1 files changed, 19 insertions, 0 deletions
diff --git a/lib/util.php b/lib/util.php
index ed7e10f6b..bf7282858 100644
--- a/lib/util.php
+++ b/lib/util.php
@@ -1370,9 +1370,28 @@ function common_memcache()
     }
 }
 
+function common_license_terms($uri)
+{
+    if(preg_match('/creativecommons.org\/licenses\/([^\/]+)/', $uri, $matches)) {
+        return explode('-',$matches[1]);
+    }
+    return array($uri);
+}
+
 function common_compatible_license($from, $to)
 {
+    $from_terms = common_license_terms($from);
+    // public domain and cc-by are compatible with everything
+    if(count($from_terms) == 1 && ($from_terms[0] == 'publicdomain' || $from_terms[0] == 'by')) {
+        return true;
+    }
+    $to_terms = common_license_terms($to);
+    // sa is compatible across versions. IANAL
+    if(in_array('sa',$from_terms) || in_array('sa',$to_terms)) {
+        return count(array_diff($from_terms, $to_terms)) == 0;
+    }
     // XXX: better compatibility check needed here!
+    // Should at least normalise URIs
     return ($from == $to);
 }