consolidate font-features attributes (closes #45)

quad/qtest/test-ot-features.rkt

@@ -1,5 +1,6 @@
 #lang quadwriter
 
+
 '(q ((font-features "liga 0")) "No ligs: fifle")
 
 '(q ((break "para")))
@@ -12,7 +13,7 @@
 
 '(q ((break "para")))
 
-'(q ((font-features "liga 1")) (q ((font-features-adjust "liga 0")) "No ligs: fifle"))
+'(q ((font-features "liga 1")) (q ((font-features "+ liga 0")) "No ligs: fifle"))
 
 '(q ((break "para")))
 
@@ -20,4 +21,4 @@
 
 '(q ((break "para")))
 
-'(q ((font-features "zero 1")) (q ((font-features-adjust "liga 0")) "No ligs, slashed zero: fifle0"))
+'(q ((font-features "zero 1")) (q ((font-features "+ liga 0")) "No ligs, slashed zero: fifle0"))

quad/quad/scribblings/quad.scrbl

@@ -645,9 +645,8 @@ Whether the quad has bold styling applied. Activated only when value is @racket[
 Whether the quad has italic styling applied. Activated only when value is @racket["true"]. 
 }
 
-@deftogether[(@defthing[#:kind "attribute" font-features symbol?]
-              @defthing[#:kind "attribute" font-features-adjust symbol?])]{
-Two ways of setting OpenType layout features. @racket[font-features] takes a @deftech{feature string}, which is an alternating list of OT feature tags and values, separated by white space. For instance, @racket{liga 0 smcp 1} would deactivate the ligature feature and activate the small-cap feature. @racket[font-features-adjust] works the same way, but rather than replacing the current feature settings, it amends them.
+@defthing[#:kind "attribute" font-features symbol?]{
+Sets OpenType layout features. @racket[font-features] takes a @deftech{feature string}, which is an alternating list of OT feature tags and values, separated by white space. For instance, @racket{liga 0 smcp 1} would deactivate the ligature feature and activate the small-cap feature. If the feature string is prefixed with @racket["+"], rather than replacing the current feature settings, it amends the features that would otherwise apply.
 
 @margin-note{Fonts with OpenType layout features may be configured so that certain features, like ligatures, are activated by default. Your font will display these layout features even though there is no @racket[font-features] attribute in your Q-expression. You can, however, still turn them off with @racket[font-features].}
 }

quad/quadwriter/render.rkt

@@ -50,25 +50,44 @@
 
 
 (define (string->feature-list str)
-  (for/list ([kv (in-slice 2 (string-split str))])
-    (cons (string->bytes/utf-8 (first kv)) (string->number (second kv)))))
+  (define pcs (string-split str))
+  (unless (even? (length pcs))
+    (raise-argument-error 'string->feature-list "even number of tags and values" pcs))
+  (for/list ([kv (in-slice 2 pcs)])
+            (cons (match (first kv)
+                    [(? string? k) (string->bytes/utf-8 k)]
+                    [k (raise-argument-error 'string->feature-list "string" k)])
+                  (match (string->number (second kv))
+                    [(? number? num) num]
+                    [v (raise-argument-error 'string->feature-list "number string" v)]))))
 
 (define (parse-font-features! attrs)
   ;; `font-features` are OpenType font feature specifiers.
-  (match (hash-ref attrs :font-features-adjust #false)
-    [(? string? str)
-     ;; adjustment: parse the feature string and append to the current feature set
-     (define parsed-features (string->feature-list str))
-     (hash-update! attrs :font-features (λ (fs) (remove-duplicates (append parsed-features fs) equal? #:key car)))
-     ;; once adjustment is incorporated, delete it
-     (hash-set! attrs :font-features-adjust #false)]
-    [_ (match (hash-ref attrs :font-features #false)
-         ;; override: parse features & replace current set
-         [(? string? str)
-          (define parsed-features (string->feature-list str))
-          (hash-set! attrs :font-features parsed-features)]
-         [_ (void)])]))
-
+  (define font-features-previous-key 'font-features-previous)
+  (define features-previous (hash-ref attrs font-features-previous-key empty))
+  (define val (hash-ref attrs :font-features #false))
+  (when (string? val)
+    (hash-set! attrs :font-features
+               (cond
+                 [(regexp-match #px"^\\s*\\+ " val)
+                  ;; adjustment: parse the feature string and append to the previous feature set
+                  (define parsed-features (string->feature-list (string-trim (string-trim val) "+")))
+                  (remove-duplicates (append parsed-features features-previous) bytes=? #:key car)]
+                 ;; replacement of previous feature string
+                 [else (string->feature-list val)]))
+    (hash-set! attrs font-features-previous-key (hash-ref attrs :font-features))))
+
+(module+ test
+  (require rackunit)
+  ;; feature replacement
+  (define attrs (make-hash '((font-features-previous '((#"smcp" . 1))))))
+  (hash-set! attrs :font-features " liga 0 ")
+  (parse-font-features! attrs)
+  (check-equal? (hash-ref attrs :font-features) '((#"liga" . 0)))
+  ;; feature append
+  (hash-set! attrs :font-features "  + calt 1 ")
+  (parse-font-features! attrs)
+  (check-equal? (sort (hash-ref attrs :font-features) bytes<? #:key car) '((#"calt" . 1)(#"liga" . 0))))
 
 (define (parse-dimension-strings! attrs)
   ;; certain attributes can be "dimension strings", which are strings like "3in" or "4.2cm"