Refactor HyperLogLogPlusPlus to separate algorithms and internal data…

… representation (#60104) (#60109)

server/src/main/java/org/elasticsearch/search/aggregations/metrics/AbstractCardinalityAlgorithm.java

@@ -0,0 +1,53 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch 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.elasticsearch.search.aggregations.metrics;
+
+/**
+ * Base class for cardinality related algorithms.
+ */
+abstract class AbstractCardinalityAlgorithm {
+
+    public static final int MIN_PRECISION = 4;
+    public static final int MAX_PRECISION = 18;
+
+    protected final int p;
+
+    AbstractCardinalityAlgorithm(int precision) {
+        if (precision < MIN_PRECISION) {
+            throw new IllegalArgumentException("precision must be >= 4");
+        }
+        if (precision > MAX_PRECISION) {
+            throw new IllegalArgumentException("precision must be <= 18");
+        }
+        p = precision;
+    }
+
+    /** Precision of the algorithm */
+    public int precision() {
+        return p;
+    }
+
+    /** Returns the current computed cardinality */
+    public abstract long cardinality();
+
+    static long linearCounting(long m, long v) {
+        return Math.round(m * Math.log((double) m / v));
+    }
+}