(module (downstroke entity)
    *
  (import scheme
	      (chicken base)
	      (only srfi-1 fold alist-delete))

  ;; Entities = alists with shared keys (#:type, #:x, #:y, #:width, #:height, ...).

  (define (make-entity x y w h)
    `((#:type . none) (#:x . ,x) (#:y . ,y) (#:width . ,w) (#:height . ,h)))

  (define (entity-ref entity key #!optional default)
    (cond ((assq key entity) => cdr)
	      ((procedure? default) (default))
	      (else default)))

  (define (entity-type entity)
    (entity-ref entity #:type #f))

  (define (entity-set entity key val)
    (cons (cons key val) (alist-delete key entity eq?)))

  (define (entity-set-many entity pairs)
    (fold (lambda (pair working-ent)
	        (entity-set working-ent (car pair) (cdr pair)))
	      entity
	      pairs))

  (define (entity-update entity key proc #!optional default)
    (entity-set entity key (proc (entity-ref entity key default))))

  ;; #:skip-pipelines — list of symbols naming frame pipeline steps to skip for this
  ;; entity. Physics documents the built-in step names (see docs/physics.org). Other
  ;; subsystems (e.g. animation, rendering) may reserve additional symbols later and
  ;; use the same predicate and define-pipeline macro.

  (define (entity-skips-pipeline? entity step)
    (let ((skips (entity-ref entity #:skip-pipelines '())))
      (and (pair? skips) (memq step skips))))

  (define-syntax define-pipeline
    (syntax-rules ()
      ((define-pipeline (identifier name) (scene entity dt) :guard guard (body ...))
       (define (identifier scene entity dt)
         (if (or (not guard) (entity-skips-pipeline? entity (quote name)))
	         entity
	         (body ...))))
      ((define-pipeline (identifier name) (scene entity dt) (body ...))
       (define (identifier scene entity dt)
         (if (entity-skips-pipeline? entity (quote name))
	         entity
	         (body ...))))))

  ) ;; End of entity