/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.juneau.svl;

import java.io.*;

import org.apache.juneau.common.utils.*;

/**
 * Abstract superclass of all Simple Var Language variables.
 *
 * <p>
 * Vars are used to convert simple variables of the form <js>"$varName{varKey}"</js> into something else by the
 * {@link VarResolver} class.
 *
 * <p>
 * Subclasses must implement one of the following two methods:
 * <ul class='javatree'>
 *    <li class='jm'>{@link #resolve(VarResolverSession,String)} - For simple vars.
 *    <li class='jm'>{@link #resolveTo(VarResolverSession,Writer,String)} - For streamed vars.
 * </ul>
 *
 * <p>
 * Subclasses MUST implement a no-arg constructor so that class names can be passed to the
 * {@link VarResolver.Builder#vars(Class...)} method.
 * <br><b>They must also be thread safe!</b>
 *
 * <p>
 * Two direct abstract subclasses are provided to differentiated between simple and streamed vars:
 * <ul class='javatree'>
 *    <li class='jac'>{@link SimpleVar}
 *    <li class='jac'>{@link StreamedVar}
 * </ul>
 *
 * <h5 class='section'>See Also:</h5><ul>
 *    <li class='link'><a class="doclink" href="https://juneau.apache.org/docs/topics/SimpleVariableLanguageBasics">Simple Variable Language Basics</a>

 * </ul>
 */
public abstract class Var {

   private final String name;
   final boolean streamed;

   /**
    * Constructor.
    *
    * @param name The name of this variable.
    * @param streamed
    *    Whether this variable is 'streamed', meaning the {@link #resolveTo(VarResolverSession, Writer, String)} method
    *    is implemented.
    *    If <jk>false</jk>, then the {@link #resolve(VarResolverSession, String)} method is implemented.
    */
   public Var(String name, boolean streamed) {
      Utils.assertArgNotNull("name", name);
      this.name = name;
      this.streamed = streamed;

      for (int i = 0; i < name.length(); i++) {
         // Need to make sure only ASCII characters are used.
         char c = name.charAt(i);
         if (c < 'A' || c > 'z' || (c > 'Z' && c < 'a'))
            throw new IllegalArgumentException("Invalid var name.  Must consist of only uppercase and lowercase ASCII letters.");
      }
   }

   /**
    * Return the name of this variable.
    *
    * <p>
    * For example, the system property variable returns <js>"S"</js> since the format of the variable is
    * <js>"$S{system.property}"</js>.
    *
    * @return The name of this variable.
    */
   protected String getName() {
      return name;
   }

   /**
    * Returns whether nested variables are supported by this variable.
    *
    * <p>
    * For example, in <js>"$X{$Y{xxx}}"</js>, $Y is a nested variable that will be resolved if this method returns
    * <jk>true</jk>.
    *
    * <p>
    * The default implementation of this method always returns <jk>true</jk>.
    * Subclasses can override this method to override the default behavior.
    *
    * @return <jk>true</jk> if nested variables are supported by this variable.
    */
   protected boolean allowNested() {
      return true;
   }

   /**
    * Returns whether variables in the resolved contents of this variable should also be resolved.
    *
    * <p>
    * For example, if <js>"$X{xxx}"</js> resolves to <js>"$Y{xxx}"</js>, then the $Y variable will be recursively
    * resolved if this method returns <jk>true</jk>.
    *
    * <p>
    * The default implementation of this method always returns <jk>true</jk>.
    * <br>Subclasses can override this method to override the default behavior.
    *
    * <div class='warn'>
    * As a general rule, variables that resolve user-entered data should not be recursively resolved as this may
    * cause a security hole.
    * </div>
    *
    * @return <jk>true</jk> if resolved variables should be recursively resolved.
    */
   protected boolean allowRecurse() {
      return true;
   }

   /**
    * Returns <jk>true</jk> if this variable can be resolved in the specified session.
    *
    * <p>
    * For example, some variable cannot resolve unless specific context or session objects are available.
    *
    * @param session The current session.
    * @return <jk>true</jk> if this variable can be resolved in the specified session.
    */
   protected boolean canResolve(VarResolverSession session) {
      return true;
   }

   /**
    * The method called from {@link VarResolver}.
    *
    * <p>
    * Can be overridden to intercept the request and do special handling.
    * <br>Default implementation simply calls resolve(String).
    *
    * @param session The session object used for a single instance of a string resolution.
    * @param arg The inside argument of the variable.
    * @return The resolved value.
    * @throws Exception Any exception can be thrown.
    */
   protected String doResolve(VarResolverSession session, String arg) throws Exception {
      return resolve(session, arg);
   }

   /**
    * The interface that needs to be implemented for subclasses of {@link SimpleVar}.
    *
    * @param session The session object used for a single instance of a var resolution.
    * @param arg The inside argument of the variable.
    * @return The resolved value.
    * @throws Exception Any exception can be thrown.
    */
   public abstract String resolve(VarResolverSession session, String arg) throws Exception;

   /**
    * The interface that needs to be implemented for subclasses of {@link StreamedVar}.
    *
    * @param session The session object used for a single instance of a var resolution.
    * @param w The writer to send the resolved value to.
    * @param arg The inside argument of the variable.
    * @throws Exception Any exception can be thrown.
    */
   public abstract void resolveTo(VarResolverSession session, Writer w, String arg) throws Exception;
}