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;;; Chickadee Game Toolkit
;;; Copyright © 2016, 2018 David Thompson <davet@gnu.org>
;;;
;;; Chickadee is free software: you can redistribute it and/or modify
;;; it under the terms of the GNU General Public License as published
;;; by the Free Software Foundation, either version 3 of the License,
;;; or (at your option) any later version.
;;;
;;; Chickadee is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;;; General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with this program. If not, see
;;; <http://www.gnu.org/licenses/>.
(define-module (chickadee)
#:export (run-game
abort-game))
;;;
;;; Error handling
;;;
(define (call-with-error-handling handler thunk)
(if handler
(let ((stack #f))
(catch #t
(lambda ()
(thunk)
#f)
(lambda (key . args)
(handler stack key args)
#t)
(lambda (key . args)
(set! stack (make-stack #t 3)))))
(begin
(thunk)
#f)))
(define-syntax-rule (with-error-handling handler body ...)
(call-with-error-handling handler (lambda () body ...)))
(define (default-error-handler stack key args)
(apply throw key args))
;;;
;;; Game loop kernel
;;;
(define game-loop-prompt-tag (make-prompt-tag 'game-loop))
(define (abort-game)
(abort-to-prompt game-loop-prompt-tag #f))
(define* (run-game #:key update render time error
(update-hz 60))
(let ((timestep (round (/ 1000 update-hz))))
(call-with-prompt game-loop-prompt-tag
(lambda ()
;; Catch SIGINT and kill the loop.
(sigaction SIGINT
(lambda (signum)
(abort-game)))
;; A simple analogy is that we are filling up a bucket
;; with water. When the bucket fills up to a marked
;; line, we dump it out. Our water is time, and each
;; time we dump the bucket we update the game. Updating
;; the game on a fixed timestep like this yields a
;; stable simulation.
(let loop ((previous-time (time))
(buffer 0))
(let* ((current-time (time))
(delta (- current-time previous-time)))
(let update-loop ((buffer (+ buffer delta)))
(if (>= buffer timestep)
;; Short-circuit the update loop if an error
;; occurred, and reset the current time to now in
;; order to discard the undefined amount of time
;; that was spent handling the error.
(if (with-error-handling error (update timestep))
(loop (time) 0)
(begin
(usleep 1)
(update-loop (- buffer timestep))))
(begin
;; We render upon every iteration of the loop, and
;; thus rendering is decoupled from updating.
;; It's possible to render multiple times before
;; an update is performed.
(if (with-error-handling error (render (/ buffer timestep)))
(loop (time) 0)
(loop current-time buffer))))))))
(lambda (cont callback)
#f))))
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