// Copyright 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef CHROME_BROWSER_COMPONENT_UPDATER_CRX_DOWNLOADER_H_
#define CHROME_BROWSER_COMPONENT_UPDATER_CRX_DOWNLOADER_H_

#include <vector>

#include "base/basictypes.h"
#include "base/callback.h"
#include "base/files/file_path.h"
#include "base/memory/ref_counted.h"
#include "base/memory/scoped_ptr.h"
#include "base/sequenced_task_runner.h"
#include "url/gurl.h"

namespace net {
class URLRequestContextGetter;
}

namespace component_updater {

// Defines a download interface for downloading components, with retrying on
// fallback urls in case of errors. This class implements a chain of
// responsibility design pattern. It can give successors in the chain a chance
// to handle a download request, until one of them succeeds, or there are no
// more urls or successors to try. A callback is always called at the end of
// the download, one time only.
// When multiple urls and downloaders exists, first all the urls are tried, in
// the order they are provided in the StartDownload function argument. After
// that, the download request is routed to the next downloader in the chain.
// The members of this class expect to be called from the UI thread only.
class CrxDownloader {
 public:
  struct DownloadMetrics {
    enum Downloader {
      kNone = 0,
      kUrlFetcher,
      kBits
    };

    DownloadMetrics();

    GURL url;

    Downloader downloader;

    int error;

    int64 bytes_downloaded;   // -1 means that the byte count is unknown.
    int64 bytes_total;

    uint64 download_time_ms;
  };

  // Contains the outcome of the download.
  struct Result {
    Result();

    // Download error: 0 indicates success.
    int error;

    // Path of the downloaded file if the download was successful.
    base::FilePath response;
  };

  // The callback fires only once, regardless of how many urls are tried, and
  // how many successors in the chain of downloaders have handled the
  // download. The callback interface can be extended if needed to provide
  // more visibility into how the download has been handled, including
  // specific error codes and download metrics.
  typedef base::Callback<void (const Result& result)> DownloadCallback;

  // Factory method to create an instance of this class and build the
  // chain of responsibility. |is_background_download| specifies that a
  // background downloader be used, if the platform supports it.
  static CrxDownloader* Create(
      bool is_background_download,
      net::URLRequestContextGetter* context_getter,
      scoped_refptr<base::SequencedTaskRunner> task_runner,
      const DownloadCallback& download_callback);
  virtual ~CrxDownloader();

  // Starts the download. One instance of the class handles one download only.
  // One instance of CrxDownloader can only be started once, otherwise the
  // behavior is undefined. The callback gets invoked if the download can't
  // be started.
  void StartDownloadFromUrl(const GURL& url);
  void StartDownload(const std::vector<GURL>& urls);

  const std::vector<DownloadMetrics> download_metrics() const;

 protected:
  CrxDownloader(scoped_ptr<CrxDownloader> successor,
                const DownloadCallback& download_callback);

  // Handles the fallback in the case of multiple urls and routing of the
  // download to the following successor in the chain. Derived classes must call
  // this function after each attempt at downloading the urls provided
  // in the StartDownload function.
  // In case of errors, |is_handled| indicates that a server side error has
  // occured for the current url and the url should not be retried down
  // the chain to avoid DDOS of the server. This url will be removed from the
  // list of url and never tried again.
  void OnDownloadComplete(bool is_handled,
                          const Result& result,
                          const DownloadMetrics& download_metrics);

  // Returns the url which is currently downloaded from.
  GURL url() const;

 private:
  virtual void DoStartDownload(const GURL& url) = 0;

  std::vector<GURL> urls_;
  scoped_ptr<CrxDownloader> successor_;
  DownloadCallback download_callback_;

  std::vector<GURL>::iterator current_url_;

  std::vector<DownloadMetrics> download_metrics_;

  DISALLOW_COPY_AND_ASSIGN(CrxDownloader);
};

}  // namespace component_updater

#endif  // CHROME_BROWSER_COMPONENT_UPDATER_CRX_DOWNLOADER_H_