/*
 * Copyright 2017 Graz University of Technology EAAF-Core Components has been developed in a
 * cooperation between EGIZ, A-SIT Plus, A-SIT, and Graz University of Technology.
 *
 * Licensed under the EUPL, Version 1.2 or - as soon they will be approved by the European
 * Commission - subsequent versions of the EUPL (the "Licence"); You may not use this work except in
 * compliance with the Licence. You may obtain a copy of the Licence at:
 * https://joinup.ec.europa.eu/news/understanding-eupl-v12
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the Licence
 * is distributed on an "AS IS" basis, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the Licence for the specific language governing permissions and limitations under
 * the Licence.
 *
 * This product combines work with different licenses. See the "NOTICE" text file for details on the
 * various modules and licenses. The "NOTICE" text file is part of the distribution. Any derivative
 * works that you distribute must include a readable copy of the "NOTICE" text file.
*/

package at.gv.egiz.eaaf.core.impl.idp.process.springweb;

import java.io.Serializable;
import java.util.Map;
import java.util.Objects;

import javax.annotation.PostConstruct;
import javax.servlet.http.HttpServletRequest;

import org.apache.commons.lang3.BooleanUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.expression.BeanFactoryResolver;
import org.springframework.expression.Expression;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.spel.standard.SpelExpressionParser;
import org.springframework.expression.spel.support.StandardEvaluationContext;

import at.gv.egiz.eaaf.core.api.idp.process.ExecutionContext;
import at.gv.egiz.eaaf.core.api.idp.process.ExpressionEvaluationContext;
import at.gv.egiz.eaaf.core.api.idp.process.ExpressionEvaluator;
import at.gv.egiz.eaaf.core.impl.idp.process.model.Transition;

/**
 * Expression evaluator for processing {@link Transition} conditions allowing
 * to.
 * <ul>
 * <li>reference Spring beans from the application context using
 * {@code @myBeanName...},</li>
 * <li>{@link ExecutionContext} properties using {@code ctx['property']},</li>
 * <li>Multi valued {@link HttpServletRequest} parameters using
 * {@code requestParameters['foo']} (keep in mind that this expression returns
 * an array of String values) and</li>
 * <li>Single valued {@link HttpServletRequest} parameters using
 * {@code requestParameter['foo']}</li>
 * </ul>
 *
 * @author tknall
 *
 */
public class SpringWebExpressionEvaluator implements ExpressionEvaluator {

  private final Logger log = LoggerFactory.getLogger(getClass());
  private final ExpressionParser parser = new SpelExpressionParser();
  private final StandardEvaluationContext evaluationContext = new StandardEvaluationContext();

  @Autowired(required = false)
  private ApplicationContext ctx;

  @PostConstruct
  private void init() {
    if (ctx != null) {
      evaluationContext.setBeanResolver(new BeanFactoryResolver(ctx));
    }
  }

  /**
   * Evaluation context that provides access to {@link HttpServletRequest}
   * parameters using {@code requestParameter['foo']} for single value parameters
   * or {@code requestParameters['foo']} for multi value parameters. Basic calls
   * to {@code ctx} will be delegated.
   *
   * @author tknall
   *
   */
  private static class SpringWebExpressionEvaluationContext implements ExpressionEvaluationContext {

    private static final long serialVersionUID = 1L;

    /**
     * Creates a new expression evaluation context, providing access to
     * HttpServletRequest parameter(s).
     *
     * @param delegate The original {@link ExpressionEvaluationContext} to be
     *                 delegated to for {@code ctx['foo']} expressions.
     */
    public SpringWebExpressionEvaluationContext(final ExpressionEvaluationContext delegate) {
      this.delegate = delegate;
    }

    private final ExpressionEvaluationContext delegate;

    @Override
    public Map<String, Serializable> getCtx() {
      return delegate.getCtx();
    }

  }

  @Override
  public boolean evaluate(final ExpressionEvaluationContext expressionContext,
      final String expression) {
    Objects.requireNonNull(expression, "Expression must not be null.");
    log.trace("Evaluating '{}'.", expression);

    final Expression expr = parser.parseExpression(expression);
    Boolean result = null;
    try {
      result = expr.getValue(evaluationContext,
          new SpringWebExpressionEvaluationContext(expressionContext), Boolean.class);
      if (result == null) {
        log.debug("Evaluation of '{}' results in null-value.", expression);
      } else {
        log.debug("Expression '{}' -> {}", expression, result);
      }
    } catch (final Exception e) {
      log.warn("Expression '{}' could not be processed.", expression, e);
    }

    return BooleanUtils.isTrue(result);
  }

}