/*
 * Copyright Elasticsearch B.V., and/or licensed to Elasticsearch B.V.
 * under one or more license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch B.V. 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.
 *
 * This file is based on a modification of https://github.com/open-telemetry/opentelemetry-java which is licensed under the Apache 2.0 License.
 */

package org.elasticsearch.exponentialhistogram;

import java.util.function.LongBinaryOperator;

/**
 * An iterator that merges two bucket iterators, aligning them to a common scale and combining buckets with the same index.
 */
final class MergingBucketIterator implements BucketIterator {

    private final BucketIterator itA;
    private final BucketIterator itB;

    private boolean endReached;
    private long currentIndex;
    private long currentCount;

    private final LongBinaryOperator countMergeOperator;

    /**
     * Creates a new merging iterator.
     *
     * @param itA         the first iterator to merge
     * @param itB         the second iterator to merge
     * @param targetScale the histogram scale to which both iterators should be aligned
     */
    MergingBucketIterator(BucketIterator itA, BucketIterator itB, int targetScale) {
        this(itA, itB, targetScale, Long::sum);
    }

    /**
     * Creates a new merging iterator, using the provided operator to merge the counts.
     * Note that the resulting count can be negative if the operator produces negative results.
     *
     * @param itA         the first iterator to merge
     * @param itB         the second iterator to merge
     * @param countMergeOperator the operator to use to merge counts of buckets with the same index
     * @param targetScale the histogram scale to which both iterators should be aligned
     */
    MergingBucketIterator(BucketIterator itA, BucketIterator itB, int targetScale, LongBinaryOperator countMergeOperator) {
        this.itA = new ScaleAdjustingBucketIterator(itA, targetScale);
        this.itB = new ScaleAdjustingBucketIterator(itB, targetScale);
        this.countMergeOperator = countMergeOperator;
        endReached = false;
        advance();
    }

    @Override
    public void advance() {
        boolean hasNextA = itA.hasNext();
        boolean hasNextB = itB.hasNext();
        endReached = hasNextA == false && hasNextB == false;
        if (endReached) {
            return;
        }
        long idxA = 0;
        long idxB = 0;
        if (hasNextA) {
            idxA = itA.peekIndex();
        }
        if (hasNextB) {
            idxB = itB.peekIndex();
        }

        boolean advanceA = hasNextA && (hasNextB == false || idxA <= idxB);
        boolean advanceB = hasNextB && (hasNextA == false || idxB <= idxA);
        long countA = 0;
        long countB = 0;
        if (advanceA) {
            currentIndex = idxA;
            countA = itA.peekCount();
            itA.advance();
        }
        if (advanceB) {
            currentIndex = idxB;
            countB = itB.peekCount();
            itB.advance();
        }
        currentCount = countMergeOperator.applyAsLong(countA, countB);
    }

    @Override
    public boolean hasNext() {
        return endReached == false;
    }

    @Override
    public long peekCount() {
        assertEndNotReached();
        return currentCount;
    }

    @Override
    public long peekIndex() {
        assertEndNotReached();
        return currentIndex;
    }

    @Override
    public int scale() {
        return itA.scale();
    }

    private void assertEndNotReached() {
        if (endReached) {
            throw new IllegalStateException("Iterator has no more buckets");
        }
    }

}