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

import java.io.Serializable;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Objects;

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

import org.apache.commons.lang3.ArrayUtils;
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 Logger log = LoggerFactory.getLogger(getClass());
	private ExpressionParser parser = new SpelExpressionParser();
	private StandardEvaluationContext evaluationContext = new StandardEvaluationContext();

	@Autowired(required = false)
	private ApplicationContext ctx;

	@Autowired(required = false)
	private HttpServletRequest request;

	@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 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(ExpressionEvaluationContext delegate) {
			this.delegate = delegate;
		}

		private ExpressionEvaluationContext delegate;

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

		@SuppressWarnings("unused")
		public Map<String, String> getRequestParameter() {
			if (request != null) {
				Map<String, String> singleValueMap = new HashMap<String, String>();
				Iterator<Entry<String, String[]>> it = request.getParameterMap().entrySet().iterator();
				while (it.hasNext()) {
					Entry<String, String[]> entry = it.next();
					if (ArrayUtils.isNotEmpty(entry.getValue())) {
						singleValueMap.put(entry.getKey(), entry.getValue()[0]);
					}
				}
				return singleValueMap;
			} else {
				return Collections.<String, String> emptyMap();
			}
		}

		@SuppressWarnings("unused")
		public Map<String, String[]> getRequestParameters() {
			if (request != null) {
				return request.getParameterMap();
			} else {
				return Collections.<String, String[]> emptyMap();
			}
		}

	}

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

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

		return BooleanUtils.isTrue(result);
	}

}