1 files changed, 55 insertions, 0 deletions
diff --git a/ldp/precondition.scm b/ldp/precondition.scm
new file mode 100644
index 0000000..990193a
--- /dev/null
+++ b/ldp/precondition.scm
@@ -0,0 +1,55 @@
+(define-module (ldp precondition)
+  #:use-module (ldp etag)
+  #:use-module (web request)
+  #:use-module (oop goops))
+
+(define-class <precondition> ()
+  (if-match #:init-keyword #:if-match #:getter precondition-if-match)
+  (if-none-match #:init-keyword #:if-match #:getter precondition-if-none-match))
+
+(define (the-precondition x)
+  (unless (is-a? x <precondition>)
+    (scm-error 'wrong-type-arg
+	       "the-precondition"
+	       "Expected a precondition."
+	       '()
+	       (list x)))
+  x)
+
+(define (the-string x)
+  (unless (string? x)
+    (scm-error 'wrong-type-arg
+	       "the-string"
+	       "Expected a string."
+	       '()
+	       (list x)))
+  x)
+
+(define-public (make-precondition if-match if-none-match)
+  (unless if-match
+    (set! if-match '("*")))
+  (unless if-none-match
+    (set! if-none-match '()))
+  (set! if-match (map the-string if-match))
+  (set! if-none-match (map the-string if-none-match))
+  (make <precondition>
+    #:if-match if-match
+    #:if-none-match if-none-match))
+
+(define-public (request->precondition request)
+  (make-precondition
+   (request-if-match request)
+   (request-if-none-match request)))
+
+(define-public (precondition-valid? x etag)
+  (define (check-matching list)
+    (and (not (null? list))
+	 (or (string=? etag (car list))
+	     (string=? (car list) "*")
+	     (check-matching (cdr list)))))
+  (define (check-non-matching list)
+    (or (null? list)
+	(and (not (string=? etag (car list)))
+	     (check-non-matching (cdr list)))))
+  (and (check-matching (precondition-if-match x))
+       (check-non-matching (precondition-if-none-match x))))