Data commit

Task/ISBN13-check-digit/Red/isbn13-check-digit.red

@@ -0,0 +1,23 @@
+check_valid_isbn13: function [str] [
+  is_digit: charset [#"0" - #"9"]
+  remove-each i str [not pick is_digit i] ; remove non-digits
+
+  either 13 = length? str [               ; reject strings of incorrect length
+    sum: 0
+    repeat i 13 [
+      mul: either even? i [3] [1]         ; multiplier for odd/even digits
+      sum: sum + (mul * to integer! to string! pick str i)
+    ]
+
+    zero? (sum % 10)                      ; check if remainder mod 10 is zero
+  ] [
+    false
+  ]
+]
+
+; check given examples
+foreach [str] ["978-1734314502" "978-1734314509" "978-1788399081" "978-1788399083"] [
+  prin str
+  prin " - "
+  print check_valid_isbn13 str
+]