SOLR-17076: Optimize OrderedNodePlacementPlugin#getAllReplicasOnNode (

apache#2076)

* 1. Changed getAllReplicasOnNode to just return a copy of `allReplicas` field, which keeps track of all replicas. Instead of computing a new list every time.
2. Added a getAllReplicaCount method to avoid creating new list of replicas if only the count is required

* ./gradlew tidy

* minor refactoring

solr/core/src/java/org/apache/solr/cluster/placement/plugins/OrderedNodePlacementPlugin.java

@@ -404,20 +404,25 @@ public abstract static class WeightedNode implements Comparable<WeightedNode> {
     private final Node node;
     private final Map<String, Map<String, Set<Replica>>> replicas;
 
+    // a flattened list of all replicas, as computing from the map could be costly
+    private final Set<Replica> allReplicas;
+
     public WeightedNode(Node node) {
       this.node = node;
       this.replicas = new HashMap<>();
+      this.allReplicas = new HashSet<>();
     }
 
     public Node getNode() {
       return node;
     }
 
     public Set<Replica> getAllReplicasOnNode() {
-      return replicas.values().stream()
-          .flatMap(shard -> shard.values().stream())
-          .flatMap(Collection::stream)
-          .collect(Collectors.toSet());
+      return new HashSet<>(allReplicas);
+    }
+
+    public int getAllReplicaCount() {
+      return allReplicas.size();
     }
 
     public Set<String> getCollectionsOnNode() {
@@ -454,6 +459,7 @@ public boolean canAddReplica(Replica replica) {
     }
 
     private boolean addReplicaToInternalState(Replica replica) {
+      allReplicas.add(replica);
       return replicas
           .computeIfAbsent(replica.getShard().getCollection().getName(), k -> new HashMap<>())
           .computeIfAbsent(replica.getShard().getShardName(), k -> CollectionUtil.newHashSet(1))
@@ -508,6 +514,7 @@ public final void removeReplica(Replica replica) {
                 (shard, reps) -> {
                   if (reps.remove(replica)) {
                     hasReplica.set(true);
+                    allReplicas.remove(replica);
                   }
                   return reps.isEmpty() ? null : reps;
                 });

solr/core/src/java/org/apache/solr/cluster/placement/plugins/SimplePlacementFactory.java

@@ -129,7 +129,7 @@ public int calcRelevantWeightWithReplica(Replica replica) {
       int colReplicaCount =
           collectionReplicas.getOrDefault(replica.getShard().getCollection().getName(), 0);
       int shardReplicaCount = getReplicasForShardOnNode(replica.getShard()).size();
-      return getAllReplicasOnNode().size()
+      return getAllReplicaCount()
           + 1
           + colReplicaCount * SAME_COL_MULT
           + shardReplicaCount * SAME_SHARD_MULT;