level-helper.js

/**
 * Level Helper class, providing methods dealing with playlist sliding and drift
*/

import { logger } from '../utils/logger';

export function updatePTS (fragments, fromIdx, toIdx) {
  let fragFrom = fragments[fromIdx], fragTo = fragments[toIdx], fragToPTS = fragTo.startPTS;
  // if we know startPTS[toIdx]
  if (!isNaN(fragToPTS)) {
    // update fragment duration.
    // it helps to fix drifts between playlist reported duration and fragment real duration
    if (toIdx > fromIdx) {
      fragFrom.duration = fragToPTS - fragFrom.start;
      if (fragFrom.duration < 0) {
        logger.warn(`negative duration computed for frag ${fragFrom.sn},level ${fragFrom.level}, there should be some duration drift between playlist and fragment!`);
      }
    } else {
      fragTo.duration = fragFrom.start - fragToPTS;
      if (fragTo.duration < 0) {
        logger.warn(`negative duration computed for frag ${fragTo.sn},level ${fragTo.level}, there should be some duration drift between playlist and fragment!`);
      }
    }
  } else {
    // we dont know startPTS[toIdx]
    if (toIdx > fromIdx) {
      fragTo.start = fragFrom.start + fragFrom.duration;
    } else {
      fragTo.start = Math.max(fragFrom.start - fragTo.duration, 0);
    }
  }
}

export function updateFragPTSDTS (details, frag, startPTS, endPTS, startDTS, endDTS) {
  // update frag PTS/DTS
  let maxStartPTS = startPTS;
  if (!isNaN(frag.startPTS)) {
    // delta PTS between audio and video
    let deltaPTS = Math.abs(frag.startPTS - startPTS);
    if (isNaN(frag.deltaPTS)) {
      frag.deltaPTS = deltaPTS;
    } else {
      frag.deltaPTS = Math.max(deltaPTS, frag.deltaPTS);
    }

    maxStartPTS = Math.max(startPTS, frag.startPTS);
    startPTS = Math.min(startPTS, frag.startPTS);
    endPTS = Math.max(endPTS, frag.endPTS);
    startDTS = Math.min(startDTS, frag.startDTS);
    endDTS = Math.max(endDTS, frag.endDTS);
  }

  const drift = startPTS - frag.start;
  frag.start = frag.startPTS = startPTS;
  frag.maxStartPTS = maxStartPTS;
  frag.endPTS = endPTS;
  frag.startDTS = startDTS;
  frag.endDTS = endDTS;
  frag.duration = endPTS - startPTS;

  const sn = frag.sn;
  // exit if sn out of range
  if (!details || sn < details.startSN || sn > details.endSN) {
    return 0;
  }

  let fragIdx, fragments, i;
  fragIdx = sn - details.startSN;
  fragments = details.fragments;
  // update frag reference in fragments array
  // rationale is that fragments array might not contain this frag object.
  // this will happpen if playlist has been refreshed between frag loading and call to updateFragPTSDTS()
  // if we don't update frag, we won't be able to propagate PTS info on the playlist
  // resulting in invalid sliding computation
  fragments[fragIdx] = frag;
  // adjust fragment PTS/duration from seqnum-1 to frag 0
  for (i = fragIdx; i > 0; i--) {
    updatePTS(fragments, i, i - 1);
  }

  // adjust fragment PTS/duration from seqnum to last frag
  for (i = fragIdx; i < fragments.length - 1; i++) {
    updatePTS(fragments, i, i + 1);
  }

  details.PTSKnown = true;
  // logger.log(`                                            frag start/end:${startPTS.toFixed(3)}/${endPTS.toFixed(3)}`);

  return drift;
}

export function mergeDetails (oldDetails, newDetails) {
  let start = Math.max(oldDetails.startSN, newDetails.startSN) - newDetails.startSN,
    end = Math.min(oldDetails.endSN, newDetails.endSN) - newDetails.startSN,
    delta = newDetails.startSN - oldDetails.startSN,
    oldfragments = oldDetails.fragments,
    newfragments = newDetails.fragments,
    ccOffset = 0,
    PTSFrag;

  // potentially retrieve cached initsegment
  if (newDetails.initSegment && oldDetails.initSegment) {
    newDetails.initSegment = oldDetails.initSegment;
  }

  // check if old/new playlists have fragments in common
  if (end < start) {
    newDetails.PTSKnown = false;
    return;
  }
  // loop through overlapping SN and update startPTS , cc, and duration if any found
  for (var i = start; i <= end; i++) {
    let oldFrag = oldfragments[delta + i],
      newFrag = newfragments[i];
    if (newFrag && oldFrag) {
      ccOffset = oldFrag.cc - newFrag.cc;
      if (!isNaN(oldFrag.startPTS)) {
        newFrag.start = newFrag.startPTS = oldFrag.startPTS;
        newFrag.endPTS = oldFrag.endPTS;
        newFrag.duration = oldFrag.duration;
        newFrag.backtracked = oldFrag.backtracked;
        newFrag.dropped = oldFrag.dropped;
        PTSFrag = newFrag;
      }
    }
  }

  if (ccOffset) {
    logger.log('discontinuity sliding from playlist, take drift into account');
    for (i = 0; i < newfragments.length; i++) {
      newfragments[i].cc += ccOffset;
    }
  }

  // if at least one fragment contains PTS info, recompute PTS information for all fragments
  if (PTSFrag) {
    updateFragPTSDTS(newDetails, PTSFrag, PTSFrag.startPTS, PTSFrag.endPTS, PTSFrag.startDTS, PTSFrag.endDTS);
  } else {
    // ensure that delta is within oldfragments range
    // also adjust sliding in case delta is 0 (we could have old=[50-60] and new=old=[50-61])
    // in that case we also need to adjust start offset of all fragments
    if (delta >= 0 && delta < oldfragments.length) {
      // adjust start by sliding offset
      let sliding = oldfragments[delta].start;
      for (i = 0; i < newfragments.length; i++) {
        newfragments[i].start += sliding;
      }
    }
  }
  // if we are here, it means we have fragments overlapping between
  // old and new level. reliable PTS info is thus relying on old level
  newDetails.PTSKnown = oldDetails.PTSKnown;
}