feature tags are byte strings

fontland/fontland/unsafe/harfbuzz-helper.rkt

@@ -13,9 +13,7 @@
     HB_DIRECTION_BTT = 7))
 
 (define (->tag bstr)
-  (define bs (bytes->list bstr))
-  (for/sum ([(b i) (in-indexed (reverse bs))])
-    (* b (expt 2 (* i 8)))))
+  (integer-bytes->integer bstr #f #;(= unsigned) 'big-endian))
 
 (define hb-script-values
   `(HB_SCRIPT_COMMON = ,(->tag #"Zyyy")

fontland/fontland/unsafe/harfbuzz.rkt

@@ -182,18 +182,16 @@
 
 (define HB_FEATURE_GLOBAL_START 0)
 (define HB_FEATURE_GLOBAL_END 4294967295)
-(define (tag->hb-feature tag)
-  (define str (symbol->string tag))
-  (define bs (string->bytes/utf-8 str))
-  (unless (= (bytes-length bs) (string-length str))
-    (error 'invalid-tag-char))
+(define (tag->hb-feature bs)
+  (unless (and (bytes? bs) (= (bytes-length bs) 4))
+    (raise-argument-error 'tag->hb-feature "4 bytes" bs))
   (hb_feature_from_string bs))
   
-(define liga_on (tag->hb-feature 'liga))
+(define liga_on (tag->hb-feature #"liga"))
 (define liga_off (make-hb_feature_t (->tag #"liga") 0 0 4294967295))
-(define kern_on (tag->hb-feature 'kern))
+(define kern_on (tag->hb-feature #"kern"))
 (define kern_off (make-hb_feature_t (->tag #"kern") 0 0 4294967295))
-(define onum_on (tag->hb-feature 'onum))
+(define onum_on (tag->hb-feature #"onum"))
 (define onum_off (make-hb_feature_t (->tag #"onum") 0 0 4294967295))
 
 (define (shape font text [feats null])