[PATCH] (issue1803) Use MD5 instead of a homegrown hashing algorithm

[Wald Commits]

# HG changeset patch
# User Andre Heinecke <andre.heinecke at intevation.de>
# Date 1427989218 -7200
# Node ID af415396d9cad17b7fbbfc1541df51eef90bf96d
# Parent  3531f0cee5e10dd435064a7b2ae59aad00f205a4
(issue1803) Use MD5 instead of a homegrown hashing algorithm

    For creating a digest of the parametrization we should use an
    algorithm that does not create collisions if there are small changes
    in the parametrization so that wrong results are returned.

diff -r 3531f0cee5e1 -r af415396d9ca artifacts/src/main/java/org/dive4elements/river/artifacts/D4EArtifact.java
--- a/artifacts/src/main/java/org/dive4elements/river/artifacts/D4EArtifact.java	Thu Apr 02 17:17:51 2015 +0200
+++ b/artifacts/src/main/java/org/dive4elements/river/artifacts/D4EArtifact.java	Thu Apr 02 17:40:18 2015 +0200
@@ -18,7 +18,14 @@
 import java.util.Set;
 import java.util.TreeMap;
 
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.io.ByteArrayOutputStream;
+import java.io.ObjectOutputStream;
+import java.io.IOException;
+
 import javax.xml.xpath.XPathConstants;
+import javax.xml.bind.DatatypeConverter;
 
 import net.sf.ehcache.Cache;
 
@@ -1135,20 +1142,21 @@
      */
     @Override
     public String hash() {
-
-        long hash  = 0L;
-        int  shift = 3;
-
-        for (Map.Entry<String, StateData> entry: data.entrySet()) {
-            String key   = entry.getKey();
-            Object value = entry.getValue().getValue();
-
-            hash ^= ((long)key.hashCode() << shift)
-                 |  ((long)value.hashCode() << (shift + 3));
-            shift += 2;
+        try {
+            ByteArrayOutputStream ba = new ByteArrayOutputStream();
+            ObjectOutputStream oa = new ObjectOutputStream(ba);
+            for (Map.Entry<String, StateData> entry: data.entrySet()) {
+                oa.writeObject(entry.getKey());
+                oa.writeObject(entry.getValue().getValue());
+            }
+            MessageDigest md = MessageDigest.getInstance("MD5");
+            byte[] md5sum = md.digest(ba.toByteArray());
+            return DatatypeConverter.printBase64Binary(md5sum);
+        } catch (NoSuchAlgorithmException e) {
+            throw new RuntimeException("MD5 unavailable. Can't happen.");
+        } catch (IOException e) {
+            throw new RuntimeException("Cant write parameter. Can't happen.");
         }
-
-        return getCurrentStateId() + hash;
     }
 
 
@@ -1509,7 +1517,7 @@
             if (cache != null) {
                 net.sf.ehcache.Element element = cache.get(key);
                 if (element != null) {
-                    log.debug("Got computation result from cache.");
+                    log.debug("Got computation result from cache for key: " + key);
                     old = element.getValue();
                 }
             }