improve some errors

pitfall/xenomorph/private/pointer.rkt

@@ -71,6 +71,9 @@ https://github.com/mbutterick/restructure/blob/master/src/Pointer.coffee
                  
 
   (define/augment (encode port val [ctx #f])
+    (unless ctx
+      ;; todo: furnish default pointer context? adapt from Struct?
+      (raise-argument-error 'Pointer:encode "valid pointer context" ctx))
     (if (not val)
         (send offset-type encode port null-value)
         (let* ([parent ctx]

pitfall/xenomorph/private/string.rkt

@@ -57,8 +57,11 @@ https://github.com/mbutterick/restructure/blob/master/src/String.coffee
            [encoding (if (procedure? encoding)
                          (or (encoding (and parent (· parent val)) 'ascii))
                          encoding)])
+      (define encoded-length (byte-length val encoding))
+      (when (and (exact-nonnegative-integer? len) (> encoded-length len))
+        (raise-argument-error 'String:encode (format "string no longer than ~a" len) val)) 
       (when (NumberT? len)
-        (send len encode port (byte-length val encoding)))
+        (send len encode port encoded-length))
       (write-encoded-string port val encoding)
       (when (not len) (write-byte #x00 port)))) ; null terminated when no len
 
@@ -89,6 +92,9 @@ https://github.com/mbutterick/restructure/blob/master/src/String.coffee
 
 
 (test-module
+ (define S-fixed (+String 4 'utf8))
+ (check-equal? (encode S-fixed "Mike" #f) #"Mike")
+ (check-exn exn:fail? (λ () (encode S-fixed "Mikes" #f))) ; too long for fixed string 
  (define S (+String uint8 'utf8))
  (check-equal? (decode S #"\2BCDEF") "BC")
  (check-equal? (encode S "Mike" #f) #"\4Mike")

pitfall/xenomorph/private/struct.rkt

@@ -114,6 +114,11 @@ https://github.com/mbutterick/restructure/blob/master/src/Struct.coffee
     (unless (dict? val)
       (raise-argument-error 'Struct:encode "dict" val))
 
+    ;; check keys first, since `size` also relies on keys being valid
+    (unless (andmap (λ (key) (memq key (dict-keys val))) (dict-keys fields))
+      (raise-argument-error 'Struct:encode
+                            (format "dict that contains superset of Struct keys: ~a" (dict-keys fields)) (dict-keys val)))
+
     (define ctx (mhash 'pointers empty
                        'startOffset (pos port)
                        'parent parent
@@ -121,9 +126,6 @@ https://github.com/mbutterick/restructure/blob/master/src/Struct.coffee
                        'pointerSize 0))
     (ref-set! ctx 'pointerOffset (+ (pos port) (size val ctx #f)))
 
-    (unless (andmap (λ (key) (memq key (dict-keys val))) (dict-keys fields))
-      (raise-argument-error 'Struct:encode
-                            (format "dict that contains superset of Struct keys: ~a" (dict-keys fields)) (dict-keys val)))
     (for ([(key type) (in-dict fields)])
          (send type encode port (ref val key) ctx))
     (for ([ptr (in-list (· ctx pointers))])