1 files changed, 43 insertions, 0 deletions
diff --git a/ldp/etag.scm b/ldp/etag.scm
new file mode 100644
index 0000000..756766e
--- /dev/null
+++ b/ldp/etag.scm
@@ -0,0 +1,43 @@
+(define-module (ldp etag))
+
+(define alphabet
+  (string-join
+   '("abcdefghijklmnopqrstuvwxyz"
+     "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+     "0123456789"
+     "-_")
+   ""))
+
+(define (generate-etag-letter)
+  (string-ref alphabet (random (string-length alphabet))))
+
+(define (generate-etag-letters n)
+  (if (<= n 0)
+      '()
+      (cons (generate-etag-letter)
+	    (generate-etag-letters (- n 1)))))
+
+(define-public (generate-etag)
+  (list->string (generate-etag-letters 16)))
+
+(define-public (etag? x)
+  (define (aux i)
+    (or (>= i (string-length x))
+	(and (let ((c (string-ref x i)))
+	       (or (and (char>=? c #\a) (char<=? c #\z))
+		   (and (char>=? c #\A) (char<=? c #\Z))
+		   (and (char>=? c #\0) (char<=? c #\9))
+		   (char=? c #\-)
+		   (char=? c #\_)))
+	     (aux (+ i 1)))))
+  (and (string? x)
+       (aux 0)))
+
+(define-public (the-etag x)
+  (unless (etag? x)
+    (scm-error 'wrong-type-arg
+	       "the-etag"
+	       "Expected a string satisfying etag? from (ldp etag)."
+	       '()
+	       (list x)))
+  x)