/*
 * 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 it.cavallium.dbengine.lucene;

import java.util.Objects;
import java.util.concurrent.atomic.LongAccumulator;

/**
 * Maintains the maximum score and its corresponding document id concurrently
 *
 * This class must mirror this changes:
 * <a href="https://github.com/apache/lucene/commits/94b66c0ed279fe23656d451fecd56fdfd106e1ea/lucene/core/src/java/org/apache/lucene/search/MaxScoreAccumulator.java">
 *   Lucene MaxScoreAccumulator changes on GitHub</a>
 */
public final class MaxScoreAccumulator {
	// we use 2^10-1 to check the remainder with a bitwise operation
	static final int DEFAULT_INTERVAL = 0x3ff;

	// scores are always positive
	final LongAccumulator acc = new LongAccumulator(MaxScoreAccumulator::maxEncode, Long.MIN_VALUE);

	// non-final and visible for tests
	public long modInterval;

	public MaxScoreAccumulator() {
		this.modInterval = DEFAULT_INTERVAL;
	}

	/**
	 * Return the max encoded DocAndScore in a way that is consistent with {@link
	 * DocAndScore#compareTo}.
	 */
	private static long maxEncode(long v1, long v2) {
		float score1 = Float.intBitsToFloat((int) (v1 >> 32));
		float score2 = Float.intBitsToFloat((int) (v2 >> 32));
		int cmp = Float.compare(score1, score2);
		if (cmp == 0) {
			// tie-break on the minimum doc base
			return (int) v1 < (int) v2 ? v1 : v2;
		} else if (cmp > 0) {
			return v1;
		}
		return v2;
	}

	public void accumulate(int docBase, float score) {
		assert docBase >= 0 && score >= 0;
		long encode = (((long) Float.floatToIntBits(score)) << 32) | docBase;
		acc.accumulate(encode);
	}

	public DocAndScore get() {
		long value = acc.get();
		if (value == Long.MIN_VALUE) {
			return null;
		}
		float score = Float.intBitsToFloat((int) (value >> 32));
		int docBase = (int) value;
		return new DocAndScore(docBase, score);
	}

	public static class DocAndScore implements Comparable<DocAndScore> {
		public final int docBase;
		public final float score;

		public DocAndScore(int docBase, float score) {
			this.docBase = docBase;
			this.score = score;
		}

		@Override
		public int compareTo(DocAndScore o) {
			int cmp = Float.compare(score, o.score);
			if (cmp == 0) {
				// tie-break on the minimum doc base
				// For a given minimum competitive score, we want to know the first segment
				// where this score occurred, hence the reverse order here.
				// On segments with a lower docBase, any document whose score is greater
				// than or equal to this score would be competitive, while on segments with a
				// higher docBase, documents need to have a strictly greater score to be
				// competitive since we tie break on doc ID.
				return Integer.compare(o.docBase, docBase);
			}
			return cmp;
		}

		@Override
		public boolean equals(Object o) {
			if (this == o) return true;
			if (o == null || getClass() != o.getClass()) return false;
			DocAndScore result = (DocAndScore) o;
			return docBase == result.docBase && Float.compare(result.score, score) == 0;
		}

		@Override
		public int hashCode() {
			return Objects.hash(docBase, score);
		}

		@Override
		public String toString() {
			return "DocAndScore{" + "docBase=" + docBase + ", score=" + score + '}';
		}
	}
}