/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */

package org.elasticsearch.index;

import org.apache.lucene.index.ConcurrentMergeScheduler;
import org.elasticsearch.common.settings.Setting;
import org.elasticsearch.common.settings.Setting.Property;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.util.concurrent.EsExecutors;

/**
 * The merge scheduler (<code>ConcurrentMergeScheduler</code>) controls the execution of
 * merge operations once they are needed (according to the merge policy).  Merges
 * run in separate threads, and when the maximum number of threads is reached,
 * further merges will wait until a merge thread becomes available.
 *
 * <p>The merge scheduler supports the following <b>dynamic</b> settings:
 *
 * <ul>
 * <li> <code>index.merge.scheduler.max_thread_count</code>:
 *
 *     The maximum number of threads that may be merging at once. Defaults to
 *     <code>Math.max(1, Math.min(4, {@link EsExecutors#allocatedProcessors(Settings)} / 2))</code>
 *     which works well for a good solid-state-disk (SSD).  If your index is on
 *     spinning platter drives instead, decrease this to 1.
 *
 * <li><code>index.merge.scheduler.auto_throttle</code>:
 *
 *     If this is true (the default), then the merge scheduler will rate-limit IO
 *     (writes) for merges to an adaptive value depending on how many merges are
 *     requested over time.  An application with a low indexing rate that
 *     unluckily suddenly requires a large merge will see that merge aggressively
 *     throttled, while an application doing heavy indexing will see the throttle
 *     move higher to allow merges to keep up with ongoing indexing.
 * </ul>
 */
public final class MergeSchedulerConfig {

    public static final Setting<Integer> MAX_THREAD_COUNT_SETTING = new Setting<>(
        "index.merge.scheduler.max_thread_count",
        (s) -> Integer.toString(Math.max(1, Math.min(4, EsExecutors.allocatedProcessors(s) / 2))),
        (s) -> Setting.parseInt(s, 1, "index.merge.scheduler.max_thread_count"),
        Property.Dynamic,
        Property.IndexScope
    );
    public static final Setting<Integer> MAX_MERGE_COUNT_SETTING = new Setting<>(
        "index.merge.scheduler.max_merge_count",
        (s) -> Integer.toString(MAX_THREAD_COUNT_SETTING.get(s) + 5),
        (s) -> Setting.parseInt(s, 1, "index.merge.scheduler.max_merge_count"),
        Property.Dynamic,
        Property.IndexScope
    );
    public static final Setting<Boolean> AUTO_THROTTLE_SETTING = Setting.boolSetting(
        "index.merge.scheduler.auto_throttle",
        true,
        Property.Dynamic,
        Property.IndexScope
    );

    private volatile boolean autoThrottle;
    private volatile int maxThreadCount;
    private volatile int maxMergeCount;

    MergeSchedulerConfig(IndexSettings indexSettings) {
        int maxThread = indexSettings.getValue(MAX_THREAD_COUNT_SETTING);
        int maxMerge = indexSettings.getValue(MAX_MERGE_COUNT_SETTING);
        setMaxThreadAndMergeCount(maxThread, maxMerge);
        this.autoThrottle = indexSettings.getValue(AUTO_THROTTLE_SETTING);
    }

    /**
     * Returns <code>true</code> iff auto throttle is enabled.
     *
     * @see ConcurrentMergeScheduler#enableAutoIOThrottle()
     */
    public boolean isAutoThrottle() {
        return autoThrottle;
    }

    /**
     * Enables / disables auto throttling on the {@link ConcurrentMergeScheduler}
     */
    void setAutoThrottle(boolean autoThrottle) {
        this.autoThrottle = autoThrottle;
    }

    /**
     * Returns {@code maxThreadCount}.
     */
    public int getMaxThreadCount() {
        return maxThreadCount;
    }

    /**
     * Expert: directly set the maximum number of merge threads and
     * simultaneous merges allowed.
     */
    void setMaxThreadAndMergeCount(int maxThreadCount, int maxMergeCount) {
        if (maxThreadCount < 1) {
            throw new IllegalArgumentException("maxThreadCount should be at least 1");
        }
        if (maxMergeCount < 1) {
            throw new IllegalArgumentException("maxMergeCount should be at least 1");
        }
        if (maxThreadCount > maxMergeCount) {
            throw new IllegalArgumentException(
                "maxThreadCount (= " + maxThreadCount + ") should be <= maxMergeCount (= " + maxMergeCount + ")"
            );
        }
        this.maxThreadCount = maxThreadCount;
        this.maxMergeCount = maxMergeCount;
    }

    /**
     * Returns {@code maxMergeCount}.
     */
    public int getMaxMergeCount() {
        return maxMergeCount;
    }
}
