Simplify the parsing of an OCapN address

goblins/ocapn/captp-test.rkt

@@ -177,7 +177,7 @@
 (define m2-location
   (string->ocapn-machine "ocapn://m2.fake"))
 (define m3-location
-  (string->ocapn-machine "ocapn://.m3.fake"))
+  (string->ocapn-machine "ocapn://m3.fake"))
 
 (define m1-fakenet
   (m1-run (spawn ^fake-netlayer "m1" network m1-incoming)))

goblins/ocapn/structs-urls.rkt

@@ -18,6 +18,7 @@
          "../utils/url-base64.rkt"
          "define-recordable-struct.rkt"
          racket/string
+         racket/list
          racket/match
          racket/contract
          syrup)
@@ -171,28 +172,17 @@
 (define (error-not-an-ocapn-url something-else)
   (error 'not-an-ocapn-url "Not an OCapN URL: ~a" something-else))
 
-(define (string-rsplit-once str find-char)
-  (define (list-rsplit-once lst elem-to-find match-func)
-    (for/fold ([left-side '()]
-               [right-side '()])
-              ([elem lst])
-      (if (match-func elem elem-to-find)
-          (values (append left-side
-                          (if (null? left-side) '() (list elem))
-                          right-side)
-                  '())
-          (values left-side (append right-side (list elem))))))
-
-  (let*-values ([(chars) (string->list str)]
-                [(left-side right-side) (list-rsplit-once chars find-char char=?)])
-    (values (list->string left-side)
-            (list->string right-side))))
+(define (url-host->transport-parts url-host)
+  (let* ((url-host-length (string-length url-host))
+         (transport-str (last (string-split url-host ".")))
+         (address (substring url-host 0 (- url-host-length (string-length transport-str) 1))))
+    (values (string->symbol transport-str) address)))
 
 (define/contract (url->ocapn-machine ocapn-url)
   (-> url? ocapn-machine?)
   (let*-values ([(host) (url-host ocapn-url)]
-                [(address transport) (string-rsplit-once host #\.)])
-    (ocapn-machine (string->symbol transport) address #f)))
+                [(transport address) (url-host->transport-parts host)])
+    (ocapn-machine transport address #f)))
 
 (define (url->ocapn-struct ocapn-url)
   (unless (and (string? (url-scheme ocapn-url))