/*
 * Copyright (c) 2012, 2017, 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 com.sun.tools.doclint;

import java.io.IOException;
import java.io.StringWriter;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.Deque;
import java.util.EnumSet;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import javax.lang.model.element.Element;
import javax.lang.model.element.ElementKind;
import javax.lang.model.element.ExecutableElement;
import javax.lang.model.element.Name;
import javax.lang.model.element.VariableElement;
import javax.lang.model.type.TypeKind;
import javax.lang.model.type.TypeMirror;
import javax.tools.Diagnostic.Kind;
import javax.tools.JavaFileObject;

import com.sun.source.doctree.AttributeTree;
import com.sun.source.doctree.AuthorTree;
import com.sun.source.doctree.DocCommentTree;
import com.sun.source.doctree.DocRootTree;
import com.sun.source.doctree.DocTree;
import com.sun.source.doctree.EndElementTree;
import com.sun.source.doctree.EntityTree;
import com.sun.source.doctree.ErroneousTree;
import com.sun.source.doctree.IdentifierTree;
import com.sun.source.doctree.IndexTree;
import com.sun.source.doctree.InheritDocTree;
import com.sun.source.doctree.InlineTagTree;
import com.sun.source.doctree.LinkTree;
import com.sun.source.doctree.LiteralTree;
import com.sun.source.doctree.ParamTree;
import com.sun.source.doctree.ProvidesTree;
import com.sun.source.doctree.ReferenceTree;
import com.sun.source.doctree.ReturnTree;
import com.sun.source.doctree.SerialDataTree;
import com.sun.source.doctree.SerialFieldTree;
import com.sun.source.doctree.SinceTree;
import com.sun.source.doctree.StartElementTree;
import com.sun.source.doctree.SummaryTree;
import com.sun.source.doctree.SystemPropertyTree;
import com.sun.source.doctree.TextTree;
import com.sun.source.doctree.ThrowsTree;
import com.sun.source.doctree.UnknownBlockTagTree;
import com.sun.source.doctree.UnknownInlineTagTree;
import com.sun.source.doctree.UsesTree;
import com.sun.source.doctree.ValueTree;
import com.sun.source.doctree.VersionTree;
import com.sun.source.tree.Tree;
import com.sun.source.util.DocTreePath;
import com.sun.source.util.DocTreePathScanner;
import com.sun.source.util.TreePath;
import com.sun.tools.doclint.HtmlTag.AttrKind;
import com.sun.tools.javac.tree.DocPretty;
import com.sun.tools.javac.util.Assert;
import com.sun.tools.javac.util.DefinedBy;
import com.sun.tools.javac.util.DefinedBy.Api;
import com.sun.tools.javac.util.StringUtils;

import static com.sun.tools.doclint.Messages.Group.*;


Validate a doc comment.
This is NOT part of any supported API.
If you write code that depends on this, you do so at your own
risk.  This code and its internal interfaces are subject to change
or deletion without notice.
/**
 * Validate a doc comment.
 *
 * <p><b>This is NOT part of any supported API.
 * If you write code that depends on this, you do so at your own
 * risk.  This code and its internal interfaces are subject to change
 * or deletion without notice.</b></p>
 */
public class Checker extends DocTreePathScanner<Void, Void> {
    final Env env;

    Set<Element> foundParams = new HashSet<>();
    Set<TypeMirror> foundThrows = new HashSet<>();
    Map<Element, Set<String>> foundAnchors = new HashMap<>();
    boolean foundInheritDoc = false;
    boolean foundReturn = false;
    boolean hasNonWhitespaceText = false;

    public enum Flag {
        TABLE_HAS_CAPTION,
        HAS_ELEMENT,
        HAS_HEADING,
        HAS_INLINE_TAG,
        HAS_TEXT,
        REPORTED_BAD_INLINE
    }

    static class TagStackItem {
        final DocTree tree; // typically, but not always, StartElementTree
        final HtmlTag tag;
        final Set<HtmlTag.Attr> attrs;
        final Set<Flag> flags;
        TagStackItem(DocTree tree, HtmlTag tag) {
            this.tree = tree;
            this.tag = tag;
            attrs = EnumSet.noneOf(HtmlTag.Attr.class);
            flags = EnumSet.noneOf(Flag.class);
        }
        @Override
        public String toString() {
            return String.valueOf(tag);
        }
    }

    private final Deque<TagStackItem> tagStack; // TODO: maybe want to record starting tree as well
    private HtmlTag currHeaderTag;

    private final int implicitHeaderLevel;

    // <editor-fold defaultstate="collapsed" desc="Top level">

    Checker(Env env) {
        this.env = Assert.checkNonNull(env);
        tagStack = new LinkedList<>();
        implicitHeaderLevel = env.implicitHeaderLevel;
    }

    public Void scan(DocCommentTree tree, TreePath p) {
        env.initTypes();
        env.setCurrent(p, tree);

        boolean isOverridingMethod = !env.currOverriddenMethods.isEmpty();
        JavaFileObject fo = p.getCompilationUnit().getSourceFile();

        if (p.getLeaf().getKind() == Tree.Kind.PACKAGE) {
            // If p points to a package, the implied declaration is the
            // package declaration (if any) for the compilation unit.
            // Handle this case specially, because doc comments are only
            // expected in package-info files.
            boolean isPkgInfo = fo.isNameCompatible("package-info", JavaFileObject.Kind.SOURCE);
            if (tree == null) {
                if (isPkgInfo)
                    reportMissing("dc.missing.comment");
                return null;
            } else {
                if (!isPkgInfo)
                    reportReference("dc.unexpected.comment");
            }
        } else if (tree != null && fo.isNameCompatible("package", JavaFileObject.Kind.HTML)) {
            // a package.html file with a DocCommentTree
            if (tree.getFullBody().isEmpty()) {
                reportMissing("dc.missing.comment");
                return null;
            }
        } else {
            if (tree == null) {
                if (!isSynthetic() && !isOverridingMethod)
                    reportMissing("dc.missing.comment");
                return null;
            }
        }

        tagStack.clear();
        currHeaderTag = null;

        foundParams.clear();
        foundThrows.clear();
        foundInheritDoc = false;
        foundReturn = false;
        hasNonWhitespaceText = false;

        scan(new DocTreePath(p, tree), null);

        if (!isOverridingMethod) {
            switch (env.currElement.getKind()) {
                case METHOD:
                case CONSTRUCTOR: {
                    ExecutableElement ee = (ExecutableElement) env.currElement;
                    checkParamsDocumented(ee.getTypeParameters());
                    checkParamsDocumented(ee.getParameters());
                    switch (ee.getReturnType().getKind()) {
                        case VOID:
                        case NONE:
                            break;
                        default:
                            if (!foundReturn
                                    && !foundInheritDoc
                                    && !env.types.isSameType(ee.getReturnType(), env.java_lang_Void)) {
                                reportMissing("dc.missing.return");
                            }
                    }
                    checkThrowsDocumented(ee.getThrownTypes());
                }
            }
        }

        return null;
    }

    private void reportMissing(String code, Object... args) {
        env.messages.report(MISSING, Kind.WARNING, env.currPath.getLeaf(), code, args);
    }

    private void reportReference(String code, Object... args) {
        env.messages.report(REFERENCE, Kind.WARNING, env.currPath.getLeaf(), code, args);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitDocComment(DocCommentTree tree, Void ignore) {
        super.visitDocComment(tree, ignore);
        for (TagStackItem tsi: tagStack) {
            warnIfEmpty(tsi, null);
            if (tsi.tree.getKind() == DocTree.Kind.START_ELEMENT
                    && tsi.tag.endKind == HtmlTag.EndKind.REQUIRED) {
                StartElementTree t = (StartElementTree) tsi.tree;
                env.messages.error(HTML, t, "dc.tag.not.closed", t.getName());
            }
        }
        return null;
    }
    // </editor-fold>

    // <editor-fold defaultstate="collapsed" desc="Text and entities.">

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitText(TextTree tree, Void ignore) {
        hasNonWhitespaceText = hasNonWhitespace(tree);
        if (hasNonWhitespaceText) {
            checkAllowsText(tree);
            markEnclosingTag(Flag.HAS_TEXT);
        }
        return null;
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitEntity(EntityTree tree, Void ignore) {
        checkAllowsText(tree);
        markEnclosingTag(Flag.HAS_TEXT);
        String name = tree.getName().toString();
        if (name.startsWith("#")) {
            int v = StringUtils.toLowerCase(name).startsWith("#x")
                    ? Integer.parseInt(name.substring(2), 16)
                    : Integer.parseInt(name.substring(1), 10);
            if (!Entity.isValid(v)) {
                env.messages.error(HTML, tree, "dc.entity.invalid", name);
            }
        } else if (!Entity.isValid(name)) {
            env.messages.error(HTML, tree, "dc.entity.invalid", name);
        }
        return null;
    }

    void checkAllowsText(DocTree tree) {
        TagStackItem top = tagStack.peek();
        if (top != null
                && top.tree.getKind() == DocTree.Kind.START_ELEMENT
                && !top.tag.acceptsText()) {
            if (top.flags.add(Flag.REPORTED_BAD_INLINE)) {
                env.messages.error(HTML, tree, "dc.text.not.allowed",
                        ((StartElementTree) top.tree).getName());
            }
        }
    }

    // </editor-fold>

    // <editor-fold defaultstate="collapsed" desc="HTML elements">

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitStartElement(StartElementTree tree, Void ignore) {
        final Name treeName = tree.getName();
        final HtmlTag t = HtmlTag.get(treeName);
        if (t == null) {
            env.messages.error(HTML, tree, "dc.tag.unknown", treeName);
        } else if (t.allowedVersion != HtmlVersion.ALL && t.allowedVersion != env.htmlVersion) {
            env.messages.error(HTML, tree, "dc.tag.not.supported", treeName);
        } else {
            boolean done = false;
            for (TagStackItem tsi: tagStack) {
                if (tsi.tag.accepts(t)) {
                    while (tagStack.peek() != tsi) {
                        warnIfEmpty(tagStack.peek(), null);
                        tagStack.pop();
                    }
                    done = true;
                    break;
                } else if (tsi.tag.endKind != HtmlTag.EndKind.OPTIONAL) {
                    done = true;
                    break;
                }
            }
            if (!done && HtmlTag.BODY.accepts(t)) {
                while (!tagStack.isEmpty()) {
                    warnIfEmpty(tagStack.peek(), null);
                    tagStack.pop();
                }
            }

            markEnclosingTag(Flag.HAS_ELEMENT);
            checkStructure(tree, t);

            // tag specific checks
            switch (t) {
                // check for out of sequence headers, such as <h1>...</h1>  <h3>...</h3>
                case H1: case H2: case H3: case H4: case H5: case H6:
                    checkHeader(tree, t);
                    break;
            }

            if (t.flags.contains(HtmlTag.Flag.NO_NEST)) {
                for (TagStackItem i: tagStack) {
                    if (t == i.tag) {
                        env.messages.warning(HTML, tree, "dc.tag.nested.not.allowed", treeName);
                        break;
                    }
                }
            }
        }

        // check for self closing tags, such as <a id="name"/>
        if (tree.isSelfClosing()) {
            env.messages.error(HTML, tree, "dc.tag.self.closing", treeName);
        }

        try {
            TagStackItem parent = tagStack.peek();
            TagStackItem top = new TagStackItem(tree, t);
            tagStack.push(top);

            super.visitStartElement(tree, ignore);

            // handle attributes that may or may not have been found in start element
            if (t != null) {
                switch (t) {
                    case CAPTION:
                        if (parent != null && parent.tag == HtmlTag.TABLE)
                            parent.flags.add(Flag.TABLE_HAS_CAPTION);
                        break;

                    case H1: case H2: case H3: case H4: case H5: case H6:
                        if (parent != null && (parent.tag == HtmlTag.SECTION || parent.tag == HtmlTag.ARTICLE)) {
                            parent.flags.add(Flag.HAS_HEADING);
                        }
                        break;

                    case IMG:
                        if (!top.attrs.contains(HtmlTag.Attr.ALT))
                            env.messages.error(ACCESSIBILITY, tree, "dc.no.alt.attr.for.image");
                        break;
                }
            }

            return null;
        } finally {

            if (t == null || t.endKind == HtmlTag.EndKind.NONE)
                tagStack.pop();
        }
    }

    private void checkStructure(StartElementTree tree, HtmlTag t) {
        Name treeName = tree.getName();
        TagStackItem top = tagStack.peek();
        switch (t.blockType) {
            case BLOCK:
                if (top == null || top.tag.accepts(t))
                    return;

                switch (top.tree.getKind()) {
                    case START_ELEMENT: {
                        if (top.tag.blockType == HtmlTag.BlockType.INLINE) {
                            Name name = ((StartElementTree) top.tree).getName();
                            env.messages.error(HTML, tree, "dc.tag.not.allowed.inline.element",
                                    treeName, name);
                            return;
                        }
                    }
                    break;

                    case LINK:
                    case LINK_PLAIN: {
                        String name = top.tree.getKind().tagName;
                        env.messages.error(HTML, tree, "dc.tag.not.allowed.inline.tag",
                                treeName, name);
                        return;
                    }
                }
                break;

            case INLINE:
                if (top == null || top.tag.accepts(t))
                    return;
                break;

            case LIST_ITEM:
            case TABLE_ITEM:
                if (top != null) {
                    // reset this flag so subsequent bad inline content gets reported
                    top.flags.remove(Flag.REPORTED_BAD_INLINE);
                    if (top.tag.accepts(t))
                        return;
                }
                break;

            case OTHER:
                switch (t) {
                    case SCRIPT:
                        // <script> may or may not be allowed, depending on --allow-script-in-comments
                        // but we allow it here, and rely on a separate scanner to detect all uses
                        // of JavaScript, including <script> tags, and use in attributes, etc.
                        break;

                    default:
                        env.messages.error(HTML, tree, "dc.tag.not.allowed", treeName);
                }
                return;
        }

        env.messages.error(HTML, tree, "dc.tag.not.allowed.here", treeName);
    }

    private void checkHeader(StartElementTree tree, HtmlTag tag) {
        // verify the new tag
        if (getHeaderLevel(tag) > getHeaderLevel(currHeaderTag) + 1) {
            if (currHeaderTag == null) {
                env.messages.error(ACCESSIBILITY, tree, "dc.tag.header.sequence.1", tag);
            } else {
                env.messages.error(ACCESSIBILITY, tree, "dc.tag.header.sequence.2",
                    tag, currHeaderTag);
            }
        }

        currHeaderTag = tag;
    }

    private int getHeaderLevel(HtmlTag tag) {
        if (tag == null)
            return implicitHeaderLevel;
        switch (tag) {
            case H1: return 1;
            case H2: return 2;
            case H3: return 3;
            case H4: return 4;
            case H5: return 5;
            case H6: return 6;
            default: throw new IllegalArgumentException();
        }
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitEndElement(EndElementTree tree, Void ignore) {
        final Name treeName = tree.getName();
        final HtmlTag t = HtmlTag.get(treeName);
        if (t == null) {
            env.messages.error(HTML, tree, "dc.tag.unknown", treeName);
        } else if (t.endKind == HtmlTag.EndKind.NONE) {
            env.messages.error(HTML, tree, "dc.tag.end.not.permitted", treeName);
        } else {
            boolean done = false;
            while (!tagStack.isEmpty()) {
                TagStackItem top = tagStack.peek();
                if (t == top.tag) {
                    switch (t) {
                        case TABLE:
                            if (!top.attrs.contains(HtmlTag.Attr.SUMMARY)
                                    && !top.flags.contains(Flag.TABLE_HAS_CAPTION)) {
                                env.messages.error(ACCESSIBILITY, tree,
                                        "dc.no.summary.or.caption.for.table");
                            }
                            break;

                        case SECTION:
                        case ARTICLE:
                            if (env.htmlVersion == HtmlVersion.HTML5 && !top.flags.contains(Flag.HAS_HEADING)) {
                                env.messages.error(HTML, tree, "dc.tag.requires.heading", treeName);
                            }
                            break;
                    }
                    warnIfEmpty(top, tree);
                    tagStack.pop();
                    done = true;
                    break;
                } else if (top.tag == null || top.tag.endKind != HtmlTag.EndKind.REQUIRED) {
                    tagStack.pop();
                } else {
                    boolean found = false;
                    for (TagStackItem si: tagStack) {
                        if (si.tag == t) {
                            found = true;
                            break;
                        }
                    }
                    if (found && top.tree.getKind() == DocTree.Kind.START_ELEMENT) {
                        env.messages.error(HTML, top.tree, "dc.tag.start.unmatched",
                                ((StartElementTree) top.tree).getName());
                        tagStack.pop();
                    } else {
                        env.messages.error(HTML, tree, "dc.tag.end.unexpected", treeName);
                        done = true;
                        break;
                    }
                }
            }

            if (!done && tagStack.isEmpty()) {
                env.messages.error(HTML, tree, "dc.tag.end.unexpected", treeName);
            }
        }

        return super.visitEndElement(tree, ignore);
    }

    void warnIfEmpty(TagStackItem tsi, DocTree endTree) {
        if (tsi.tag != null && tsi.tree instanceof StartElementTree) {
            if (tsi.tag.flags.contains(HtmlTag.Flag.EXPECT_CONTENT)
                    && !tsi.flags.contains(Flag.HAS_TEXT)
                    && !tsi.flags.contains(Flag.HAS_ELEMENT)
                    && !tsi.flags.contains(Flag.HAS_INLINE_TAG)) {
                DocTree tree = (endTree != null) ? endTree : tsi.tree;
                Name treeName = ((StartElementTree) tsi.tree).getName();
                env.messages.warning(HTML, tree, "dc.tag.empty", treeName);
            }
        }
    }

    // </editor-fold>

    // <editor-fold defaultstate="collapsed" desc="HTML attributes">

    @Override @DefinedBy(Api.COMPILER_TREE) @SuppressWarnings("fallthrough")
    public Void visitAttribute(AttributeTree tree, Void ignore) {
        HtmlTag currTag = tagStack.peek().tag;
        if (currTag != null) {
            Name name = tree.getName();
            HtmlTag.Attr attr = currTag.getAttr(name);
            if (attr != null) {
                if (env.htmlVersion == HtmlVersion.HTML4 && attr.name().contains("-")) {
                    env.messages.error(HTML, tree, "dc.attr.not.supported.html4", name);
                }
                boolean first = tagStack.peek().attrs.add(attr);
                if (!first)
                    env.messages.error(HTML, tree, "dc.attr.repeated", name);
            }
            // for now, doclint allows all attribute names beginning with "on" as event handler names,
            // without checking the validity or applicability of the name
            if (!name.toString().startsWith("on")) {
                AttrKind k = currTag.getAttrKind(name);
                switch (env.htmlVersion) {
                    case HTML4:
                        validateHtml4Attrs(tree, name, k);
                        break;

                    case HTML5:
                        validateHtml5Attrs(tree, name, k);
                        break;
                }
            }

            if (attr != null) {
                switch (attr) {
                    case NAME:
                        if (currTag != HtmlTag.A) {
                            break;
                        }
                        // fallthrough
                    case ID:
                        String value = getAttrValue(tree);
                        if (value == null) {
                            env.messages.error(HTML, tree, "dc.anchor.value.missing");
                        } else {
                            if (!validName.matcher(value).matches()) {
                                env.messages.error(HTML, tree, "dc.invalid.anchor", value);
                            }
                            if (!checkAnchor(value)) {
                                env.messages.error(HTML, tree, "dc.anchor.already.defined", value);
                            }
                        }
                        break;

                    case HREF:
                        if (currTag == HtmlTag.A) {
                            String v = getAttrValue(tree);
                            if (v == null || v.isEmpty()) {
                                env.messages.error(HTML, tree, "dc.attr.lacks.value");
                            } else {
                                Matcher m = docRoot.matcher(v);
                                if (m.matches()) {
                                    String rest = m.group(2);
                                    if (!rest.isEmpty())
                                        checkURI(tree, rest);
                                } else {
                                    checkURI(tree, v);
                                }
                            }
                        }
                        break;

                    case VALUE:
                        if (currTag == HtmlTag.LI) {
                            String v = getAttrValue(tree);
                            if (v == null || v.isEmpty()) {
                                env.messages.error(HTML, tree, "dc.attr.lacks.value");
                            } else if (!validNumber.matcher(v).matches()) {
                                env.messages.error(HTML, tree, "dc.attr.not.number");
                            }
                        }
                        break;

                    case BORDER:
                        if (currTag == HtmlTag.TABLE) {
                            String v = getAttrValue(tree);
                            try {
                                if (env.htmlVersion == HtmlVersion.HTML5
                                        && (v == null || (!v.isEmpty() && Integer.parseInt(v) != 1))) {
                                    env.messages.error(HTML, tree, "dc.attr.table.border.html5", attr);
                                }
                            } catch (NumberFormatException ex) {
                                env.messages.error(HTML, tree, "dc.attr.table.border.html5", attr);
                            }
                        }
                        break;
                }
            }
        }

        // TODO: basic check on value

        return super.visitAttribute(tree, ignore);
    }

    private void validateHtml4Attrs(AttributeTree tree, Name name, AttrKind k) {
        switch (k) {
            case ALL:
            case HTML4:
                break;

            case INVALID:
                env.messages.error(HTML, tree, "dc.attr.unknown", name);
                break;

            case OBSOLETE:
                env.messages.warning(ACCESSIBILITY, tree, "dc.attr.obsolete", name);
                break;

            case USE_CSS:
                env.messages.warning(ACCESSIBILITY, tree, "dc.attr.obsolete.use.css", name);
                break;

            case HTML5:
                env.messages.error(HTML, tree, "dc.attr.not.supported.html4", name);
                break;
        }
    }

    private void validateHtml5Attrs(AttributeTree tree, Name name, AttrKind k) {
        switch (k) {
            case ALL:
            case HTML5:
                break;

            case INVALID:
            case OBSOLETE:
            case USE_CSS:
            case HTML4:
                env.messages.error(HTML, tree, "dc.attr.not.supported.html5", name);
                break;
        }
    }

    private boolean checkAnchor(String name) {
        Element e = getEnclosingPackageOrClass(env.currElement);
        if (e == null)
            return true;
        Set<String> set = foundAnchors.get(e);
        if (set == null)
            foundAnchors.put(e, set = new HashSet<>());
        return set.add(name);
    }

    private Element getEnclosingPackageOrClass(Element e) {
        while (e != null) {
            switch (e.getKind()) {
                case CLASS:
                case ENUM:
                case INTERFACE:
                case PACKAGE:
                    return e;
                default:
                    e = e.getEnclosingElement();
            }
        }
        return e;
    }

    // http://www.w3.org/TR/html401/types.html#type-name
    private static final Pattern validName = Pattern.compile("[A-Za-z][A-Za-z0-9-_:.]*");

    private static final Pattern validNumber = Pattern.compile("-?[0-9]+");

    // pattern to remove leading {@docRoot}/?
    private static final Pattern docRoot = Pattern.compile("(?i)(\\{@docRoot *\\}/?)?(.*)");

    private String getAttrValue(AttributeTree tree) {
        if (tree.getValue() == null)
            return null;

        StringWriter sw = new StringWriter();
        try {
            new DocPretty(sw).print(tree.getValue());
        } catch (IOException e) {
            // cannot happen
        }
        // ignore potential use of entities for now
        return sw.toString();
    }

    private void checkURI(AttributeTree tree, String uri) {
        // allow URIs beginning with javascript:, which would otherwise be rejected by the URI API.
        if (uri.startsWith("javascript:"))
            return;
        try {
            URI u = new URI(uri);
        } catch (URISyntaxException e) {
            env.messages.error(HTML, tree, "dc.invalid.uri", uri);
        }
    }
    // </editor-fold>

    // <editor-fold defaultstate="collapsed" desc="javadoc tags">

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitAuthor(AuthorTree tree, Void ignore) {
        warnIfEmpty(tree, tree.getName());
        return super.visitAuthor(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitDocRoot(DocRootTree tree, Void ignore) {
        markEnclosingTag(Flag.HAS_INLINE_TAG);
        return super.visitDocRoot(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitIndex(IndexTree tree, Void ignore) {
        for (TagStackItem tsi : tagStack) {
            if (tsi.tag == HtmlTag.A) {
                env.messages.warning(HTML, tree, "dc.tag.a.within.a",
                        "{@" + tree.getTagName() + "}");
                break;
            }
        }
        return super.visitIndex(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitInheritDoc(InheritDocTree tree, Void ignore) {
        markEnclosingTag(Flag.HAS_INLINE_TAG);
        // TODO: verify on overridden method
        foundInheritDoc = true;
        return super.visitInheritDoc(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitLink(LinkTree tree, Void ignore) {
        markEnclosingTag(Flag.HAS_INLINE_TAG);
        // simulate inline context on tag stack
        HtmlTag t = (tree.getKind() == DocTree.Kind.LINK)
                ? HtmlTag.CODE : HtmlTag.SPAN;
        tagStack.push(new TagStackItem(tree, t));
        try {
            return super.visitLink(tree, ignore);
        } finally {
            tagStack.pop();
        }
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitLiteral(LiteralTree tree, Void ignore) {
        markEnclosingTag(Flag.HAS_INLINE_TAG);
        if (tree.getKind() == DocTree.Kind.CODE) {
            for (TagStackItem tsi: tagStack) {
                if (tsi.tag == HtmlTag.CODE) {
                    env.messages.warning(HTML, tree, "dc.tag.code.within.code");
                    break;
                }
            }
        }
        return super.visitLiteral(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    @SuppressWarnings("fallthrough")
    public Void visitParam(ParamTree tree, Void ignore) {
        boolean typaram = tree.isTypeParameter();
        IdentifierTree nameTree = tree.getName();
        Element paramElement = nameTree != null ? env.trees.getElement(new DocTreePath(getCurrentPath(), nameTree)) : null;

        if (paramElement == null) {
            switch (env.currElement.getKind()) {
                case CLASS: case INTERFACE: {
                    if (!typaram) {
                        env.messages.error(REFERENCE, tree, "dc.invalid.param");
                        break;
                    }
                }
                case METHOD: case CONSTRUCTOR: {
                    env.messages.error(REFERENCE, nameTree, "dc.param.name.not.found");
                    break;
                }

                default:
                    env.messages.error(REFERENCE, tree, "dc.invalid.param");
                    break;
            }
        } else {
            boolean unique = foundParams.add(paramElement);

            if (!unique) {
                env.messages.warning(REFERENCE, tree, "dc.exists.param", nameTree);
            }
        }

        warnIfEmpty(tree, tree.getDescription());
        return super.visitParam(tree, ignore);
    }

    private void checkParamsDocumented(List<? extends Element> list) {
        if (foundInheritDoc)
            return;

        for (Element e: list) {
            if (!foundParams.contains(e)) {
                CharSequence paramName = (e.getKind() == ElementKind.TYPE_PARAMETER)
                        ? "<" + e.getSimpleName() + ">"
                        : e.getSimpleName();
                reportMissing("dc.missing.param", paramName);
            }
        }
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitProvides(ProvidesTree tree, Void ignore) {
        Element e = env.trees.getElement(env.currPath);
        if (e.getKind() != ElementKind.MODULE) {
            env.messages.error(REFERENCE, tree, "dc.invalid.provides");
        }
        ReferenceTree serviceType = tree.getServiceType();
        Element se = env.trees.getElement(new DocTreePath(getCurrentPath(), serviceType));
        if (se == null) {
            env.messages.error(REFERENCE, tree, "dc.service.not.found");
        }
        return super.visitProvides(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitReference(ReferenceTree tree, Void ignore) {
        String sig = tree.getSignature();
        if (sig.contains("<") || sig.contains(">")) {
            env.messages.error(REFERENCE, tree, "dc.type.arg.not.allowed");
        } else {
            Element e = env.trees.getElement(getCurrentPath());
            if (e == null)
                env.messages.error(REFERENCE, tree, "dc.ref.not.found");
        }
        return super.visitReference(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitReturn(ReturnTree tree, Void ignore) {
        if (foundReturn) {
            env.messages.warning(REFERENCE, tree, "dc.exists.return");
        }

        Element e = env.trees.getElement(env.currPath);
        if (e.getKind() != ElementKind.METHOD
                || ((ExecutableElement) e).getReturnType().getKind() == TypeKind.VOID)
            env.messages.error(REFERENCE, tree, "dc.invalid.return");
        foundReturn = true;
        warnIfEmpty(tree, tree.getDescription());
        return super.visitReturn(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitSerialData(SerialDataTree tree, Void ignore) {
        warnIfEmpty(tree, tree.getDescription());
        return super.visitSerialData(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitSerialField(SerialFieldTree tree, Void ignore) {
        warnIfEmpty(tree, tree.getDescription());
        return super.visitSerialField(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitSince(SinceTree tree, Void ignore) {
        warnIfEmpty(tree, tree.getBody());
        return super.visitSince(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitSummary(SummaryTree node, Void aVoid) {
        int idx = env.currDocComment.getFullBody().indexOf(node);
        // Warn if the node is preceded by non-whitespace characters,
        // or other non-text nodes.
        if ((idx == 1 && hasNonWhitespaceText) || idx > 1) {
            env.messages.warning(SYNTAX, node, "dc.invalid.summary", node.getTagName());
        }
        return super.visitSummary(node, aVoid);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitSystemProperty(SystemPropertyTree tree, Void ignore) {
        for (TagStackItem tsi : tagStack) {
            if (tsi.tag == HtmlTag.A) {
                env.messages.warning(HTML, tree, "dc.tag.a.within.a",
                        "{@" + tree.getTagName() + "}");
                break;
            }
        }
        return super.visitSystemProperty(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitThrows(ThrowsTree tree, Void ignore) {
        ReferenceTree exName = tree.getExceptionName();
        Element ex = env.trees.getElement(new DocTreePath(getCurrentPath(), exName));
        if (ex == null) {
            env.messages.error(REFERENCE, tree, "dc.ref.not.found");
        } else if (isThrowable(ex.asType())) {
            switch (env.currElement.getKind()) {
                case CONSTRUCTOR:
                case METHOD:
                    if (isCheckedException(ex.asType())) {
                        ExecutableElement ee = (ExecutableElement) env.currElement;
                        checkThrowsDeclared(exName, ex.asType(), ee.getThrownTypes());
                    }
                    break;
                default:
                    env.messages.error(REFERENCE, tree, "dc.invalid.throws");
            }
        } else {
            env.messages.error(REFERENCE, tree, "dc.invalid.throws");
        }
        warnIfEmpty(tree, tree.getDescription());
        return scan(tree.getDescription(), ignore);
    }

    private boolean isThrowable(TypeMirror tm) {
        switch (tm.getKind()) {
            case DECLARED:
            case TYPEVAR:
                return env.types.isAssignable(tm, env.java_lang_Throwable);
        }
        return false;
    }

    private void checkThrowsDeclared(ReferenceTree tree, TypeMirror t, List<? extends TypeMirror> list) {
        boolean found = false;
        for (TypeMirror tl : list) {
            if (env.types.isAssignable(t, tl)) {
                foundThrows.add(tl);
                found = true;
            }
        }
        if (!found)
            env.messages.error(REFERENCE, tree, "dc.exception.not.thrown", t);
    }

    private void checkThrowsDocumented(List<? extends TypeMirror> list) {
        if (foundInheritDoc)
            return;

        for (TypeMirror tl: list) {
            if (isCheckedException(tl) && !foundThrows.contains(tl))
                reportMissing("dc.missing.throws", tl);
        }
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitUnknownBlockTag(UnknownBlockTagTree tree, Void ignore) {
        checkUnknownTag(tree, tree.getTagName());
        return super.visitUnknownBlockTag(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitUnknownInlineTag(UnknownInlineTagTree tree, Void ignore) {
        checkUnknownTag(tree, tree.getTagName());
        return super.visitUnknownInlineTag(tree, ignore);
    }

    private void checkUnknownTag(DocTree tree, String tagName) {
        if (env.customTags != null && !env.customTags.contains(tagName))
            env.messages.error(SYNTAX, tree, "dc.tag.unknown", tagName);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitUses(UsesTree tree, Void ignore) {
        Element e = env.trees.getElement(env.currPath);
        if (e.getKind() != ElementKind.MODULE) {
            env.messages.error(REFERENCE, tree, "dc.invalid.uses");
        }
        ReferenceTree serviceType = tree.getServiceType();
        Element se = env.trees.getElement(new DocTreePath(getCurrentPath(), serviceType));
        if (se == null) {
            env.messages.error(REFERENCE, tree, "dc.service.not.found");
        }
        return super.visitUses(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitValue(ValueTree tree, Void ignore) {
        ReferenceTree ref = tree.getReference();
        if (ref == null || ref.getSignature().isEmpty()) {
            if (!isConstant(env.currElement))
                env.messages.error(REFERENCE, tree, "dc.value.not.allowed.here");
        } else {
            Element e = env.trees.getElement(new DocTreePath(getCurrentPath(), ref));
            if (!isConstant(e))
                env.messages.error(REFERENCE, tree, "dc.value.not.a.constant");
        }

        markEnclosingTag(Flag.HAS_INLINE_TAG);
        return super.visitValue(tree, ignore);
    }

    private boolean isConstant(Element e) {
        if (e == null)
            return false;

        switch (e.getKind()) {
            case FIELD:
                Object value = ((VariableElement) e).getConstantValue();
                return (value != null); // can't distinguish "not a constant" from "constant is null"
            default:
                return false;
        }
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitVersion(VersionTree tree, Void ignore) {
        warnIfEmpty(tree, tree.getBody());
        return super.visitVersion(tree, ignore);
    }

    @Override @DefinedBy(Api.COMPILER_TREE)
    public Void visitErroneous(ErroneousTree tree, Void ignore) {
        env.messages.error(SYNTAX, tree, null, tree.getDiagnostic().getMessage(null));
        return null;
    }
    // </editor-fold>

    // <editor-fold defaultstate="collapsed" desc="Utility methods">

    private boolean isCheckedException(TypeMirror t) {
        return !(env.types.isAssignable(t, env.java_lang_Error)
                || env.types.isAssignable(t, env.java_lang_RuntimeException));
    }

    private boolean isSynthetic() {
        switch (env.currElement.getKind()) {
            case CONSTRUCTOR:
                // A synthetic default constructor has the same pos as the
                // enclosing class
                TreePath p = env.currPath;
                return env.getPos(p) == env.getPos(p.getParentPath());
        }
        return false;
    }

    void markEnclosingTag(Flag flag) {
        TagStackItem top = tagStack.peek();
        if (top != null)
            top.flags.add(flag);
    }

    String toString(TreePath p) {
        StringBuilder sb = new StringBuilder("TreePath[");
        toString(p, sb);
        sb.append("]");
        return sb.toString();
    }

    void toString(TreePath p, StringBuilder sb) {
        TreePath parent = p.getParentPath();
        if (parent != null) {
            toString(parent, sb);
            sb.append(",");
        }
       sb.append(p.getLeaf().getKind()).append(":").append(env.getPos(p)).append(":S").append(env.getStartPos(p));
    }

    void warnIfEmpty(DocTree tree, List<? extends DocTree> list) {
        for (DocTree d: list) {
            switch (d.getKind()) {
                case TEXT:
                    if (hasNonWhitespace((TextTree) d))
                        return;
                    break;
                default:
                    return;
            }
        }
        env.messages.warning(SYNTAX, tree, "dc.empty", tree.getKind().tagName);
    }

    boolean hasNonWhitespace(TextTree tree) {
        String s = tree.getBody();
        for (int i = 0; i < s.length(); i++) {
            Character c = s.charAt(i);
            if (!Character.isWhitespace(s.charAt(i)))
                return true;
        }
        return false;
    }

    // </editor-fold>

}