package uk.ac.starlink.ttools.taplint;

import java.io.BufferedInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import javax.xml.XMLConstants;
import javax.xml.transform.sax.SAXResult;
import javax.xml.transform.sax.SAXSource;
import javax.xml.validation.SchemaFactory;
import javax.xml.validation.Validator;
import org.xml.sax.Attributes;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

/**
 * Methods to perform validation against XSD schemas.
 * This is intended for use where the schemas are known by the
 * IvoaSchemaResolver.
 *
 * @author   Mark Taylor
 * @since    24 Sep 2018
 */
public class XsdValidation {

    /**
     * Validates a given document XML document against its declared schema.
     * If an expected top element name is supplied, a check is also made
     * that the top element of the validated document matches it.
     *
     * <p>Getting this right seems to be remarkably painful.
     * See comments in the implementation for details.
     *
     * @param  reporter  destination for validation messages
     * @param  docUrl   URL of XML document to validate
     * @param  topElName   expected name for top element; may be null
     * @param  topElNamespaceUri  expected namespace for top element;
     *                            only used for reporting messages,
     *                            and ignored if topElName is null
     * @param  includeSummary  if true, report an end-of-stage type summary
     * @return  validation result
     */
    public static Result validateDoc( Reporter reporter, URL docUrl,
                                      String topElName,
                                      String topElNamespaceUri,
                                      boolean includeSummary ) {

        /* Open the document. */
        final InputStream docStream;
        try {
            docStream = docUrl.openStream();
        }
        catch ( FileNotFoundException e ) {
            return Result.NOT_FOUND;
        }
        catch ( IOException e ) {
            reporter.report( FixedCode.E_DCER, "Error reading document", e );
            return Result.FAILURE;
        }

        /* Prepare a validator.  We use a schema factory with no schema
         * sources here (SchemaFactory.newSchema()), which means that
         * any required schemas are read following location hints
         * in the document itself (schemaLocation attributes).
         * The alternative (which used to be here) would be to feed a
         * hardwired schema URL to the schema factory.  The disadvantage of
         * that is that the validator then does not pick up any schemas
         * corresponding to namespaces appearing in the document which
         * have not appeared in the hard-wired schema.  There doesn't
         * appear to be any way to mix and match (feed it a hard-wired
         * schema but also get it to pick up new ones as required).
         * The reason we might want both is because it can be legitimate
         * to include items from a non-IVOA namespace in validated
         * documents, for instance <xs:anyAttribute namespace="##other"/>
         * attributes from VODataService 1.1. */
        final Validator val;
        try {
            val = SchemaFactory
                 .newInstance( XMLConstants.W3C_XML_SCHEMA_NS_URI )
                 .newSchema()
                 .newValidator();
        }
        catch ( SAXException e ) {
            reporter.report( FixedCode.F_XVAL, "Can't prepare validator", e );
            return Result.FAILURE;
        }

        /* Given that we have not fed a hardwired schema to the validator,
         * it's possible that the document is using a non-standard version
         * of the schema for IVOA namespaces.  To ensure that is not the
         * case, install our own ResourceResolver so that given namespaces
         * are reliably mapped to the official locations.
         * I played around with this quite a bit, and this (installing an
         * org.w3c.dom.ls.LSResourceResolver on the validator) seems to be the
         * only way to get it to work.  It would be simpler to just install
         * an org.xml.sax.EntityResolver on a SAX parser and do a validating
         * parse, but in that case the entity resolver doesn't seem to
         * get invoked for schema entities.  Even like this, it only seems
         * to work for java6 and greater (I'm using Oracle J2SE). */
        IvoaSchemaResolver resolver = new IvoaSchemaResolver( reporter );
        val.setResourceResolver( resolver );

        /* Install a reporting error handler on the validator. */
        ReporterErrorHandler errHandler = new ReporterErrorHandler( reporter );
        val.setErrorHandler( errHandler );

        /* Perform the validation, passing the validated output to a custom
         * SAX handler that records the top-level document element. */
        TopElementHandler topelHandler = new TopElementHandler();
        try {
            val.validate( new SAXSource(
                              new InputSource(
                                  new BufferedInputStream( docStream ) ) ),
                          new SAXResult( topelHandler ) );

            /* If we get this far, the validating parse succeeded.
             * Now check to see what the top-level document element was,
             * to ensure that the document is not only valid, but contains
             * the kind of structure we're expecting for this document.
             * I did experiment with testing whether the validated
             * namespace of the top-level element matched the expected one.
             * But frankly, I don't understand under what circumstances
             * the namespaces are supposed to match, so drop that.
             * In principle this means that the document could pass the
             * checks by having a similarly-named element with a completely
             * non-standard definition, but even in that case you'd get
             * a warning that a non-standard schema was being read.
             * I'm not even 100% certain that the top-level element local
             * names have to match, but I think they do. */
            if ( topElName != null &&
                 ! topElName.equals( topelHandler.topName_ ) ) {
                String msg = new StringBuffer()
                   .append( "Wrong top-level element: " )
                   .append( "{" )
                   .append( topelHandler.topUri_ )
                   .append( "}" )
                   .append( topelHandler.topName_ )
                   .append( " != " )
                   .append( "{" )
                   .append( topElNamespaceUri )
                   .append( "}" )
                   .append( topElName )
                   .toString();
                reporter.report( FixedCode.E_ELDF, msg );
            }

            /* Check that the resolver was actually asked to resolve some
             * schemas.  If it wasn't (resolvedCount==0) there are two
             * likely explanations, either
             * (1) the validated document uses some completely non-standard
             *     namespace(s) (though it might just be misspelt etc)
             * (2) the resolution mechanism was not getting invoked by the
             *     validator
             * If it's (1) I think that's an error in the document, though
             * I'm not entirely sure about the legitimacy of using alternative
             * namespaces for the same schema types.
             * But it might be (2) depending on the details of the XSD
             * validator implementation.  In my tests, Oracle J2SE5 doesn't
             * invoke the entity resolver in a way that makes this work,
             * but java 6 and 7 do.  
             * It looks to me like a bug or misfeature for this to happen,
             * but I don't see JAXP documentation detailed enough to
             * say that's definitely the case.  Java 5 is pretty ancient
             * though, so it's probably not a serious problem. */
            if ( resolver.getResolvedCount() == 0 ) {
                String msg = new StringBuffer()
                    .append( "No resources from known namespaces resolved" )
                    .append( "; resolver not used??" )
                    .append( " (can happen for JRE version<=5)" )
                    .toString();
                reporter.report( FixedCode.W_ZRES, msg );
            }
            return Result.SUCCESS;
        }

        /* IO error during parsing. */
        catch ( IOException e ) {
            reporter.report( FixedCode.E_IOER,
                             "Error reading document to parse" );
            return Result.FAILURE;
        }

        /* The validator should only throw a SAX exception in case of
         * a fatal parse error. */
        catch ( SAXException e ) {
            if ( errHandler.getFatalCount() == 0 ) {
                reporter.report( FixedCode.F_SXER,
                                 "Unexpected document parse error", e );
                return Result.SUCCESS;
            }
            return Result.FAILURE;
        }

        /* Summarise results. */
        finally {
            if ( includeSummary ) {
                if ( reporter instanceof OutputReporter ) {
                    OutputReporter orep = (OutputReporter) reporter;
                    orep.summariseUnreportedMessages( orep.getSectionCode() );
                }
                reporter.report( FixedCode.S_VALI, errHandler.getSummary() );
            }
        }
    }

    /**
     * Enumerates possible results of the parse.
     */
    public static enum Result {

        /** The document did not exist (for instance an HTTP 404). */
        NOT_FOUND,

        /** A serious error prevented the validation from completing. */
        FAILURE,

        /** The validation completed, possibly with some errors or warnings. */
        SUCCESS;
    }

    /**
     * SAX content handler that records the top-level element of the
     * document it is parsing.
     */
    private static class TopElementHandler extends DefaultHandler {
        private boolean isTop_;
        String topUri_;
        String topName_;
        @Override
        public void startElement( String uri, String localName,
                                  String qName, Attributes atts ) {
            if ( ! isTop_ ) {
                isTop_ = true;
                topUri_ = uri;
                topName_ = localName;
            }
        }
    }

    /**
     * Main method.
     * Usage: &lt;url-to-validate&gt; [&lt;expected-top-level-element&gt;].
     */
    public static void main( String[] args ) throws IOException {
        URL url = new URL( args[ 0 ] );
        String topElName = args.length > 1 ? args[ 1 ] : null;
        Reporter reporter =
            new TextOutputReporter( System.out, ReportType.values(),
                                    10, false, 1024 );
        Result result = validateDoc( reporter, url, topElName, "??", false );
        System.out.println( result );
    }
}
