderedCollection new.

	private native signalNatively throws NativeMethodFailure.
	private native waitNatively   throws NativeMethodFailure.

	"Answer whether there have been more signals than waits."
	public hasSignals ^Boolean 
	[
		^( signalCount > 0 )
	]

	"Increment the receiver's signal count. If there are processes
	 waiting on the semaphore, resume the longest waiting. Upon exit,
	 interrupts are always enabled."
	public signal
	[
		Exception context
			"build the context"
			catch: NativeMethodFailure with:
			[	: exception is NativeMethodFailure !
				Process withoutInterruptsDo:
				[	( signalCount := signalCount + 1 ) < 1 ifTrue:
					[	Processor resume: waitingProcesses removeFirst
					]
				]
			];
			"try a block that might throw an exception"
			try: [ self signalNatively ].
	]

	"Force the current process to be suspended until the receiver
	 semaphore is signaled. Upon exit, interrupts are always enabled."
	public wait
	[
		Processor currentProcessIsRecursive ifTrue:
		[	self error: 'wait not allowed during call-back'
		].
		Exception context
			catch: NativeMethodFailure with:
			[	: exception is NativeMethodFailure !
				Process withoutInterruptsDo:
				[	( signalCount := signalCount - 1 ) < 0 ifTrue:
					[	waitingProcesses addLast: Processor currentProcess.
						Processor suspendCurrentProcess.
					]
				]
			];
			try: [ self waitNatively ].
	]
]