Presence detector

pattern.rkt

@@ -7,7 +7,8 @@
 	 wildcard?
 	 specialization?
 	 ground?
-	 intersect)
+	 intersect
+	 intersect?)
 
 (struct exn:unification-failure ())
 (define unification-failure (exn:unification-failure))
@@ -87,3 +88,9 @@
   (if ok?
       (ks result)
       (kf)))
+
+;; Any Any -> Boolean
+(define (intersect? a b)
+  (with-handlers ([exn:unification-failure? (lambda (e) #f)])
+    (unify a b)
+    #t))

presence-detector.rkt

@@ -0,0 +1,28 @@
+#lang racket/base
+
+(require racket/set)
+(require "core.rkt")
+(require "pattern.rkt")
+
+(provide (except-out (struct-out presence-detector) presence-detector)
+	 (rename-out [make-presence-detector presence-detector])
+	 presence-detector-update
+	 presence-exists-for?)
+
+(struct presence-detector (route-set) #:transparent)
+
+(define (make-presence-detector [initial-routes '()])
+  (presence-detector (list->set initial-routes)))
+
+(define (presence-detector-update p rs)
+  (define old-route-set (presence-detector-route-set p))
+  (define new-route-set (list->set rs))
+  (values (struct-copy presence-detector p [route-set new-route-set])
+	  (set-subtract new-route-set old-route-set)
+	  (set-subtract old-route-set new-route-set)))
+
+(define (presence-exists-for? p probe-route)
+  (for/or ((existing-route (in-set (presence-detector-route-set p))))
+    (and (equal? (route-subscription? probe-route) (route-subscription? existing-route))
+	 (equal? (route-meta-level probe-route) (route-meta-level existing-route))
+	 (intersect? (route-pattern probe-route) (route-pattern existing-route)))))