/*
 * Copyright (c) 1998, 2023, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package org.jdrupes.mdoclet.internal.doclets.toolkit.util;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URISyntaxException;
import java.net.URL;
import java.net.URLConnection;
import java.util.HashMap;
import java.util.Map;
import java.util.Properties;
import java.util.TreeMap;

import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.ModuleElement;
import javax.lang.model.element.PackageElement;
import javax.tools.Diagnostic;
import javax.tools.Diagnostic.Kind;

import org.jdrupes.mdoclet.internal.doclets.toolkit.AbstractDoclet;
import org.jdrupes.mdoclet.internal.doclets.toolkit.BaseConfiguration;
import org.jdrupes.mdoclet.internal.doclets.toolkit.Resources;

import javax.tools.DocumentationTool;

import jdk.javadoc.doclet.Reporter;

/**
 * Process and manage "-link" and "-linkoffline" to external packages. The
 * options "-link" and "-linkoffline" both depend on the fact that Javadoc now
 * generates "package-list"(lists all the packages which are getting
 * documented) file in the current or the destination directory, while
 * generating the documentation.
 */
public class Extern {

    /**
     * Map element names onto Extern Item objects.
     * Lazily initialized.
     */
    private Map<String, Item> moduleItems = new HashMap<>();
    private Map<String, Map<String, Item>> packageItems = new HashMap<>();

    /**
     * The global configuration information for this run.
     */
    private final BaseConfiguration configuration;

    private final Resources resources;

    private final Utils utils;

    /**
     * True if we are using -linkoffline and false if -link is used instead.
     */
    private boolean linkoffline = false;

    /**
     * Stores the info for one external doc set
     */
    private static class Item {

        /**
         * Element name, found in the "element-list" file in the {@link #path}.
         */
        final String elementName;

        /**
         * The URL or the directory path at which the element documentation will be
         * available.
         */
        final DocPath path;

        /**
         * If given path is directory path then true else if it is a URL then false.
         */
        final boolean relative;

        /**
         * Indicates that docs use old-form of anchors.
         */
        final boolean useOldFormId;

        /**
         * Constructor to build a Extern Item object and map it with the element name.
         * If the same element name is found in the map, then the first mapped
         * Item object or offline location will be retained.
         *
         * @param elementName Element name found in the "element-list" file.
         * @param path        URL or Directory path from where the "element-list"
         * file is picked.
         * @param relative    True if path is URL, false if directory path.
         */
        Item(String elementName, DocPath path, boolean relative,
                boolean useOldFormId) {
            this.elementName = elementName;
            this.path = path;
            this.relative = relative;
            this.useOldFormId = useOldFormId;
        }

        /**
         * String representation of "this" with elementname and the path.
         */
        @Override
        public String toString() {
            return elementName + (relative ? " -> " : " => ") + path.getPath();
        }
    }

    public Extern(BaseConfiguration configuration) {
        this.configuration = configuration;
        this.resources = configuration.getDocResources();
        this.utils = configuration.utils;
    }

    /**
     * Determine if a element item is externally documented.
     *
     * @param element an Element.
     * @return true if the element is externally documented
     */
    public boolean isExternal(Element element) {
        if (packageItems.isEmpty()) {
            return false;
        }
        PackageElement pe = utils.containingPackage(element);
        if (pe.isUnnamed()) {
            return false;
        }

        return findElementItem(pe) != null;
    }

    /**
     * Determine if a element item is a module or not.
     *
     * @param elementName name of the element.
     * @return true if the element is a module
     */
    public boolean isModule(String elementName) {
        Item elem = moduleItems.get(elementName);
        return elem != null;
    }

    /**
     * Convert a link to be an external link if appropriate.
     *
     * @param element The element .
     * @param relativepath    The relative path.
     * @param filename    The link to convert.
     * @return if external return converted link else return null
     */
    public DocLink getExternalLink(Element element, DocPath relativepath,
            String filename) {
        return getExternalLink(element, relativepath, filename, null);
    }

    public DocLink getExternalLink(Element element, DocPath relativepath,
            String filename,
            String memberName) {
        Item fnd = findElementItem(element);
        if (fnd == null)
            return null;

        // The following is somewhat questionable since we are using DocPath
        // to contain external URLs!
        DocPath p
            = fnd.relative ? relativepath.resolve(fnd.path).resolve(filename)
                : fnd.path.resolve(filename);
        return new DocLink(p,
            fnd.useOldFormId ? getOldFormHtmlName(memberName) : memberName);
    }

    /**
     * Build the extern element list from given URL or the directory path,
     * as specified with the "-link" flag.
     * Flag error if the "-link" or "-linkoffline" option is already used.
     *
     * @param url        URL or Directory path.
     * @param reporter   The <code>DocErrorReporter</code> used to report errors.
     * @return true if successful, false otherwise
     * @throws DocFileIOException if there is a problem reading a element list file
     */
    public boolean link(String url, Reporter reporter)
            throws DocFileIOException {
        return link(url, url, reporter, false);
    }

    /**
     * Build the extern element list from given URL or the directory path,
     * as specified with the "-linkoffline" flag.
     * Flag error if the "-link" or "-linkoffline" option is already used.
     *
     * @param url        URL or Directory path.
     * @param elemlisturl This can be another URL for "element-list" or ordinary
     *                   file.
     * @param reporter   The <code>DocErrorReporter</code> used to report errors.
     * @return true if successful, false otherwise
     * @throws DocFileIOException if there is a problem reading the element list file
     */
    public boolean link(String url, String elemlisturl, Reporter reporter)
            throws DocFileIOException {
        return link(url, elemlisturl, reporter, true);
    }

    /**
     * Check whether links to platform documentation are configured. If not then configure
     * links using the documentation URL defined in {@code linkPlatformProperties} or the
     * default documentation URL if that parameter is {@code null}.
     *
     * @param linkPlatformProperties path or URL to properties file containing
     *                               platform documentation URLs, or null
     * @param reporter the {@code DocErrorReporter} used to report errors
     */
    public void checkPlatformLinks(String linkPlatformProperties,
            Reporter reporter) {
        PackageElement javaLang
            = utils.elementUtils.getPackageElement("java.lang");
        if (utils.isIncluded(javaLang)) {
            return;
        }
        DocLink link = getExternalLink(javaLang, DocPath.empty,
            DocPaths.PACKAGE_SUMMARY.getPath());
        if (link != null) {
            // Links to platform docs are already configure, nothing to do here.
            return;
        }
        try {
            int versionNumber = getSourceVersionNumber();
            String docUrl;

            if (linkPlatformProperties != null) {
                docUrl = getCustomPlatformDocs(versionNumber,
                    linkPlatformProperties);
            } else {
                docUrl = getDefaultPlatformDocs(versionNumber);
            }
            if (docUrl == null) {
                return;
            }
            DocPath elementListPath = getPlatformElementList(versionNumber);
            URL elementListUrl
                = jdk.javadoc.internal.doclets.toolkit.AbstractDoclet.class
                    .getResource(elementListPath.getPath());
            if (elementListUrl == null) {
                reporter.print(Kind.WARNING, resources.getText(
                    "doclet.Resource_error", elementListPath.getPath()));
            } else {
                try (InputStream in = open(elementListUrl)) {
                    readElementList(in, docUrl, false, versionNumber,
                        isOldFormPlatformDocs(versionNumber));
                } catch (IOException exc) {
                    throw new Fault(resources.getText(
                        "doclet.Resource_error", elementListPath.getPath()),
                        exc);
                }
            }
        } catch (Fault f) {
            reporter.print(Kind.ERROR, f.getMessage());
        }
    }

    /**
     * Checks if platform docs for the specified version use old-form anchors.
     * Old-form anchors are used by Oracle docs for JDKs 8 and 9.
     * It can be checked on https://docs.oracle.com/javase/<version>/docs/api
     *
     * @param version
     * @return True if docs use old-form anchors
     */
    private boolean isOldFormPlatformDocs(int version) {
        return 8 == version || 9 == version;
    }

    /**
     * Return the resource path for the package or element list for the given {@code version}.
     * @param version the platform version number
     * @return the resource path
     */
    private DocPath getPlatformElementList(int version) {
        String filename = version <= 8
            ? "package-list-" + version + ".txt"
            : "element-list-" + version + ".txt";
        return DocPaths.RESOURCES.resolve("releases").resolve(filename);
    }

    /**
     * Return the default URL for the platform API documentation for the given {@code version}.
     * @param version the platform version number
     * @return the URL as String
     */
    private String getDefaultPlatformDocs(int version) {
        Resources resources = configuration.getDocResources();
        return version <= 10
            ? resources.getText("doclet.platform.docs.old", version)
            : isPrerelease(version)
                ? resources.getText("doclet.platform.docs.ea", version)
                : resources.getText("doclet.platform.docs.new", version);
    }

    /**
     * Retrieve and return the custom URL for the platform API documentation for the given
     * {@code version} from the properties file at {@code linkPlatformProperties}.
     * @param version the platform version number
     * @param linkPlatformProperties path pointing to a properties file
     * @return the custom URL as String
     */
    private String getCustomPlatformDocs(int version,
            String linkPlatformProperties) throws Fault {
        String url;
        try {
            Properties props = new Properties();
            InputStream inputStream;
            if (isUrl(linkPlatformProperties)) {
                inputStream = toURL(linkPlatformProperties).openStream();
            } else {
                inputStream = DocFile
                    .createFileForInput(configuration, linkPlatformProperties)
                    .openInputStream();
            }
            try (inputStream) {
                props.load(inputStream);
            }
            url = props.getProperty("doclet.platform.docs." + version);
        } catch (MalformedURLException exc) {
            throw new Fault(resources.getText("doclet.MalformedURL",
                linkPlatformProperties), exc);
        } catch (IOException exc) {
            throw new Fault(
                resources.getText("doclet.URL_error", linkPlatformProperties),
                exc);
        } catch (DocFileIOException exc) {
            throw new Fault(
                resources.getText("doclet.File_error", linkPlatformProperties),
                exc);
        }
        return url;
    }

    /**
     * Return the source version number used in the current execution of javadoc.
     * @return the source version number
     */
    private int getSourceVersionNumber() {
        SourceVersion sourceVersion = configuration.docEnv.getSourceVersion();
        // TODO it would be nice if this was provided by SourceVersion
        String versionNumber = sourceVersion.name().substring(8);
        assert SourceVersion
            .valueOf("RELEASE_" + versionNumber) == sourceVersion;
        return Integer.parseInt(versionNumber);
    }

    /**
     * Return true if the given {@code sourceVersion} is the same as the current doclet version
     * and is a pre-release version.
     * @param sourceVersion the source version number
     * @return true if it is a pre-release version
     */
    private boolean isPrerelease(int sourceVersion) {
        Runtime.Version docletVersion = configuration.getDocletVersion();
        return docletVersion.feature() == sourceVersion
            && docletVersion.pre().isPresent();
    }

    /*
     * Build the extern element list from given URL or the directory path.
     * Flag error if the "-link" or "-linkoffline" option is already used.
     *
     * @param url URL or Directory path.
     * 
     * @param elemlisturl This can be another URL for "element-list" or ordinary
     * file.
     * 
     * @param reporter The <code>DocErrorReporter</code> used to report errors.
     * 
     * @param linkoffline True if -linkoffline is used and false if -link is
     * used.
     * 
     * @return true if successful, false otherwise
     * 
     * @throws DocFileIOException if there is a problem reading the element list
     * file
     */
    private boolean link(String url, String elemlisturl, Reporter reporter,
            boolean linkoffline)
            throws DocFileIOException {
        this.linkoffline = linkoffline;
        try {
            url = adjustEndFileSeparator(url);
            if (isUrl(elemlisturl)) {
                readElementListFromURL(url,
                    toURL(adjustEndFileSeparator(elemlisturl)));
            } else {
                readElementListFromFile(url,
                    DocFile.createFileForInput(configuration, elemlisturl));
            }
            return true;
        } catch (Fault f) {
            reporter.print(Diagnostic.Kind.ERROR, f.getMessage());
            return false;
        }
    }

    private static class Fault extends Exception {
        private static final long serialVersionUID = 0;

        Fault(String msg, Exception cause) {
            super(msg + (cause == null ? "" : " (" + cause + ")"), cause);
        }
    }

    /**
     * Get the Extern Item object associated with this element name.
     *
     * @param element Element
     */
    private Item findElementItem(Element element) {
        Item item = null;
        if (element instanceof ModuleElement me) {
            item = moduleItems.get(utils.getModuleName(me));
        } else if (element instanceof PackageElement pkg) {
            ModuleElement moduleElement = utils.containingModule(pkg);
            Map<String, Item> pkgMap
                = packageItems.get(utils.getModuleName(moduleElement));
            item = (pkgMap != null) ? pkgMap.get(utils.getPackageName(pkg))
                : null;
        }
        return item;
    }

    /**
     * If the URL or Directory path is missing end file separator, add that.
     */
    private String adjustEndFileSeparator(String url) {
        return url.endsWith("/") ? url : url + '/';
    }

    /**
     * Fetch the URL and read the "element-list" file.
     *
     * @param urlpath        Path to the elements.
     * @param elemlisturlpath URL or the path to the "element-list" file.
     */
    private void readElementListFromURL(String urlpath, URL elemlisturlpath)
            throws Fault {
        try {
            URL link = elemlisturlpath.toURI()
                .resolve(DocPaths.ELEMENT_LIST.getPath()).toURL();
            try (InputStream in = open(link)) {
                readElementList(in, urlpath, false, 0, false);
            }
        } catch (URISyntaxException | MalformedURLException exc) {
            throw new Fault(resources.getText("doclet.MalformedURL",
                elemlisturlpath.toString()), exc);
        } catch (IOException exc) {
            readPackageListFromURL(urlpath, elemlisturlpath);
        }
    }

    /**
     * Fetch the URL and read the "package-list" file.
     *
     * @param urlpath        Path to the packages.
     * @param elemlisturlpath URL or the path to the "package-list" file.
     */
    private void readPackageListFromURL(String urlpath, URL elemlisturlpath)
            throws Fault {
        try {
            URL link = elemlisturlpath.toURI()
                .resolve(DocPaths.PACKAGE_LIST.getPath()).toURL();
            try (InputStream in = open(link)) {
                readElementList(in, urlpath, false, 0, true);
            }
        } catch (URISyntaxException | MalformedURLException exc) {
            throw new Fault(resources.getText("doclet.MalformedURL",
                elemlisturlpath.toString()), exc);
        } catch (IOException exc) {
            throw new Fault(resources.getText("doclet.URL_error",
                elemlisturlpath.toString()), exc);
        }
    }

    /**
     * Read the "element-list" file which is available locally.
     *
     * @param path URL or directory path to the elements.
     * @param elemListPath Path to the local "element-list" file.
     * @throws Fault if an error occurs that can be treated as a warning
     * @throws DocFileIOException if there is a problem opening the element list file
     */
    private void readElementListFromFile(String path, DocFile elemListPath)
            throws Fault, DocFileIOException {
        DocFile file = elemListPath.resolve(DocPaths.ELEMENT_LIST);
        if (!(file.isAbsolute() || linkoffline)) {
            file = file.resolveAgainst(
                DocumentationTool.Location.DOCUMENTATION_OUTPUT);
        }
        if (file.exists()) {
            readElementList(file, path, false);
        } else {
            DocFile file1 = elemListPath.resolve(DocPaths.PACKAGE_LIST);
            if (!(file1.isAbsolute() || linkoffline)) {
                file1 = file1.resolveAgainst(
                    DocumentationTool.Location.DOCUMENTATION_OUTPUT);
            }
            if (file1.exists()) {
                readElementList(file1, path, true);
            } else {
                throw new Fault(
                    resources.getText("doclet.File_error", file.getPath()),
                    null);
            }
        }
    }

    private void readElementList(DocFile file, String path,
            boolean isOldFormDoc) throws Fault, DocFileIOException {
        try {
            if (file.canRead()) {
                boolean pathIsRelative
                    = !isUrl(path)
                        && !DocFile.createFileForInput(configuration, path)
                            .isAbsolute();
                readElementList(file.openInputStream(), path, pathIsRelative, 0,
                    isOldFormDoc);
            } else {
                throw new Fault(
                    resources.getText("doclet.File_error", file.getPath()),
                    null);
            }
        } catch (IOException exc) {
            throw new Fault(
                resources.getText("doclet.File_error", file.getPath()), exc);
        }
    }

    /**
     * Read the file "element-list" and for each element name found, create
     * Extern object and associate it with the element name in the map.
     *
     * @param input     InputStream from the "element-list" file.
     * @param path     URL or the directory path to the elements.
     * @param relative Is path relative?
     * @param platformVersion The version of platform libraries the element list belongs to,
     *                        or {@code 0} if it does not belong to a platform libraries doc bundle.
     * @throws IOException if there is a problem reading or closing the stream
     */
    private void readElementList(InputStream input, String path,
            boolean relative, int platformVersion,
            boolean isOldFormDoc)
            throws IOException {
        try (BufferedReader in
            = new BufferedReader(new InputStreamReader(input))) {
            String elemname;
            DocPath elempath;
            String moduleName = null;
            DocPath basePath = DocPath.create(path);
            boolean showDiagnostic = true;
            while ((elemname = in.readLine()) != null) {
                if (elemname.length() > 0) {
                    elempath = basePath;
                    if (elemname.startsWith(DocletConstants.MODULE_PREFIX)) {
                        moduleName = elemname
                            .replace(DocletConstants.MODULE_PREFIX, "");
                        Item item = new Item(moduleName, elempath, relative,
                            isOldFormDoc);
                        moduleItems.put(moduleName, item);
                    } else {
                        DocPath pkgPath
                            = DocPath.create(elemname.replace('.', '/'));
                        // Although being modular, JDKs 9 and 10 do not use
                        // module names in javadoc URL paths.
                        if (moduleName != null && platformVersion != 9
                            && platformVersion != 10) {
                            elempath = elempath.resolve(
                                DocPath.create(moduleName).resolve(pkgPath));
                        } else {
                            elempath = elempath.resolve(pkgPath);
                        }
                        String actualModuleName;
                        // For user provided libraries we check whether
                        // modularity matches the actual library.
                        // We trust modularity to be correct for platform
                        // library element lists.
                        if (platformVersion == 0) {
                            actualModuleName = checkLinkCompatibility(elemname,
                                moduleName, path, showDiagnostic);
                        } else {
                            actualModuleName = moduleName == null
                                ? DocletConstants.DEFAULT_ELEMENT_NAME
                                : moduleName;
                        }
                        Item item = new Item(elemname, elempath, relative,
                            isOldFormDoc);
                        packageItems
                            .computeIfAbsent(actualModuleName,
                                k -> new TreeMap<>())
                            .putIfAbsent(elemname, item); // first-one-wins
                                                          // semantics
                        showDiagnostic = false;
                    }
                }
            }
        }
    }

    /**
     * Check if the external documentation format matches our internal model of the code.
     * Returns the module name to use for external reference lookup according to the actual
     * modularity of the external package (and regardless of modularity of documentation).
     *
     * @param packageName the package name
     * @param moduleName the module name or null
     * @param path the documentation path
     * @param showDiagnostic whether to print a diagnostic message in case of modularity mismatch
     * @return the module name to use according to actual modularity of the package
     */
    private String checkLinkCompatibility(String packageName, String moduleName,
            String path, boolean showDiagnostic) {
        PackageElement pe = utils.elementUtils.getPackageElement(packageName);
        if (pe != null) {
            ModuleElement me = (ModuleElement) pe.getEnclosingElement();
            if (me == null || me.isUnnamed()) {
                if (moduleName != null && showDiagnostic) {
                    printModularityMismatchDiagnostic(
                        "doclet.linkMismatch_PackagedLinkedtoModule", path);
                }
                // library is not modular, ignore module name even if
                // documentation is modular
                return DocletConstants.DEFAULT_ELEMENT_NAME;
            } else if (moduleName == null) {
                // suppress the diagnostic message in the case of automatic
                // modules
                if (!utils.elementUtils.isAutomaticModule(me)
                    && showDiagnostic) {
                    printModularityMismatchDiagnostic(
                        "doclet.linkMismatch_ModuleLinkedtoPackage", path);
                }
                // library is modular, use module name for lookup even though
                // documentation is not
                return utils.getModuleName(me);
            }
        }
        return moduleName == null ? DocletConstants.DEFAULT_ELEMENT_NAME
            : moduleName;
    }

    public boolean isUrl(String urlCandidate) {
        try {
            @SuppressWarnings("deprecation")
            var _unused = new URL(urlCandidate);
            // No exception was thrown, so this must really be a URL.
            return true;
        } catch (MalformedURLException e) {
            // Since exception is thrown, this must be a directory path.
            return false;
        }
    }

    @SuppressWarnings("deprecation")
    private URL toURL(String url) throws Fault {
        try {
            return new URL(url);
        } catch (MalformedURLException e) {
            throw new Fault(resources.getText("doclet.MalformedURL", url), e);
        }
    }

    /**
     * Open a stream to a URL, following a limited number of redirects
     * if necessary.
     *
     * @param url the URL
     * @return the stream
     * @throws IOException if an error occurred accessing the URL
     */
    private InputStream open(URL url) throws IOException {
        URLConnection conn = url.openConnection();

        boolean redir;
        int redirects = 0;
        InputStream in;

        do {
            // Open the input stream before getting headers,
            // because getHeaderField() et al swallow IOExceptions.
            in = conn.getInputStream();
            redir = false;

            if (conn instanceof HttpURLConnection http) {
                int stat = http.getResponseCode();
                // See:
                // https://developer.mozilla.org/en-US/docs/Web/HTTP/Status
                // https://en.wikipedia.org/wiki/List_of_HTTP_status_codes#3xx_Redirection
                switch (stat) {
                case 300: // Multiple Choices
                case 301: // Moved Permanently
                case 302: // Found (previously Moved Temporarily)
                case 303: // See Other
                case 307: // Temporary Redirect
                case 308: // Permanent Redirect
                    URL base = http.getURL();
                    String loc = http.getHeaderField("Location");
                    URL target = null;
                    if (loc != null) {
                        @SuppressWarnings("deprecation")
                        var _unused = target = new URL(base, loc);
                    }
                    http.disconnect();
                    if (target == null || redirects >= 5) {
                        throw new IOException("illegal URL redirect");
                    }
                    redir = true;
                    conn = target.openConnection();
                    redirects++;
                }
            }
        } while (redir);

        if (!url.equals(conn.getURL())) {
            configuration.getReporter().print(Kind.WARNING,
                resources.getText("doclet.urlRedirected", url, conn.getURL()));
        }

        return in;
    }

    private void printModularityMismatchDiagnostic(String key, Object arg) {
        switch (configuration.getOptions().linkModularityMismatch()) {
        case INFO -> configuration.getMessages().notice(key, arg);
        case WARN -> configuration.getMessages().warning(key, arg);
        }
    }

    /**
     * Converts a name to an old-form HTML name (old-form id).
     *
     * @param name the string that needs to be converted to a valid HTML name
     * @return old-form HTML name
     */
    private String getOldFormHtmlName(String name) {
        /*
         * The HTML 4 spec at http://www.w3.org/TR/html4/types.html#h-6.2
         * mentions
         * that the name/id should begin with a letter followed by other valid
         * characters.
         * The HTML 5 spec (draft) is more permissive on names/ids where the
         * only restriction
         * is that it should be at least one character long and should not
         * contain spaces.
         * The spec draft is @
         * http://www.w3.org/html/wg/drafts/html/master/dom.html#the-id-
         * attribute.
         *
         * For HTML 4, we need to check for non-characters at the beginning of
         * the name and
         * substitute it accordingly, "_" and "$" can appear at the beginning of
         * a member name.
         * The method substitutes "$" with "Z:Z:D" and will prefix "_" with
         * "Z:Z".
         */

        if (null == name)
            return name;

        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < name.length(); i++) {
            char ch = name.charAt(i);
            switch (ch) {
            case '(':
            case ')':
            case '<':
            case '>':
            case ',':
                sb.append('-');
                break;
            case ' ':
            case '[':
                break;
            case ']':
                sb.append(":A");
                break;
            // Any appearance of $ needs to be substituted with ":D" and not
            // with hyphen
            // since a field name "P$$ and a method P(), both valid member
            // names, can end
            // up as "P--". A member name beginning with $ needs to be
            // substituted with
            // "Z:Z:D".
            case '$':
                if (i == 0)
                    sb.append("Z:Z");
                sb.append(":D");
                break;
            // A member name beginning with _ needs to be prefixed with "Z:Z"
            // since valid anchor
            // names can only begin with a letter.
            case '_':
                if (i == 0)
                    sb.append("Z:Z");
                sb.append(ch);
                break;
            default:
                sb.append(ch);
            }
        }
        return sb.toString();
    }
}