/**

 * Back 2 Browser Bytecode Translator

 * Copyright (C) 2012-2015 Jaroslav Tulach <jaroslav.tulach@apidesign.org>

 *

 * This program is free software: you can redistribute it and/or modify

 * it under the terms of the GNU General Public License as published by

 * the Free Software Foundation, version 2 of the License.

 *

 * This program 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 for more details.

 *

 * You should have received a copy of the GNU General Public License

 * along with this program. Look for COPYING file in the top folder.

 * If not, see http://opensource.org/licenses/GPL-2.0.

 */

package org.apidesign.vm4brwsr;

import java.io.IOException;

import java.io.InputStream;

import org.apidesign.bck2brwsr.core.JavaScriptBody;

import org.apidesign.vm4brwsr.ByteCodeParser.ClassData;

import org.apidesign.vm4brwsr.ByteCodeParser.FieldData;

import org.apidesign.vm4brwsr.ByteCodeParser.MethodData;

/** Generator of JavaScript from bytecode of classes on classpath of the VM.

 *

 * @author Jaroslav Tulach <jtulach@netbeans.org>

 */

abstract class VM extends ByteCodeToJavaScript {

    protected final ClassDataCache classDataCache;

    private final Bck2Brwsr.Resources resources;

    private final ExportedSymbols exportedSymbols;

    private final StringBuilder invokerMethods;

    private final StringArray asBinary;

    int exportedCount;

    private VM(

        Appendable out, Bck2Brwsr.Resources resources, 

        StringArray explicitlyExported, StringArray asBinary

    ) {

        super(out);

        this.resources = resources;

        this.classDataCache = new ClassDataCache(resources);

        this.exportedSymbols = new ExportedSymbols(resources, explicitlyExported);

        this.invokerMethods = new StringBuilder();

        this.asBinary = asBinary;

    }

    static {

        // uses VMLazy to load dynamic classes

        boolean assertsOn = false;

        assert assertsOn = true;

        if (assertsOn) {

            VMLazy.init();

            ClassPath.init();

        }

    }

    @Override

    boolean debug(String msg) throws IOException {

        return false;

    }

    static void compile(Appendable out, 

        Bck2Brwsr config

    ) throws IOException {

        String[] both = config.classes().toArray();

        final StringArray fixedNames = new StringArray();

        fixedNames.add(Object.class.getName().replace('.', '/'));

        fixedNames.add(Class.class.getName().replace('.', '/'));

        fixedNames.add(ArithmeticException.class.getName().replace('.', '/'));

        VM vm;

        if (config.isExtension()) {

            fixedNames.add(VM.class.getName().replace('.', '/'));

            vm = new Extension(out, 

                config.getResources(), both, config.exported(),

                config.allResources(), config.classpath()

            );

        } else {

            if (config.includeVM()) {

                fixedNames.add(VM.class.getName().replace('.', '/'));

            }

            vm = new Standalone(out, 

                config.getResources(), config.exported(),

                config.allResources()

            );

        }            

        vm.doCompile(fixedNames.addAndNew(both));

    }

    private void doCompile(StringArray names) throws IOException {

        generatePrologue();

        append(

                "\n  var invoker = {};");

        generateBody(names);

        append(invokerMethods);

        for (String r : asBinary.toArray()) {

            append("\n  ").append(getExportsObject()).append("['registerResource']('");

            append(r).append("', '");

            InputStream is = this.resources.get(r);

            byte[] arr = new byte[is.available()];

            int offset = 0;

            for (;;) {

                if (offset == arr.length) {

                    byte[] tmp = new byte[arr.length * 2];

                    System.arraycopy(arr, 0, tmp, 0, arr.length);

                    arr = tmp;

                }

                int len = is.read(arr, offset, arr.length - offset);

                if (len == -1) {

                    break;

                }

                offset += len;

            }

            if (offset != arr.length) {

                byte[] tmp = new byte[offset];

                System.arraycopy(arr, 0, tmp, 0, offset);

                arr = tmp;

            }

            append(btoa(arr));

            append("');");

        }

        append("\n");

        generateEpilogue();

    }

    @JavaScriptBody(args = { "arr" }, body = "return btoa(arr);")

    private static String btoa(byte[] arr) {

        return javax.xml.bind.DatatypeConverter.printBase64Binary(arr);

    }

    protected abstract void generatePrologue() throws IOException;

    protected abstract void generateEpilogue() throws IOException;

    protected abstract String getExportsObject();

    protected abstract boolean isExternalClass(String className);

    protected abstract void lazyReference(Appendable out, String n) throws IOException;

    @Override

    protected final void declaredClass(ClassData classData, String mangledName)

            throws IOException {

        if (exportedSymbols.isExported(classData)) {

            append("\n").append(getExportsObject()).append("['")

                                               .append(mangledName)

                                               .append("'] = ")

                            .append(accessClass(mangledName))

               .append(";\n");

            exportedCount++;

        }

    }

    protected String generateClass(String className) throws IOException {

        ClassData classData = classDataCache.getClassData(className);

        if (classData == null) {

            throw new IOException("Can't find class " + className);

        }

        return compile(classData);

    }

    @Override

    protected void declaredField(FieldData fieldData,

                                 String destObject,

                                 String mangledName) throws IOException {

        if (exportedSymbols.isExported(fieldData)) {

            exportMember(destObject, mangledName);

        }

    }

    @Override

    protected void declaredMethod(MethodData methodData,

                                  String destObject,

                                  String mangledName) throws IOException {

        if (isHierarchyExported(methodData)) {

            exportMember(destObject, mangledName);

        }

    }

    private void exportMember(String destObject, String memberName)

            throws IOException {

        append("\n").append(destObject).append("['")

        .append(memberName)

        .append("'] = m;\n");

    }

    private void generateBody(StringArray names) throws IOException {

        StringArray processed = new StringArray();

        StringArray initCode = new StringArray();

        StringArray skipClass = new StringArray();

        for (String baseClass : names.toArray()) {

            references.add(baseClass);

            for (;;) {

                String name = null;

                for (String n : references.toArray()) {

                    if (skipClass.contains(n)) {

                        continue;

                    }

                    if (processed.contains(n)) {

                        continue;

                    }

                    name = n;

                }

                if (name == null) {

                    break;

                }

                InputStream is = resources.get(name + ".class");

                if (is == null) {

                    lazyReference(this, name);

                    skipClass.add(name);

                    continue;

                }

                try {

                    String ic = generateClass(name);

                    processed.add(name);

                    initCode.add(ic == null ? "" : ic);

                } catch (RuntimeException ex) {

                    throw new IOException("Error while compiling " + name + "\n", ex);

                }

            }

            for (String resource : scripts.toArray()) {

                while (resource.startsWith("/")) {

                    resource = resource.substring(1);

                }

                requireResourceImpl(resource);

                asBinary.remove(resource);

            }

            scripts = new StringArray();

            StringArray toInit = StringArray.asList(references.toArray());

            toInit.reverse();

            for (String ic : toInit.toArray()) {

                int indx = processed.indexOf(ic);

                if (indx >= 0) {

                    final String theCode = initCode.toArray()[indx];

                    if (!theCode.isEmpty()) {

                        append(theCode).append("\n");

                    }

                    initCode.toArray()[indx] = "";

                }

            }

        }

    }

    final void requireResourceImpl(String resource) throws IOException {

        InputStream emul = resources.get(resource);

        if (emul == null) {

            throw new IOException("Can't find " + resource);

        }

        append("\n// resource from ").append(resource).append("\n");

        readResource(emul, this);

        append("\n");

    }

    private static void readResource(InputStream emul, Appendable out) throws IOException {

        try {

            int state = 0;

            for (;;) {

                int ch = emul.read();

                if (ch == -1) {

                    break;

                }

                if (ch < 0 || ch > 255) {

                    throw new IOException("Invalid char in emulation " + ch);

                }

                out.append((char)ch);

            }

        } finally {

            emul.close();

        }

    }

    static String toString(String name) throws IOException {

        StringBuilder sb = new StringBuilder();

//        compile(sb, name);

        return sb.toString().toString();

    }

    private StringArray scripts = new StringArray();

    private StringArray references = new StringArray();

    @Override

    protected boolean requireReference(String cn) {

        return references.addIfMissing(cn);

    }

    @Override

    protected void requireScript(String resourcePath) {

        scripts.add(resourcePath);

    }

    @Override

    String assignClass(String className) {

        return "vm." + className + " = ";

    }

    @Override

    String accessClass(String className) {

        return "vm." + className;

    }

    @Override

    protected String accessField(String object, String mangledName,

                                 String[] fieldInfoName) throws IOException {

        final FieldData field =

                classDataCache.findField(fieldInfoName[0],

                                         fieldInfoName[1],

                                         fieldInfoName[2]);

        return accessNonVirtualMember(object, mangledName,

                                      (field != null) ? field.cls : null);

    }

    @Override

    protected String accessStaticMethod(

                             String object,

                             String mangledName,

                             String[] fieldInfoName) throws IOException {

        final MethodData method =

                classDataCache.findMethod(fieldInfoName[0],

                                          fieldInfoName[1],

                                          fieldInfoName[2]);

        return accessNonVirtualMember(object, mangledName,

                                      (method != null) ? method.cls : null);

    }

    @Override

    protected String accessVirtualMethod(

            String object, 

            String mangledName, 

            String[] fieldInfoName, 

            int params

    ) throws IOException {

        final ClassData referencedClass =

                classDataCache.getClassData(fieldInfoName[0]);

        final MethodData method =

                classDataCache.findMethod(referencedClass,

                                          fieldInfoName[1],

                                          fieldInfoName[2]);

        if ((method != null)

                && !isExternalClass(method.cls.getClassName())

                && (((method.access & ByteCodeParser.ACC_FINAL) != 0)

                        || ((referencedClass.getAccessFlags()

                                 & ByteCodeParser.ACC_FINAL) != 0)

                        || !isHierarchyExported(method))) {

            return object + "." + mangledName + '(';

        }

        return accessThroughInvoker(object, mangledName, params);

    }

    private String accessThroughInvoker(String object, String mangledName, int params) 

    throws IOException {

        String def = "\n  invoker." + mangledName + " = function(target";

        if (invokerMethods.indexOf(def) == -1) {

            invokerMethods.append(def);

            for (int j = 0; j < params; j++) {

                invokerMethods.append(", p").append(j);

            }

            invokerMethods.append(") {\n    return target['").

                append(mangledName).append("'](");

            for (int j = 0; j < params; j++) {

                if (j > 0) {

                    invokerMethods.append(",");

                }

                invokerMethods.append("p").append(j);

            }

            invokerMethods.append(");\n  };");

        }

        return "invoker." + mangledName + '(' + object + (params > 1 ? "," : "");

    }

    private boolean isHierarchyExported(final MethodData methodData)

            throws IOException {

        if (exportedSymbols.isExported(methodData)) {

            return true;

        }

        if ((methodData.access & (ByteCodeParser.ACC_PRIVATE

                                      | ByteCodeParser.ACC_STATIC)) != 0) {

            return false;

        }

        final ExportedMethodFinder exportedMethodFinder =

                new ExportedMethodFinder(exportedSymbols);

        classDataCache.findMethods(

                methodData.cls,

                methodData.getName(),

                methodData.getInternalSig(),

                exportedMethodFinder);

        return (exportedMethodFinder.getFound() != null);

    }

    private String accessNonVirtualMember(String object,

                                          String mangledName,

                                          ClassData declaringClass) {

        if (declaringClass == null) {

            return object + "['" + mangledName + "']";

        }

        final String className = declaringClass.getClassName();

        if (

            "java/lang/Object".equals(className) ||

            "java/lang/reflect/Array".equals(className) ||

            isExternalClass(className)

        ) {

            return object + "['" + mangledName + "']";

        }

        return object + "." + mangledName;

    }

    private final class ExportedMethodFinder

            implements ClassDataCache.TraversalCallback<MethodData> {

        private final ExportedSymbols exportedSymbols;

        private MethodData found;

        public ExportedMethodFinder(final ExportedSymbols exportedSymbols) {

            this.exportedSymbols = exportedSymbols;

        }

        @Override

        public boolean traverse(final MethodData methodData) {

            try {

                if (

                    exportedSymbols.isExported(methodData) ||

                    isExternalClass(methodData.cls.getClassName())

                ) {

                    found = methodData;

                    return false;

                }

            } catch (final IOException e) {

            }

            return true;

        }

        public MethodData getFound() {

            return found;

        }

    }

    private static final class Standalone extends VM {

        private Standalone(Appendable out,

            Bck2Brwsr.Resources resources, 

            StringArray explicitlyExported, StringArray asBinary

        ) {

            super(out, resources, explicitlyExported, asBinary);

        }

        @Override

        protected void generatePrologue() throws IOException {

            append("(function VM(global) {var fillInVMSkeleton = function(vm) {");

        }

        @Override

        protected void generateEpilogue() throws IOException {

            append(

                  "  return vm;\n"

                + "  };\n"

                + "  var extensions = [];\n"

                + "  function replaceAll(s, target, replacement) {\n"

                + "    var pos = 0;\n"

                + "    for (;;) {\n"

                + "      var indx = s.indexOf(target, pos);\n"

                + "      if (indx === -1) {\n"

                + "        return s;\n"

                + "      }\n"

                + "      pos = indx + replacement.length;\n"

                + "      s = s.substring(0, indx) + replacement + s.substring(indx + target.length);\n"

                + "    }\n"

                + "  }\n"

                + "  function mangleClass(name) {\n"

                + "    name = replaceAll(name, '_', '_1');\n"

                + "    name = replaceAll(name, '.', '_');\n"

                + "    return name;\n"

                + "  };\n"

                + "  var pending = [];\n"

                + "  var pendingClasses = [];\n"

                + "  function extensionLoaded(ev) {\n"

                + "    var at = pending.indexOf(ev.target);\n"

                + "    pending.splice(at, 1);\n"

                + "    if (pending.length === 0) {\n"

                + "      for (var i = 0; i < pendingClasses.length; i += 3) {\n"

                + "        invokeMethod(pendingClasses[i], pendingClasses[i + 1], pendingClasses[i + 2]);\n"

                + "      }\n"

                + "      pendingClasses = [];\n"

                + "    }\n"

                + "  }\n"

                + "  function invokeMethod(vm, n, args) {\n"

                + "    var clazz = vm.loadClass(n);\n"

                + "    if (args) {\n"

                + "      var seek = args[0];\n"

                + "      var prefix = seek.indexOf('__') == -1 ? seek + '__' : seek;\n"

                + "      args = Array.prototype.slice.call(args, 1);\n"

                + "      var found = '';\n"

                + "      for (var m in clazz) {\n"

                + "        if (m.indexOf(prefix) === 0) {\n"

                + "          return clazz[m].apply(null, args);\n"

                + "        }\n"

                + "        found += m.toString() + '\\n'\n"

                + "      }\n"

                + "      throw 'Cannot find ' + seek + ' in ' + n + ' found:\\n' + found;\n"

                + "    }\n"

                + "  }\n"

                + "  function extensionError(ev) {\n"

                + "    console.log('error loading ' + ev.target.src);\n"

                + "    extensionLoaded(ev);\n"

                + "  }\n"

                + "  function loadExtension(url) {\n"

                + "      if (url.substring(url.length - 4) == '.jar')\n"

                + "        url = url.substring(0, url.length - 4) + '.js';\n"

                + "      var script = document.createElement('script');\n"

                + "      script.type = 'text/javascript';\n"

                + "      script.src = url;\n"

                + "      script.onload = extensionLoaded;\n"

                + "      script.onerror = extensionError;\n"

                + "      document.getElementsByTagName('head')[0].appendChild(script);\n"

                + "      pending.push(script);\n"

                + "  }\n"

                + "  global.bck2brwsr = function() {\n"

                + "    var args = Array.prototype.slice.apply(arguments);\n"

                + "    var resources = {};\n"

                + "    function registerResource(n, a64) {\n"

                + "      var frm = atob || window.atob;\n"

                + "      var str = frm(a64);\n"

                + "      var arr = [];\n"

                + "      for (var i = 0; i < str.length; i++) {\n"

                + "        var ch = str.charCodeAt(i) & 0xff;\n"

                + "        if (ch > 127) ch -= 256;\n"

                + "        arr.push(ch);\n"

                + "      }\n"

                + "      if (!resources[n]) resources[n] = [arr];\n"

                + "      else resources[n].push(arr);\n"

                + "    }\n"

                + "    var vm = fillInVMSkeleton({ 'registerResource' : registerResource });\n"

                + "    function initVM() {\n"

                + "      var clsArray = vm['java_lang_reflect_Array'];\n"

                + "      if (clsArray) clsArray(false);\n"

                + "    }\n"

                + "    for (var i = 0; i < extensions.length; ++i) {\n"

                + "      extensions[i](vm);\n"

                + "    }\n"

                + "    vm['registerResource'] = null;\n"

                + "    var knownExtensions = extensions.length;\n"

                + "    var loader = {};\n"

                + "    var loadBytes = function(name, skip) {\n"

                + "      skip = typeof skip == 'number' ? skip : 0;\n"

                + "      var arr = resources[name];\n"

                + "      if (arr) {\n"

                + "        var arrSize = arr.length;\n"

                + "        if (skip < arrSize) return arr[skip];\n"

                + "        skip -= arrSize;\n"

                + "      } else {\n"

                + "        var arrSize = 0;\n"

                + "      };\n"

                + "      for (var i = 0; i < args.length; i++) {\n"

                + "        var at = args[i];\n"

                + "        if(!at) continue;\n"

                + "        var ret;\n"

                + "        if (typeof at === 'string' && at.substring(at.length - 3) === '.js') {\n"

                + "          loadExtension(at);\n"

                + "          args[i] = null;\n"

                + "        } else if (typeof at === 'function') ret = at(name, skip);\n"

                + "        else {\n"

                + "          var cp = vm['org_apidesign_vm4brwsr_ClassPath'];\n"

                + "          if (!cp) throw 'Core Java library not registered. Cannot load from ' + at;\n"

                + "          ret = cp(false).\n"

                + "            loadBytes___3BLjava_lang_String_2Ljava_lang_Object_2II(name, args, i, skip);\n"

                + "        }\n"

                + "        if (ret) return ret;\n"

                + "      }\n"

                + "      while (knownExtensions < extensions.length) {\n"

                + "        vm['registerResource'] = registerResource;\n"

                + "        extensions[knownExtensions++](vm);\n"

                + "        vm['registerResource'] = null;\n"

                + "        initVM();\n"

                + "      }\n"

                + "      var arr = resources[name];\n"

                + "      return (arr && arr.length > arrSize) ? arr[arrSize] : null;\n"

                + "    }\n"

                + "    var reload = function(name, arr, keep) {\n"

                + "      if (!arr) throw 'Cannot find ' + name;\n"

                + "      var lazy = vm['org_apidesign_vm4brwsr_VMLazy'];\n"

                + "      if (!lazy) throw 'No bck2brwsr VM module to compile ' + name;\n"

                + "      if (!keep) {\n"

                + "        var attr = mangleClass(name);\n"

                + "        delete vm[attr];\n"

                + "      }\n"

                + "      return lazy(false)\n"

                + "        ['load__Ljava_lang_Object_2Ljava_lang_Object_2Ljava_lang_String_2_3Ljava_lang_Object_2_3B']\n"

                + "        (vm, name, args, arr);\n"

                + "    };\n"

                + "    var loadClass = function(name) {\n"

                + "      var attr = mangleClass(name);\n"

                + "      var fn = vm[attr];\n"

                + "      if (fn) return fn(false);\n"

                + "      try {\n"

                + "        var arr = loadBytes(replaceAll(name, '.', '/') + '.class');\n"

                + "        return reload(name, arr, true);\n"

                + "      } catch (err) {\n"

                + "        fn = vm[attr];\n"

                + "        if (fn) return fn(false);\n"

                + "        throw err;\n"

                + "      }\n"

                + "    }\n"

                + "    if (vm['loadClass']) {\n"

                + "      throw 'Cannot initialize the bck2brwsr VM twice!';\n"

                + "    }\n"

                + "    vm['loadClass'] = loadClass;\n"

                + "    vm['_reload'] = reload;\n"

                + "    vm['loadBytes'] = loadBytes;\n"

                + "    initVM();\n"

                + "    loader.loadClass = function(name) {\n"

                + "      if (pending.length === 0) {\n"

                + "        try {\n"

                + "          var c = loadClass(name);\n"

                + "          c['invoke'] = function() {\n"

                + "            return invokeMethod(vm, name, arguments);\n"

                + "          };\n"

                + "          return c;\n"

                + "        } catch (err) {\n"

                + "          if (pending.length === 0) throw err;\n"

                + "        }\n"

                + "      }\n"

                + "      pendingClasses.push(vm);\n"

                + "      pendingClasses.push(name);\n"

                + "      pendingClasses.push(null);\n"

                + "      return {\n"

                + "        'invoke' : function() {\n"

                + "          if (pending.length === 0) {\n"

                + "            invokeMethod(vm, name, arguments);\n"

                + "            return;\n"

                + "          }\n"

                + "          pendingClasses.push(vm);\n"

                + "          pendingClasses.push(name);\n"

                + "          pendingClasses.push(arguments);\n"

                + "        }\n"

                + "      };\n"

                + "    }\n"

                + "    return loader;\n"

                + "  };\n");

            append(

                  "  global.bck2brwsr.register = function(config, extension) {\n"

                + "    if (!config || config['magic'] !== 'kafíčko') {\n"

                + "      console.log('Will not register: ' + extension);\n"

                + "      return false;\n"

                + "    }\n"

                + "    extensions.push(extension);\n"

                + "    var cp = config['classpath'];\n"

                + "    if (cp) for (var i = 0; i < cp.length; i++) {\n"

                + "      loadExtension(cp[i]);\n"

                + "    }\n"

                + "    return null;\n"

                + "  };\n");

            append("}(this));");

        }

        @Override

        protected String getExportsObject() {

            return "vm";

        }

        @Override

        protected boolean isExternalClass(String className) {

            return false;

        }

        @Override

        protected void lazyReference(Appendable out, String n) throws IOException {

            String cls = n.replace('/', '_');

            String dot = n.replace('/', '.');

            out.append("\nvm.").append(cls).append(" = function() {");

            out.append("\n  var instance = arguments.length == 0 || arguments[0] === true;");

            out.append("\n  delete vm.").append(cls).append(";");

            out.append("\n  var c = vm.loadClass('").append(dot).append("');");

            out.append("\n  return vm.").append(cls).append("(instance);");

            out.append("\n}");

        }

        @Override

        protected void requireResource(String resourcePath) throws IOException {

            requireResourceImpl(resourcePath);

        }

    }

    private static final class Extension extends VM {

        private final StringArray extensionClasses;

        private final StringArray classpath;

        private Extension(Appendable out, Bck2Brwsr.Resources resources,

            String[] extClassesArray, StringArray explicitlyExported,

            StringArray asBinary, StringArray classpath

        ) throws IOException {

            super(out, resources, explicitlyExported, asBinary);

            this.extensionClasses = StringArray.asList(extClassesArray);

            this.classpath = classpath;

        }

        @Override

        protected void generatePrologue() throws IOException {

            append(

                  "bck2brwsr.register({\n"

                + "  'magic' : 'kafíčko'"

            );

            if (classpath != null && classpath.toArray().length > 0) {

                append(

                  ",\n  'classpath' : [\n"

                );

                String sep = "    ";

                for (String s : classpath.toArray()) {

                    append(sep).append("'").append(s).append("'");

                    sep = ",\n    ";

                }

                append(

                  "\n  ]"

                );

            }

            append(

                  "\n}, function(exports) {\n"

                + "  var vm = {};\n");

            append("  function link(n, assign) {\n"

                + "    return function() {\n"

                + "      var cls = n['replace__Ljava_lang_String_2CC']"

                                       + "('/', '_').toString();\n"

                + "      var dot = n['replace__Ljava_lang_String_2CC']"

                                       + "('/', '.').toString();\n"

                + "      exports.loadClass(dot);\n"

                + "      assign(exports[cls]);\n"

                + "      return exports[cls](arguments);\n"

                + "    };\n"

                + "  };\n"

            );

        }

        @Override

        protected void generateEpilogue() throws IOException {

            append("});");

            if (exportedCount == 0) {

                throw new IOException("Creating library without any exported symbols is useless!");

            }

        }

        @Override

        protected String generateClass(String className) throws IOException {

            if (isExternalClass(className)) {

                final String cls = className.replace('/', '_');

                append("\n").append(assignClass(cls))

                   .append("link('")

                   .append(className)

                   .append("', function(f) { ").append(assignClass(cls)).append(" f; });");

                return null;

            }

            return super.generateClass(className);

        }

        @Override

        protected String getExportsObject() {

            return "exports";

        }

        @Override

        protected boolean isExternalClass(String className) {

            return !extensionClasses.contains(className);

        }

        @Override

        protected void lazyReference(Appendable out, String n) throws IOException {

            String cls = n.replace('/', '_');

            out.append("\nvm.").append(cls).append(" = function() {");

            out.append("\n  var instance = arguments.length == 0 || arguments[0] === true;");

            out.append("\n  delete vm.").append(cls).append(";");

            out.append("\n  return link('").append(n).append("', function(f) { vm.");

            out.append(cls).append(" = f;})(instance);");

            out.append("\n}");

        }

        @Override

        protected void requireResource(String resourcePath) throws IOException {

            requireResourceImpl(resourcePath);

        }

    }

}