/**
 * Copyright (C) 2008 Google Inc.
 *
 * Licensed 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 com.google.inject.servlet;

import com.google.inject.Injector;
import com.google.inject.Key;
import com.google.inject.Scopes;
import com.google.common.collect.Iterators;
import com.google.inject.spi.BindingTargetVisitor;
import com.google.inject.spi.ProviderInstanceBinding;
import com.google.inject.spi.ProviderWithExtensionVisitor;

import java.io.IOException;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.atomic.AtomicReference;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;

import static com.google.inject.servlet.ManagedServletPipeline.REQUEST_DISPATCHER_REQUEST;

/**
 * An internal representation of a servlet definition mapped to a particular URI pattern. Also
 * performs the request dispatch to that servlet. How nice and OO =)
 *
 * @author dhanji@gmail.com (Dhanji R. Prasanna)
 */
class ServletDefinition implements ProviderWithExtensionVisitor<ServletDefinition> {
  private final String pattern;
  private final Key<? extends HttpServlet> servletKey;
  private final UriPatternMatcher patternMatcher;
  private final Map<String, String> initParams;
  // set only if this was bound using a servlet instance.
  private final HttpServlet servletInstance;

  //always set in init, our servlet is always presumed to be a singleton
  private final AtomicReference<HttpServlet> httpServlet = new AtomicReference<HttpServlet>();

  public ServletDefinition(String pattern, Key<? extends HttpServlet> servletKey,
      UriPatternMatcher patternMatcher, Map<String, String> initParams, HttpServlet servletInstance) {
    this.pattern = pattern;
    this.servletKey = servletKey;
    this.patternMatcher = patternMatcher;
    this.initParams = Collections.unmodifiableMap(new HashMap<String, String>(initParams));
    this.servletInstance = servletInstance;
  }
  
  public ServletDefinition get() {
    return this;
  }
  
  public <B, V> V acceptExtensionVisitor(BindingTargetVisitor<B, V> visitor,
      ProviderInstanceBinding<? extends B> binding) {
    if(visitor instanceof ServletModuleTargetVisitor) {
      if(servletInstance != null) {
        return ((ServletModuleTargetVisitor<B, V>)visitor).visit(
            new InstanceServletBindingImpl(initParams,
                pattern,
                servletInstance,
                patternMatcher));        
      } else {
        return ((ServletModuleTargetVisitor<B, V>)visitor).visit(
            new LinkedServletBindingImpl(initParams,
                pattern,
                servletKey,
                patternMatcher));
      }
    } else {
      return visitor.visit(binding);
    }
  }

  boolean shouldServe(String uri) {
    return patternMatcher.matches(uri);
  }

  public void init(final ServletContext servletContext, Injector injector,
      Set<HttpServlet> initializedSoFar) throws ServletException {

    // This absolutely must be a singleton, and so is only initialized once.
    if (!Scopes.isSingleton(injector.getBinding(servletKey))) {
      throw new ServletException("Servlets must be bound as singletons. "
        + servletKey + " was not bound in singleton scope.");
    }

    HttpServlet httpServlet = injector.getInstance(servletKey);
    this.httpServlet.set(httpServlet);

    // Only fire init() if we have not appeared before in the filter chain.
    if (initializedSoFar.contains(httpServlet)) {
      return;
    }

    //initialize our servlet with the configured context params and servlet context
    httpServlet.init(new ServletConfig() {
      public String getServletName() {
        return servletKey.toString();
      }

      public ServletContext getServletContext() {
        return servletContext;
      }

      public String getInitParameter(String s) {
        return initParams.get(s);
      }

      public Enumeration getInitParameterNames() {
        return Iterators.asEnumeration(initParams.keySet().iterator());
      }
    });

    // Mark as initialized.
    initializedSoFar.add(httpServlet);
  }

  public void destroy(Set<HttpServlet> destroyedSoFar) {
    HttpServlet reference = httpServlet.get();

    // Do nothing if this Servlet was invalid (usually due to not being scoped
    // properly). According to Servlet Spec: it is "out of service", and does not
    // need to be destroyed.
    // Also prevent duplicate destroys to the same singleton that may appear
    // more than once on the filter chain.
    if (null == reference || destroyedSoFar.contains(reference)) {
      return;
    }

    try {
      reference.destroy();
    } finally {
      destroyedSoFar.add(reference);
    }
  }

  /**
   * Wrapper around the service chain to ensure a servlet is servicing what it must and provides it
   * with a wrapped request.
   *
   * @return Returns true if this servlet triggered for the given request. Or false if
   *          guice-servlet should continue dispatching down the servlet pipeline.
   * 
   * @throws IOException If thrown by underlying servlet
   * @throws ServletException If thrown by underlying servlet
   */
  public boolean service(ServletRequest servletRequest,
      ServletResponse servletResponse) throws IOException, ServletException {

    final HttpServletRequest request = (HttpServletRequest) servletRequest;
    final String path = request.getRequestURI().substring(request.getContextPath().length());

    final boolean serve = shouldServe(path);

    //invocations of the chain end at the first matched servlet
    if (serve) {
      doService(servletRequest, servletResponse);
    }

    //return false if no servlet matched (so we can proceed down to the web.xml servlets)
    return serve;
  }

  /**
   * Utility that delegates to the actual service method of the servlet wrapped with a contextual
   * request (i.e. with correctly computed path info).
   *
   * We need to suppress deprecation coz we use HttpServletRequestWrapper, which implements
   * deprecated API for backwards compatibility.
   */
  void doService(final ServletRequest servletRequest, ServletResponse servletResponse)
      throws ServletException, IOException {

    HttpServletRequest request = new HttpServletRequestWrapper(
        (HttpServletRequest) servletRequest) {
      private String path;
      private boolean pathComputed = false;
      //must use a boolean on the memo field, because null is a legal value (TODO no, it's not)

      private boolean pathInfoComputed = false;
      private String pathInfo;

      @Override
      public String getPathInfo() {
        if (!isPathInfoComputed()) {
          int servletPathLength = getServletPath().length();
          pathInfo = getRequestURI().substring(getContextPath().length()).replaceAll("[/]{2,}", "/");
          pathInfo = pathInfo.length() > servletPathLength ? pathInfo.substring(servletPathLength) : null;

          // Corner case: when servlet path and request path match exactly (without trailing '/'),
          // then pathinfo is null
          if ("".equals(pathInfo) && servletPathLength != 0) {
            pathInfo = null;
          }

          pathInfoComputed = true;
        }

        return pathInfo;
      }

      // NOTE(dhanji): These two are a bit of a hack to help ensure that request dipatcher-sent
      // requests don't use the same path info that was memoized for the original request.
      private boolean isPathInfoComputed() {
        return pathInfoComputed
            && !(null != servletRequest.getAttribute(REQUEST_DISPATCHER_REQUEST));
      }

      private boolean isPathComputed() {
        return pathComputed
            && !(null != servletRequest.getAttribute(REQUEST_DISPATCHER_REQUEST));
      }

      @Override
      public String getServletPath() {
        return computePath();
      }

      @Override
      public String getPathTranslated() {
        final String info = getPathInfo();

        return (null == info) ? null : getRealPath(info);
      }

      // Memoizer pattern.
      private String computePath() {
        if (!isPathComputed()) {
          String servletPath = super.getServletPath();
          path = patternMatcher.extractPath(servletPath);
          pathComputed = true;

          if (null == path) {
            path = servletPath;
          }
        }

        return path;
      }
    };

    httpServlet.get().service(request, servletResponse);
  }

  String getKey() {
    return servletKey.toString();
  }

  String getPattern() {
    return pattern;
  }
}