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/*
 * -\-\-
 * Spotify Apollo API Implementations
 * --
 * Copyright (C) 2013 - 2015 Spotify AB
 * --
 * 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.spotify.apollo.route;

import com.spotify.apollo.Request;
import com.spotify.apollo.Response;
import com.spotify.apollo.Serializer;
import com.spotify.apollo.Status;
import com.spotify.apollo.StatusType;
import com.spotify.apollo.serialization.AutoSerializer;

import java.util.Optional;

import okio.ByteString;

/**
 * Contains utility {@link Middleware} implementations.
 */
public final class Middlewares {

  private static final String CONTENT_TYPE = "Content-Type";

  private Middlewares() {
    // prevent instantiation
  }

  /**
   * Converts an AsyncHandler with unspecified return type to one that returns {@code
   * Response}. This is done through a best-effort mechanism.
   *
   * Using this middleware has the effect that your code is no longer typesafe, so it may be
   * preferable to write your own middleware that converts the domain object your handler returns
   * into a {@code Response}.
   */
  public static  AsyncHandler> autoSerialize(AsyncHandler inner) {
    return serialize(new AutoSerializer()).apply(inner);
  }

  /**
   * Applies logic to the inner {@link AsyncHandler} that makes it conform to the semantics
   * specified in HTTP regarding when to return response bodies, Content-Length headers, etc.
   */
  public static AsyncHandler> httpPayloadSemantics(
      AsyncHandler> inner) {

    return inner.flatMapSync(resp -> ctx -> applyHttpPayloadSemantics(ctx.request(), resp));

  }

  /**
   * Middleware that adds the ability to set the response's Content-Type header to a defined
   * value.
   *
   * This middleware is type-unsafe, and it might be a better idea to set the content type directly
   * in your own middleware that does response serialization.
   */
  public static Middleware, AsyncHandler>> replyContentType(
      String contentType) {

    return inner -> inner
        .map(Middlewares::ensureResponse)
        .map(response -> response.withHeader(CONTENT_TYPE, contentType));
  }

  /**
   * Middleware that applies the supplied serializer to the result of the inner handler,
   * changing the payload and optionally the Content-Type header.
   *
   * This middleware is type-unsafe, and it might be better to write your own middleware that does
   * serialization.
   */
  public static Middleware, AsyncHandler>> serialize(
      Serializer serializer) {

    return inner -> inner
        .map(Middlewares::ensureResponse)
        .flatMapSync(resp -> ctx -> serializePayload(serializer, ctx.request(), resp));
  }

  /**
   * Returns the default middlewares applied by Apollo to routes supplied by a {@link RouteProvider}.
   */
  public static Middleware, AsyncHandler>> apolloDefaults() {
    return serialize(new AutoSerializer()).and(Middlewares::httpPayloadSemantics);
  }

  private static Response applyHttpPayloadSemantics(
      Request request, Response response) {
    Response result = response;
    Optional payload = response.payload();
    if (setContentLengthForStatus(response.status())) {
      int payloadSize = payload.isPresent() ? payload.get().size() : 0;
      result = result.withHeader("Content-Length", String.valueOf(payloadSize));
    }

    if (!setPayloadForMethod(request.method()) ||
        !setPayloadForStatus(response.status())) {
      result = result.withPayload(null);
    }

    return result;
  }

  // see http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.3
  private static boolean setPayloadForStatus(StatusType statusType) {
    return statusType.code() != Status.NOT_MODIFIED.code() &&
           statusType.code() != Status.NO_CONTENT.code() &&
           statusType.family() != StatusType.Family.INFORMATIONAL;
  }

  private static boolean setPayloadForMethod(String method) {
    return !"HEAD".equalsIgnoreCase(method);
  }

  // see http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.3
  private static boolean setContentLengthForStatus(StatusType statusType) {
    return setPayloadForStatus(statusType);
  }

  private static  Response serializePayload(
      Serializer serializer, Request request, Response response) {

    if (!response.payload().isPresent()) {
      // no payload, so this cast is safe to do
      //noinspection unchecked
      return (Response) response;
    }

    final T payloadObject = response.payload().get();
    final Serializer.Payload payload =
        serializer.serialize(request, payloadObject);

    if (payload.contentType().isPresent()) {
      response = response.withHeader(CONTENT_TYPE, payload.contentType().get());
    }

    return response.withPayload(payload.byteString());
  }

  private static  Response ensureResponse(T t) {
    if (t instanceof Response) {
      //noinspection unchecked
      return (Response) t;
    } else {
      return Response.forPayload(t);
    }
  }
}