/**
 * Playlist related utilities.
 */
import {createTimeRange} from 'video.js';

let Playlist = {
  /**
   * The number of segments that are unsafe to start playback at in
   * a live stream. Changing this value can cause playback stalls.
   * See HTTP Live Streaming, "Playing the Media Playlist File"
   * https://tools.ietf.org/html/draft-pantos-http-live-streaming-18#section-6.3.3
   */
  UNSAFE_LIVE_SEGMENTS: 3
};

const backwardDuration = function(playlist, endSequence) {
  let result = 0;
  let i = endSequence - playlist.mediaSequence;
  // if a start time is available for segment immediately following
  // the interval, use it
  let segment = playlist.segments[i];

  // Walk backward until we find the latest segment with timeline
  // information that is earlier than endSequence
  if (segment) {
    if (typeof segment.start !== 'undefined') {
      return { result: segment.start, precise: true };
    }
    if (typeof segment.end !== 'undefined') {
      return {
        result: segment.end - segment.duration,
        precise: true
      };
    }
  }
  while (i--) {
    segment = playlist.segments[i];
    if (typeof segment.end !== 'undefined') {
      return { result: result + segment.end, precise: true };
    }

    result += segment.duration;

    if (typeof segment.start !== 'undefined') {
      return { result: result + segment.start, precise: true };
    }
  }
  return { result, precise: false };
};

const forwardDuration = function(playlist, endSequence) {
  let result = 0;
  let segment;
  let i = endSequence - playlist.mediaSequence;
  // Walk forward until we find the earliest segment with timeline
  // information

  for (; i < playlist.segments.length; i++) {
    segment = playlist.segments[i];
    if (typeof segment.start !== 'undefined') {
      return {
        result: segment.start - result,
        precise: true
      };
    }

    result += segment.duration;

    if (typeof segment.end !== 'undefined') {
      return {
        result: segment.end - result,
        precise: true
      };
    }

  }
  // indicate we didn't find a useful duration estimate
  return { result: -1, precise: false };
};

/**
  * Calculate the media duration from the segments associated with a
  * playlist. The duration of a subinterval of the available segments
  * may be calculated by specifying an end index.
  *
  * @param playlist {object} a media playlist object
  * @param endSequence {number} (optional) an exclusive upper boundary
  * for the playlist.  Defaults to playlist length.
  * @return {number} the duration between the first available segment
  * and end index.
  */
const intervalDuration = function(playlist, endSequence) {
  let backward;
  let forward;

  if (typeof endSequence === 'undefined') {
    endSequence = playlist.mediaSequence + playlist.segments.length;
  }

  if (endSequence < playlist.mediaSequence) {
    return 0;
  }

  // do a backward walk to estimate the duration
  backward = backwardDuration(playlist, endSequence);
  if (backward.precise) {
    // if we were able to base our duration estimate on timing
    // information provided directly from the Media Source, return
    // it
    return backward.result;
  }

  // walk forward to see if a precise duration estimate can be made
  // that way
  forward = forwardDuration(playlist, endSequence);
  if (forward.precise) {
    // we found a segment that has been buffered and so it's
    // position is known precisely
    return forward.result;
  }

  // return the less-precise, playlist-based duration estimate
  return backward.result;
};

/**
  * Calculates the duration of a playlist. If a start and end index
  * are specified, the duration will be for the subset of the media
  * timeline between those two indices. The total duration for live
  * playlists is always Infinity.
  * @param playlist {object} a media playlist object
  * @param endSequence {number} (optional) an exclusive upper
  * boundary for the playlist.  Defaults to the playlist media
  * sequence number plus its length.
  * @param includeTrailingTime {boolean} (optional) if false, the
  * interval between the final segment and the subsequent segment
  * will not be included in the result
  * @return {number} the duration between the start index and end
  * index.
  */
export const duration = function(playlist, endSequence, includeTrailingTime) {
  if (!playlist) {
    return 0;
  }

  if (typeof includeTrailingTime === 'undefined') {
    includeTrailingTime = true;
  }

  // if a slice of the total duration is not requested, use
  // playlist-level duration indicators when they're present
  if (typeof endSequence === 'undefined') {
    // if present, use the duration specified in the playlist
    if (playlist.totalDuration) {
      return playlist.totalDuration;
    }

    // duration should be Infinity for live playlists
    if (!playlist.endList) {
      return window.Infinity;
    }
  }

  // calculate the total duration based on the segment durations
  return intervalDuration(playlist,
                          endSequence,
                          includeTrailingTime);
};

/**
  * Calculates the interval of time that is currently seekable in a
  * playlist. The returned time ranges are relative to the earliest
  * moment in the specified playlist that is still available. A full
  * seekable implementation for live streams would need to offset
  * these values by the duration of content that has expired from the
  * stream.
  * @param playlist {object} a media playlist object
  * @return {TimeRanges} the periods of time that are valid targets
  * for seeking
  */
export const seekable = function(playlist) {
  let start;
  let end;

  // without segments, there are no seekable ranges
  if (!playlist.segments) {
    return createTimeRange();
  }
  // when the playlist is complete, the entire duration is seekable
  if (playlist.endList) {
    return createTimeRange(0, duration(playlist));
  }

  // live playlists should not expose three segment durations worth
  // of content from the end of the playlist
  // https://tools.ietf.org/html/draft-pantos-http-live-streaming-16#section-6.3.3
  start = intervalDuration(playlist, playlist.mediaSequence);
  end = intervalDuration(playlist,
                         playlist.mediaSequence +
                         Math.max(0, playlist.segments.length - Playlist.UNSAFE_LIVE_SEGMENTS));
  return createTimeRange(start, end);
};

Playlist.duration = duration;
Playlist.seekable = seekable;

// exports
export default Playlist;