/*
 * 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", the "GNU Affero General Public License v3.0 only", 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", the "GNU Affero General Public
 * License v3.0 only", or the "Server Side Public License, v 1".
 */

package org.elasticsearch.cluster.routing.allocation;

import org.elasticsearch.TransportVersion;
import org.elasticsearch.cluster.routing.UnassignedInfo.AllocationStatus;
import org.elasticsearch.cluster.routing.allocation.decider.Decision;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;
import org.elasticsearch.common.io.stream.Writeable;

import java.io.IOException;
import java.util.Locale;

/**
 * An enum which represents the various decision types that can be taken by the
 * allocators and deciders for allocating a shard to a node.
 */
public enum AllocationDecision implements Writeable {
    /**
     * The shard can be allocated to a node.
     */
    YES((byte) 0),
    /**
     * The allocation attempt was throttled for the shard.
     */
    THROTTLED((byte) 1),
    /**
     * It is _not_ preferred to allocate a shard to this node, preference should be given to a YES node.
     * This can happen when the shard allocation to a node is allowed, but the node resource usage is
     * already high. Preference can be overridden if a shard's current allocation is no longer allowed
     * and no other node responded YES to the shard relocation.
     */
    NOT_PREFERRED((byte) 2),
    /**
     * The shard cannot be allocated, which can happen for any number of reasons,
     * including the allocation deciders gave a NO decision for allocating.
     */
    NO((byte) 3),
    /**
     * The shard could not be rebalanced to another node despite rebalancing
     * being allowed, because moving the shard to the other node would not form
     * a better cluster balance.
     */
    WORSE_BALANCE((byte) 4),
    /**
     * Waiting on getting shard data from all nodes before making a decision
     * about where to allocate the shard.
     */
    AWAITING_INFO((byte) 5),
    /**
     * The allocation decision has been delayed waiting for a replica with a shard copy
     * that left the cluster to rejoin.
     */
    ALLOCATION_DELAYED((byte) 6),
    /**
     * The shard was denied allocation because there were no valid shard copies
     * found for it amongst the nodes in the cluster.
     */
    NO_VALID_SHARD_COPY((byte) 7),
    /**
     * No attempt was made to allocate the shard
     */
    NO_ATTEMPT((byte) 8);

    final byte id;

    public static final TransportVersion ADD_NOT_PREFERRED_ALLOCATION_DECISION = TransportVersion.fromName(
        "add_not_preferred_allocation_decision"
    );

    AllocationDecision(byte id) {
        this.id = id;
    }

    @Override
    public void writeTo(StreamOutput out) throws IOException {
        if (out.getTransportVersion().supports(ADD_NOT_PREFERRED_ALLOCATION_DECISION) == false) {
            if (id == NOT_PREFERRED.id) {
                // NOT_PREFERRED was originally hidden / unimplemented converted to YES. So for older versions continue to use YES.
                out.write(YES.id);
            } else if (id > THROTTLED.id) {
                // NOT_PREFERRED was placed after THROTTLE in the enum list, so any subsequent values were pushed +1. Shift the enum value
                // back for older versions.
                out.write(id - 1);
            } else {
                assert id == YES.id || id == THROTTLED.id;
                out.write(id);
            }
        } else {
            out.writeByte(id);
        }
    }

    public static AllocationDecision readFrom(StreamInput in) throws IOException {
        byte id = in.readByte();
        if (in.getTransportVersion().supports(ADD_NOT_PREFERRED_ALLOCATION_DECISION) == false) {
            // This is the old enum, without NOT_PREFERRED.
            return switch (id) {
                case 0 -> YES;
                case 1 -> THROTTLED;
                case 2 -> NO;
                case 3 -> WORSE_BALANCE;
                case 4 -> AWAITING_INFO;
                case 5 -> ALLOCATION_DELAYED;
                case 6 -> NO_VALID_SHARD_COPY;
                case 7 -> NO_ATTEMPT;
                default -> throw new IllegalArgumentException("Unknown value [" + id + "]");
            };
        }

        return switch (id) {
            case 0 -> YES;
            case 1 -> THROTTLED;
            case 2 -> NOT_PREFERRED;
            case 3 -> NO;
            case 4 -> WORSE_BALANCE;
            case 5 -> AWAITING_INFO;
            case 6 -> ALLOCATION_DELAYED;
            case 7 -> NO_VALID_SHARD_COPY;
            case 8 -> NO_ATTEMPT;
            default -> throw new IllegalArgumentException("Unknown value [" + id + "]");
        };
    }

    /**
     * Gets an {@link AllocationDecision} from a {@link AllocationStatus}.
     */
    public static AllocationDecision fromAllocationStatus(AllocationStatus allocationStatus) {
        if (allocationStatus == null) {
            return YES;
        } else {
            return switch (allocationStatus) {
                case DECIDERS_THROTTLED -> THROTTLED;
                case FETCHING_SHARD_DATA -> AWAITING_INFO;
                case DELAYED_ALLOCATION -> ALLOCATION_DELAYED;
                case NO_VALID_SHARD_COPY -> NO_VALID_SHARD_COPY;
                case NO_ATTEMPT -> NO_ATTEMPT;
                case DECIDERS_NO -> NO;
            };
        }
    }

    /**
     * Gets an {@link AllocationDecision} from a {@link Decision.Type}
     */
    public static AllocationDecision fromDecisionType(Decision.Type type) {
        return switch (type) {
            case YES -> YES;
            case NOT_PREFERRED -> NOT_PREFERRED;
            case THROTTLE -> THROTTLED;
            case NO -> NO;
        };
    }

    @Override
    public String toString() {
        return super.toString().toLowerCase(Locale.ROOT);
    }

}