implement primitive and bare minimum gemtext parsing

gem300.rkt

@@ -3,6 +3,83 @@
 (require openssl)
 (require net/url)
 
+(struct text (str))
+(struct link (url str))
+(struct preformatted (str))
+
+(define (parse-url line)
+  (let ([split (string-split (substring line 2))])
+    ;; there are necessarily at least two elements
+    ;; in the split (the => and the URL) otherwise
+    ;; its just text.
+    (if (>= (length split) 2)
+        (link (second split)
+              ;; if there are at least three elements in the split,
+              ;; that means that everything after the URL is text.
+              (if (>= (length split) 3)
+                  (string-join (drop split 2))
+                  #f))
+        (text line))))
+
+
+(define (gemtext-parse lines)
+  (define (iter document lines state)
+    ;; when there are no more lines, we have finished parsing.
+    (if (empty? lines)
+        document
+
+        ;; otherwise, we have a state machine to traverse.
+        (cond
+          ;; turn off preformatted mode
+          [(and (symbol=? 'preformatted state)
+                (or (empty? (cdr lines))
+                    (string-prefix? (car lines) "```")))
+
+           (iter document
+                 (cdr lines)
+                 'normal)]
+
+          ;; add line to most recent preformat block
+          [(symbol=? 'preformatted state)
+           (if (preformatted? (car document))
+               ;; little bit hard to follow: take the existing
+               ;; preformatted blocks content, cons the new line;
+               ;; then, cons the new preformatted block to the cdr of
+               ;; the document
+               (iter (cons (preformatted (cons (car lines)
+                                               (preformatted-str (car document))))
+                           (cdr document))
+                     (cdr lines)
+                     'preformatted)
+
+               (iter (cons (preformatted (list)) document)
+                     (cdr lines)
+                     'preformatted))]
+
+          ;; rest of this is normal mode
+
+          ;; link lines
+          [(string-prefix? (car lines) "=>")
+           (iter (cons (parse-url (car lines))
+                       document)
+                 (cdr lines)
+                 'normal)]
+
+          ;; preformatting toggle lines
+          [(string-prefix? (car lines) "```")
+           ;; add preformatted block to document and toggle mode
+           (iter document
+                 (cdr lines)
+                 'preformatted)]
+
+          [else
+           (iter (cons (text (car lines))
+                       document)
+                 (cdr lines)
+                 'normal)])))
+
+  (displayln (iter (list) lines 'normal)))
+
 (define (request url-str)
   (define url (string->url url-str))
   (define-values (c-in c-out)