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// Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
package com.yahoo.config.model.provision;

import com.google.common.collect.ImmutableMap;
import com.yahoo.config.model.builder.xml.XmlHelper;
import com.yahoo.net.HostName;
import com.yahoo.text.XML;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import java.io.Reader;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Set;
import java.util.logging.Logger;

/**
 * A collection of hosts
 *
 * @author bratseth
 */
public class Hosts {

    public static final Logger log = Logger.getLogger(Hosts.class.getPackage().toString());

    private final ImmutableMap hosts;

    public Hosts(Collection hosts) {
        validateAliases(hosts);
        
        ImmutableMap.Builder hostsBuilder = new ImmutableMap.Builder<>();
        for (Host host : hosts)
            hostsBuilder.put(host.hostname(), host);
        this.hosts = hostsBuilder.build();
    }

    /** Throw IllegalArgumentException if host aliases breaks invariants */
    private void validateAliases(Collection hosts) {
        Set aliases = new HashSet<>();
        for (Host host : hosts) {
            if (!host.aliases().isEmpty()) {
                if (host.aliases().isEmpty())
                    throw new IllegalArgumentException("Host '" + host.hostname() + "' must have at least one  tag.");
                for (String alias : host.aliases()) {
                    if (aliases.contains(alias))
                        throw new IllegalArgumentException("Alias '" + alias + "' is used by multiple hosts.");
                    aliases.add(alias);
                }
            }
        }
    }
    
    /**
     * Builds host system from a hosts.xml file
     *
     * @param hostsFile a reader for host from application package
     * @return the HostSystem for this application package
     */
    public static Hosts readFrom(Reader hostsFile) {
        List hosts = new ArrayList<>();
        Document doc = XmlHelper.getDocument(hostsFile);
        for (Element hostE : XML.getChildren(doc.getDocumentElement(), "host")) {
            String name = hostE.getAttribute("name");
            if (name.isEmpty()) {
                throw new IllegalArgumentException("Missing 'name' attribute for host.");
            }
            if ("localhost".equals(name)) {
                name = HostName.getLocalhost();
            }
            List hostAliases = getHostAliases(hostE.getChildNodes());
            if (hostAliases.isEmpty()) {
                throw new IllegalArgumentException("No host aliases defined for host '" + name + "'");
            }
            hosts.add(new Host(name, hostAliases));
        }
        return new Hosts(hosts);
    }

    /** Returns an immutable collection of the hosts of this */
    public Collection asCollection() { return hosts.values(); }

    @Override
    public String toString() {
        return "Hosts: " + hosts.keySet();
    }

    /**
     * Get all aliases for one host from a list of 'alias' xml nodes.
     *
     * @param hostAliases  List of xml nodes, each representing one hostalias
     * @return a list of alias strings.
     */
    private static List getHostAliases(NodeList hostAliases) {
        List aliases = new LinkedList<>();
        for (int i = 0; i < hostAliases.getLength(); i++) {
            Node n = hostAliases.item(i);
            if (! (n instanceof Element e)) {
                continue;
            }
            if (! e.getNodeName().equals("alias")) {
                throw new IllegalArgumentException("Unexpected tag: '" + e.getNodeName() + "' at node " +
                                                   XML.getNodePath(e, " > ") + ", expected 'alias'.");
            }
            String alias = e.getFirstChild().getNodeValue();
            if ((alias == null) || (alias.isEmpty())) {
                throw new IllegalArgumentException("Missing value for the alias tag at node " +
                                                   XML.getNodePath(e, " > ") + "'.");
            }
            aliases.add(alias);
        }
        return aliases;
    }

}