[GitHub] [sling-org-apache-sling-graphql-core] raducotescu commented on a change in pull request #25: SLING-10502 - lazy loading helpers

[GitBox <gi...@apache.org>]

raducotescu commented on a change in pull request #25:
URL: https://github.com/apache/sling-org-apache-sling-graphql-core/pull/25#discussion_r657775764



##########
File path: src/main/java/org/apache/sling/graphql/helpers/layzloading/LazyLoadingMap.java
##########
@@ -0,0 +1,176 @@
+/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ ~ Licensed to the Apache Software Foundation (ASF) under one
+ ~ or more contributor license agreements.  See the NOTICE file
+ ~ distributed with this work for additional information
+ ~ regarding copyright ownership.  The ASF licenses this file
+ ~ to you under the Apache License, Version 2.0 (the
+ ~ "License"); you may not use this file except in compliance
+ ~ with the License.  You may obtain a copy of the License at
+ ~
+ ~   http://www.apache.org/licenses/LICENSE-2.0
+ ~
+ ~ Unless required by applicable law or agreed to in writing,
+ ~ software distributed under the License is distributed on an
+ ~ "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ ~ KIND, either express or implied.  See the License for the
+ ~ specific language governing permissions and limitations
+ ~ under the License.
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
+
+package org.apache.sling.graphql.helpers.layzloading;
+
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.Supplier;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/** A {@link java.util.HashMap} that optionally uses Suppliers to provide its
+ *  values. Each Supplier is called at most once, if the corresponding
+ *  value is requested. Some "global" operations requires all values
+ *  to be computed, and should be considered costly.
+ * 
+ *  Like HashMap, this class is NOT thread safe. If needed, 
+ *  {@link java.util.Collections#synchronizedMap} can be used
+ *  to sychronize it.
+  */
+public class LazyLoadingMap<K, T> extends HashMap<K, T> {
+
+    private final Logger log = LoggerFactory.getLogger(getClass());
+    private final Map<K, Supplier<T>> suppliers = new HashMap<>();
+
+    public class Stats {
+        int suppliersCallCount;
+
+        public int getSuppliersCallCount() {
+            return suppliersCallCount;
+        }
+
+        public int getUnusedSuppliersCount() {
+            return suppliers.size();
+        }
+    }
+
+    private final Stats stats = new Stats();
+
+    /** Calls computeAll - should be avoided if possible */
+    @Override
+    public boolean equals(Object o) {
+        if(!(o instanceof LazyLoadingMap)) {
+            return false;
+        }
+        final LazyLoadingMap<?,?> other = (LazyLoadingMap<?,?>)o;
+
+        // Equality seems complicated to compute without this
+        computeAll();
+        return super.equals(other);
+    }
+
+    @Override
+    public int hashCode() {
+        return super.hashCode() + suppliers.hashCode();
+    }
+
+    /** Adds a Supplier that provides a lazy loaded value.
+     *  Removes existing value with the same key if it exists.
+     */
+    public Supplier<T> put(K key, Supplier<T> supplier) {
+        super.remove(key);
+        return suppliers.put(key, supplier);
+    }
+
+    @Override
+    @SuppressWarnings("unchecked")
+    public T get(Object key) {
+        computeIfAbsent((K)key, k -> {
+            final Supplier<T> s = suppliers.remove(k);
+            if(s != null) {
+                stats.suppliersCallCount++;
+                return s.get();
+            }
+            return null;
+        });
+        return super.get(key);
+    }
+
+    /** Contrary to the usual Map contract, this always
+     *  returns null, to avoid calling a supplier "for nothing".
+     *  If the value is needed, call {@link #get} first.
+     */
+    @Override
+    public T remove(Object key) {
+        super.remove(key);
+        suppliers.remove(key);

Review comment:
       Would it be too expensive to obey the `Map` contract in this case?




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