[rrd4j] Improved the internal data structure (openhab#16389)

Signed-off-by: Jörg Sautter <joerg.sautter@gmx.net>
Signed-off-by: Jørgen Austvik <jaustvik@acm.org>

bundles/org.openhab.persistence.rrd4j/src/main/java/org/openhab/persistence/rrd4j/internal/RRD4jPersistenceService.java

@@ -25,6 +25,7 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentSkipListMap;
@@ -102,6 +103,15 @@
         QueryablePersistenceService.class }, configurationPid = "org.openhab.rrd4j", configurationPolicy = ConfigurationPolicy.OPTIONAL)
 public class RRD4jPersistenceService implements QueryablePersistenceService {
 
+    private record Key(long timestamp, String name) implements Comparable<Key> {
+        @Override
+        public int compareTo(Key other) {
+            int c = Long.compare(timestamp, other.timestamp);
+
+            return (c == 0) ? Objects.compare(name, other.name, String::compareTo) : c;
+        }
+    }
+
     public static final String SERVICE_ID = "rrd4j";
 
     private static final String DEFAULT_OTHER = "default_other";
@@ -116,7 +126,7 @@ public class RRD4jPersistenceService implements QueryablePersistenceService {
 
     private final Map<String, RrdDefConfig> rrdDefs = new ConcurrentHashMap<>();
 
-    private final ConcurrentSkipListMap<Long, Map<String, Double>> storageMap = new ConcurrentSkipListMap<>();
+    private final ConcurrentSkipListMap<Key, Double> storageMap = new ConcurrentSkipListMap<>(Key::compareTo);
 
     private static final String DATASOURCE_STATE = "state";
 
@@ -318,7 +328,7 @@ public void store(final Item item, @Nullable final String alias) {
         }
 
         long now = System.currentTimeMillis() / 1000;
-        Double oldValue = storageMap.computeIfAbsent(now, t -> new ConcurrentHashMap<>()).put(name, value);
+        Double oldValue = storageMap.put(new Key(now, name), value);
         if (oldValue != null && !oldValue.equals(value)) {
             logger.debug(
                     "Discarding value {} for item {} with timestamp {} because a new value ({}) arrived with the same timestamp.",
@@ -327,14 +337,14 @@ public void store(final Item item, @Nullable final String alias) {
     }
 
     private void doStore(boolean force) {
+        long now = System.currentTimeMillis() / 1000;
         while (!storageMap.isEmpty()) {
-            long timestamp = storageMap.firstKey();
-            long now = System.currentTimeMillis() / 1000;
-            if (now > timestamp || force) {
+            Key key = storageMap.firstKey();
+            if (now > key.timestamp || force) {
                 // no new elements can be added for this timestamp because we are already past that time or the service
                 // requires forced storing
-                Map<String, Double> values = storageMap.pollFirstEntry().getValue();
-                values.forEach((name, value) -> writePointToDatabase(name, value, timestamp));
+                Double value = storageMap.pollFirstEntry().getValue();
+                writePointToDatabase(key.name, value, key.timestamp);
             } else {
                 return;
             }