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;;;; -*- Mode: Lisp; Syntax: ANSI-Common-Lisp; Base: 10; indent-tabs-mode: nil -*-
;;;; Copyright © 2009 Josh Marchan, Adlai Chandrasekhar
;;;; Simple Queues
;;; The functions in this file are dangerous. Good compilers will generate code that will
;;; do VERY funky shit when called incorrectly. Calls to these functions should be hidden
;;; behind safe code that never passes arguments of incorrect types.
;;; Unlike the standard queue implementation which you find in CL code (push to the tail of
;;; a list, pop from the head), these queues do not cons one bit. You do, however, need to
;;; "declare" a queue's size (at runtime) when you make one.
;;; This is the API required by src/channels.lisp:
;;; These don't need to be super-optimized:
;;; (queue-count queue) -- How many elements are present in a queue
;;; (queue-length queue) -- The maximum size of a queue
;;; (queue-peek queue) -- What will the next dequeued object be?
;;; This one matters for performance, but not THAT much:
;;; (make-queue size) -- Create and return a queue of given maximum size.
;;; These do matter THAT much:
;;; (queue-full-p queue) -- Is this queue full?
;;; (queue-empty-p queue) -- Is this queue empty?
;;; (enqueue object queue) -- Enqueue an object in the queue, return object.
;;; (dequeue queue) -- Dequeue queue, return dequeued object.
;;; All other functions in here should be marked as unsafe and apocalyptic.
(in-package :chanl)
;;; Queue Condition API
(define-condition queue-condition (error)
((queue :reader queue-condition-queue :initarg :queue))
(:report (lambda (c s)
(format s "Queue error in queue ~S"
(queue-condition-queue c)))))
(define-condition queue-length-error (queue-condition)
((attempted-length :reader queue-error-attempted-length :initarg :attempted-length))
(:report (lambda (c s)
(format s "Queue created with invalid length: ~S"
(queue-error-attempted-length c)))))
(define-condition queue-overflow-error (queue-condition)
((item :reader queue-overflow-extra-item :initarg :item))
(:report (lambda (c s)
(format s "Queue ~S is full, and can't have ~S stuffed into it"
(queue-condition-queue c) (queue-overflow-extra-item c)))))
(define-condition queue-underflow-error (queue-condition) ()
(:report (lambda (c s)
(format s "Queue ~S is empty, and can't be dequeued anymore"
(queue-condition-queue c)))))
(eval-when (:compile-toplevel)
(defvar queue-sentinel (make-symbol "EMPTY")))
(define-speedy-function %make-queue (length)
"Creates a new queue of maximum size LENGTH"
(when (typep length 'fixnum)
(locally (declare (fixnum length))
(when (plusp length)
(let ((queue (make-array (the fixnum (+ 2 length)))))
(setf (svref queue 1) 2
(svref queue 0) 2
(svref queue 2) '#.queue-sentinel)
(return-from %make-queue queue)))))
(error 'queue-length-error :attempted-length length))
;;; Do we need a compiler macro for the above when LENGTH is constant so that we
;;; don't add 2 at runtime? That's not very high on the priority list, although
;;; it'll probably take less time to write than this comment did. -- Adlai
(define-speedy-function %queue-length (queue)
"Returns QUEUE's maximum length"
(the fixnum (- (length (the simple-vector queue)) 2)))
(define-speedy-function queuep (x)
"If this returns NIL, X is not a queue"
(when (simple-vector-p x)
(let ((length (length x))
(head (svref x 0))
(tail (svref x 1)))
(and (typep head 'fixnum)
(typep tail 'fixnum)
(< 1 head length)
(< 1 tail length)))))
(define-speedy-function %queue-out (queue)
"QUEUE's exit pointer"
(the fixnum (svref queue 0)))
(define-speedy-function %queue-in (queue)
"QUEUE's entry pointer"
(the fixnum (svref queue 1)))
(define-speedy-function %queue-peek (queue)
"Dereference QUEUE's exit pointer"
(svref queue (%queue-out queue)))
(define-speedy-function %queue-zero-p (queue)
"Checks whether QUEUE's theoretical length is zero"
(= (the fixnum (%queue-in queue))
(the fixnum (%queue-out queue))))
(define-speedy-function %queue-empty-p (queue)
"Checks whether QUEUE is effectively empty"
(eq (svref queue (%queue-out queue)) '#.queue-sentinel))
(define-speedy-function %queue-full-p (queue)
"Checks whether QUEUE is effectively full"
;; We keep the exit reference around because we do two checks
(let ((out (%queue-out queue)))
(declare (fixnum out))
;; Are the entry and exit pointers the same?
(when (= out (the fixnum (%queue-in queue)))
;; Is there a real value at the exit pointer?
(not (eq (svref queue out) '#.queue-sentinel)))))
(define-speedy-function %queue-count (queue)
"Returns QUEUE's effective length"
;; We start with the 'raw' length -- the difference between the pointers
(let ((length (- (%queue-in queue) (%queue-out queue))))
(declare (fixnum length))
(cond ((plusp length) length) ; Raw length is OK
((or (minusp length) ; Entry pointer is before exit pointer,
(not (eq (%queue-peek queue) ; or the queue is full if the pointers
'#.queue-sentinel))) ; don't point to the sentinel value, so
(the fixnum
(+ length (%queue-length queue)))) ; Add the effective length
(t 0)))) ; Queue is empty -- return zero
(define-speedy-function %next-index (current-index queue-real-length)
(declare (fixnum current-index queue-real-length))
(let ((new-index (1+ current-index))) ; Simply increment the index
(declare (fixnum new-index))
(if (= new-index queue-real-length) 2 new-index))) ; Overflow to 2 if necessary
(define-speedy-function %enqueue (object queue &aux (in (%queue-in queue)))
(declare (fixnum in))
"Enqueue OBJECT and increment QUEUE's entry pointer"
(if (or (not (= in (the fixnum (%queue-out queue))))
(eq (svref queue in) '#.queue-sentinel))
(prog1 (setf (svref queue in) object)
(setf (svref queue 1) (%next-index in (length queue))))
(error 'queue-overflow-error :queue queue :item object)))
(define-speedy-function %dequeue (queue &aux (out (%queue-out queue)))
(declare (fixnum out))
"Sets QUEUE's tail to QUEUE, increments QUEUE's tail pointer, and returns the previous tail ref"
(let ((out-object (svref queue out)))
(if (eq out-object '#.queue-sentinel)
(error 'queue-underflow-error :queue queue)
(prog1 out-object
(setf (svref queue 0)
(if (= (the fixnum (incf out)) (the fixnum (length queue))) (setf out 2) out))
(when (= (the fixnum (%queue-in queue)) out)
(setf (svref queue out) '#.queue-sentinel))))))
;;; Now that all the backend functions are defined, we can define the API:
(defun make-queue (size)
"Makes a queue of maximum size SIZE"
(%make-queue size))
(defun queue-count (queue)
"Returns the current size of QUEUE"
(%queue-count queue))
(defun queue-length (queue)
"Returns the maximum size of QUEUE"
(%queue-length queue))
(defun queue-peek (queue)
(let ((peek (%queue-peek queue)))
(if (eq peek '#.queue-sentinel)
(values nil nil)
(values peek t))))
(defun queue-full-p (queue)
"Tests whether QUEUE is full"
(%queue-full-p queue))
(defun queue-empty-p (queue)
"Tests whether QUEUE is empty"
(%queue-empty-p queue))
(defun enqueue (object queue)
"Enqueues OBJECT in QUEUE"
(%enqueue object queue))
(defun dequeue (queue)
"Dequeues QUEUE"
(%dequeue queue))
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