src/controller/base-stream-controller.ts

import TaskLoop from '../task-loop';
import { FragmentState } from './fragment-tracker';
import { Bufferable, BufferHelper, BufferInfo } from '../utils/buffer-helper';
import { logger } from '../utils/logger';
import { Events } from '../events';
import { ErrorDetails, ErrorTypes } from '../errors';
import { ChunkMetadata } from '../types/transmuxer';
import { appendUint8Array } from '../utils/mp4-tools';
import { alignStream } from '../utils/discontinuities';
import {
  findFragmentByPDT,
  findFragmentByPTS,
  findFragWithCC,
} from './fragment-finders';
import {
  findPart,
  getFragmentWithSN,
  getPartWith,
  updateFragPTSDTS,
} from './level-helper';
import TransmuxerInterface from '../demux/transmuxer-interface';
import { Fragment, Part } from '../loader/fragment';
import FragmentLoader, {
  FragmentLoadProgressCallback,
  LoadError,
} from '../loader/fragment-loader';
import KeyLoader from '../loader/key-loader';
import { LevelDetails } from '../loader/level-details';
import Decrypter from '../crypt/decrypter';
import TimeRanges from '../utils/time-ranges';
import { PlaylistLevelType } from '../types/loader';
import type {
  BufferAppendingData,
  ErrorData,
  FragLoadedData,
  PartsLoadedData,
  KeyLoadedData,
  MediaAttachingData,
  BufferFlushingData,
  LevelSwitchingData,
} from '../types/events';
import type { FragmentTracker } from './fragment-tracker';
import type { Level } from '../types/level';
import type { RemuxedTrack } from '../types/remuxer';
import type Hls from '../hls';
import type { HlsConfig } from '../config';
import type { NetworkComponentAPI } from '../types/component-api';
import type { SourceBufferName } from '../types/buffer';
 
type ResolveFragLoaded = (FragLoadedEndData) => void;
type RejectFragLoaded = (LoadError) => void;
 
export const State = {
  STOPPED: 'STOPPED',
  IDLE: 'IDLE',
  KEY_LOADING: 'KEY_LOADING',
  FRAG_LOADING: 'FRAG_LOADING',
  FRAG_LOADING_WAITING_RETRY: 'FRAG_LOADING_WAITING_RETRY',
  WAITING_TRACK: 'WAITING_TRACK',
  PARSING: 'PARSING',
  PARSED: 'PARSED',
  ENDED: 'ENDED',
  ERROR: 'ERROR',
  WAITING_INIT_PTS: 'WAITING_INIT_PTS',
  WAITING_LEVEL: 'WAITING_LEVEL',
};
 
export default class BaseStreamController
  extends TaskLoop
  implements NetworkComponentAPI
{
  protected hls: Hls;
 
  protected fragPrevious: Fragment | null = null;
  protected fragCurrent: Fragment | null = null;
  protected fragmentTracker: FragmentTracker;
  protected transmuxer: TransmuxerInterface | null = null;
  protected _state: string = State.STOPPED;
  protected media: HTMLMediaElement | null = null;
  protected mediaBuffer: Bufferable | null = null;
  protected config: HlsConfig;
  protected bitrateTest: boolean = false;
  protected lastCurrentTime: number = 0;
  protected nextLoadPosition: number = 0;
  protected startPosition: number = 0;
  protected loadedmetadata: boolean = false;
  protected fragLoadError: number = 0;
  protected retryDate: number = 0;
  protected levels: Array<Level> | null = null;
  protected fragmentLoader: FragmentLoader;
  protected keyLoader: KeyLoader;
  protected levelLastLoaded: number | null = null;
  protected startFragRequested: boolean = false;
  protected decrypter: Decrypter;
  protected initPTS: Array<number> = [];
  protected onvseeking: EventListener | null = null;
  protected onvended: EventListener | null = null;
 
  private readonly logPrefix: string = '';
  protected log: (msg: any) => void;
  protected warn: (msg: any) => void;
 
  constructor(
    hls: Hls,
    fragmentTracker: FragmentTracker,
    keyLoader: KeyLoader,
    logPrefix: string
  ) {
    super();
    this.logPrefix = logPrefix;
    this.log = logger.log.bind(logger, `${logPrefix}:`);
    this.warn = logger.warn.bind(logger, `${logPrefix}:`);
    this.hls = hls;
    this.fragmentLoader = new FragmentLoader(hls.config);
    this.keyLoader = keyLoader;
    this.fragmentTracker = fragmentTracker;
    this.config = hls.config;
    this.decrypter = new Decrypter(hls.config);
    hls.on(Events.LEVEL_SWITCHING, this.onLevelSwitching, this);
  }
 
  protected doTick() {
    this.onTickEnd();
  }
 
  protected onTickEnd() {}
 
  // eslint-disable-next-line @typescript-eslint/no-unused-vars
  public startLoad(startPosition: number): void {}
 
  public stopLoad() {
    this.fragmentLoader.abort();
    this.keyLoader.abort();
    const frag = this.fragCurrent;
    if (frag) {
      frag.abortRequests();
      this.fragmentTracker.removeFragment(frag);
    }
    this.resetTransmuxer();
    this.fragCurrent = null;
    this.fragPrevious = null;
    this.clearInterval();
    this.clearNextTick();
    this.state = State.STOPPED;
  }
 
  protected _streamEnded(
    bufferInfo: BufferInfo,
    levelDetails: LevelDetails
  ): boolean {
    // If playlist is live, there is another buffered range after the current range, nothing buffered, media is detached,
    // of nothing loading/loaded return false
    if (
      levelDetails.live ||
      bufferInfo.nextStart ||
      !bufferInfo.end ||
      !this.media
    ) {
      return false;
    }
    const partList = levelDetails.partList;
    // Since the last part isn't guaranteed to correspond to the last playlist segment for Low-Latency HLS,
    // check instead if the last part is buffered.
    if (partList?.length) {
      const lastPart = partList[partList.length - 1];
 
      // Checking the midpoint of the part for potential margin of error and related issues.
      // NOTE: Technically I believe parts could yield content that is < the computed duration (including potential a duration of 0)
      // and still be spec-compliant, so there may still be edge cases here. Likewise, there could be issues in end of stream
      // part mismatches for independent audio and video playlists/segments.
      const lastPartBuffered = BufferHelper.isBuffered(
        this.media,
        lastPart.start + lastPart.duration / 2
      );
      return lastPartBuffered;
    }
 
    const playlistType =
      levelDetails.fragments[levelDetails.fragments.length - 1].type;
    return this.fragmentTracker.isEndListAppended(playlistType);
  }
 
  protected getLevelDetails(): LevelDetails | undefined {
    if (this.levels && this.levelLastLoaded !== null) {
      return this.levels[this.levelLastLoaded]?.details;
    }
  }
 
  protected onMediaAttached(
    event: Events.MEDIA_ATTACHED,
    data: MediaAttachingData
  ) {
    const media = (this.media = this.mediaBuffer = data.media);
    this.onvseeking = this.onMediaSeeking.bind(this) as EventListener;
    this.onvended = this.onMediaEnded.bind(this) as EventListener;
    media.addEventListener('seeking', this.onvseeking);
    media.addEventListener('ended', this.onvended);
    const config = this.config;
    if (this.levels && config.autoStartLoad && this.state === State.STOPPED) {
      this.startLoad(config.startPosition);
    }
  }
 
  protected onMediaDetaching() {
    const media = this.media;
    if (media?.ended) {
      this.log('MSE detaching and video ended, reset startPosition');
      this.startPosition = this.lastCurrentTime = 0;
    }
 
    // remove video listeners
    if (media && this.onvseeking && this.onvended) {
      media.removeEventListener('seeking', this.onvseeking);
      media.removeEventListener('ended', this.onvended);
      this.onvseeking = this.onvended = null;
    }
    if (this.keyLoader) {
      this.keyLoader.detach();
    }
    this.media = this.mediaBuffer = null;
    this.loadedmetadata = false;
    this.fragmentTracker.removeAllFragments();
    this.stopLoad();
  }
 
  protected onMediaSeeking() {
    const { config, fragCurrent, media, mediaBuffer, state } = this;
    const currentTime: number = media ? media.currentTime : 0;
    const bufferInfo = BufferHelper.bufferInfo(
      mediaBuffer ? mediaBuffer : media,
      currentTime,
      config.maxBufferHole
    );
 
    this.log(
      `media seeking to ${
        Number.isFinite(currentTime) ? currentTime.toFixed(3) : currentTime
      }, state: ${state}`
    );
 
    if (this.state === State.ENDED) {
      this.resetLoadingState();
    } else if (fragCurrent) {
      // Seeking while frag load is in progress
      const tolerance = config.maxFragLookUpTolerance;
      const fragStartOffset = fragCurrent.start - tolerance;
      const fragEndOffset =
        fragCurrent.start + fragCurrent.duration + tolerance;
      // if seeking out of buffered range or into new one
      if (
        !bufferInfo.len ||
        fragEndOffset < bufferInfo.start ||
        fragStartOffset > bufferInfo.end
      ) {
        const pastFragment = currentTime > fragEndOffset;
        // if the seek position is outside the current fragment range
        if (currentTime < fragStartOffset || pastFragment) {
          if (pastFragment && fragCurrent.loader) {
            this.log(
              'seeking outside of buffer while fragment load in progress, cancel fragment load'
            );
            fragCurrent.abortRequests();
          }
          this.resetLoadingState();
        }
      }
    }
 
    if (media) {
      this.lastCurrentTime = currentTime;
    }
 
    // in case seeking occurs although no media buffered, adjust startPosition and nextLoadPosition to seek target
    if (!this.loadedmetadata && !bufferInfo.len) {
      this.nextLoadPosition = this.startPosition = currentTime;
    }
 
    // Async tick to speed up processing
    this.tickImmediate();
  }
 
  protected onMediaEnded() {
    // reset startPosition and lastCurrentTime to restart playback @ stream beginning
    this.startPosition = this.lastCurrentTime = 0;
  }
 
  protected onLevelSwitching(
    event: Events.LEVEL_SWITCHING,
    data: LevelSwitchingData
  ): void {
    this.fragLoadError = 0;
  }
 
  protected onHandlerDestroying() {
    this.stopLoad();
    super.onHandlerDestroying();
  }
 
  protected onHandlerDestroyed() {
    this.state = State.STOPPED;
    this.hls.off(Events.LEVEL_SWITCHING, this.onLevelSwitching, this);
    if (this.fragmentLoader) {
      this.fragmentLoader.destroy();
    }
    if (this.keyLoader) {
      this.keyLoader.destroy();
    }
    if (this.decrypter) {
      this.decrypter.destroy();
    }
 
    this.hls =
      this.log =
      this.warn =
      this.decrypter =
      this.keyLoader =
      this.fragmentLoader =
      this.fragmentTracker =
        null as any;
    super.onHandlerDestroyed();
  }
 
  protected loadFragment(
    frag: Fragment,
    level: Level,
    targetBufferTime: number
  ) {
    this._loadFragForPlayback(frag, level, targetBufferTime);
  }
 
  private _loadFragForPlayback(
    frag: Fragment,
    level: Level,
    targetBufferTime: number
  ) {
    const progressCallback: FragmentLoadProgressCallback = (
      data: FragLoadedData
    ) => {
      if (this.fragContextChanged(frag)) {
        this.warn(
          `Fragment ${frag.sn}${
            data.part ? ' p: ' + data.part.index : ''
          } of level ${frag.level} was dropped during download.`
        );
        this.fragmentTracker.removeFragment(frag);
        return;
      }
      frag.stats.chunkCount++;
      this._handleFragmentLoadProgress(data);
    };
 
    this._doFragLoad(frag, level, targetBufferTime, progressCallback)
      .then((data) => {
        if (!data) {
          // if we're here we probably needed to backtrack or are waiting for more parts
          return;
        }
        this.fragLoadError = 0;
        const state = this.state;
        if (this.fragContextChanged(frag)) {
          if (
            state === State.FRAG_LOADING ||
            (!this.fragCurrent && state === State.PARSING)
          ) {
            this.fragmentTracker.removeFragment(frag);
            this.state = State.IDLE;
          }
          return;
        }
 
        if ('payload' in data) {
          this.log(`Loaded fragment ${frag.sn} of level ${frag.level}`);
          this.hls.trigger(Events.FRAG_LOADED, data);
        }
 
        // Pass through the whole payload; controllers not implementing progressive loading receive data from this callback
        this._handleFragmentLoadComplete(data);
      })
      .catch((reason) => {
        if (this.state === State.STOPPED || this.state === State.ERROR) {
          return;
        }
        this.warn(reason);
        this.resetFragmentLoading(frag);
      });
  }
 
  protected flushMainBuffer(
    startOffset: number,
    endOffset: number,
    type: SourceBufferName | null = null
  ) {
    if (!(startOffset - endOffset)) {
      return;
    }
    // When alternate audio is playing, the audio-stream-controller is responsible for the audio buffer. Otherwise,
    // passing a null type flushes both buffers
    const flushScope: BufferFlushingData = { startOffset, endOffset, type };
    // Reset load errors on flush
    this.fragLoadError = 0;
    this.hls.trigger(Events.BUFFER_FLUSHING, flushScope);
  }
 
  protected _loadInitSegment(frag: Fragment, level: Level) {
    this._doFragLoad(frag, level)
      .then((data) => {
        if (!data || this.fragContextChanged(frag) || !this.levels) {
          throw new Error('init load aborted');
        }
 
        return data;
      })
      .then((data: FragLoadedData) => {
        const { hls } = this;
        const { payload } = data;
        const decryptData = frag.decryptdata;
 
        // check to see if the payload needs to be decrypted
        if (
          payload &&
          payload.byteLength > 0 &&
          decryptData &&
          decryptData.key &&
          decryptData.iv &&
          decryptData.method === 'AES-128'
        ) {
          const startTime = self.performance.now();
          // decrypt the subtitles
          return this.decrypter
            .decrypt(
              new Uint8Array(payload),
              decryptData.key.buffer,
              decryptData.iv.buffer
            )
            .then((decryptedData) => {
              const endTime = self.performance.now();
              hls.trigger(Events.FRAG_DECRYPTED, {
                frag,
                payload: decryptedData,
                stats: {
                  tstart: startTime,
                  tdecrypt: endTime,
                },
              });
              data.payload = decryptedData;
 
              return data;
            });
        }
 
        return data;
      })
      .then((data: FragLoadedData) => {
        const { fragCurrent, hls, levels } = this;
        if (!levels) {
          throw new Error('init load aborted, missing levels');
        }
 
        const stats = frag.stats;
        this.state = State.IDLE;
        this.fragLoadError = 0;
        frag.data = new Uint8Array(data.payload);
        stats.parsing.start = stats.buffering.start = self.performance.now();
        stats.parsing.end = stats.buffering.end = self.performance.now();
 
        // Silence FRAG_BUFFERED event if fragCurrent is null
        if (data.frag === fragCurrent) {
          hls.trigger(Events.FRAG_BUFFERED, {
            stats,
            frag: fragCurrent,
            part: null,
            id: frag.type,
          });
        }
        this.tick();
      })
      .catch((reason) => {
        if (this.state === State.STOPPED || this.state === State.ERROR) {
          return;
        }
        this.warn(reason);
        this.resetFragmentLoading(frag);
      });
  }
 
  protected fragContextChanged(frag: Fragment | null) {
    const { fragCurrent } = this;
    return (
      !frag ||
      !fragCurrent ||
      frag.level !== fragCurrent.level ||
      frag.sn !== fragCurrent.sn ||
      frag.urlId !== fragCurrent.urlId
    );
  }
 
  protected fragBufferedComplete(frag: Fragment, part: Part | null) {
    const media = this.mediaBuffer ? this.mediaBuffer : this.media;
    this.log(
      `Buffered ${frag.type} sn: ${frag.sn}${
        part ? ' part: ' + part.index : ''
      } of ${this.logPrefix === '[stream-controller]' ? 'level' : 'track'} ${
        frag.level
      } ${
        media
          ? TimeRanges.toString(BufferHelper.getBuffered(media))
          : '(detached)'
      }`
    );
    this.state = State.IDLE;
    if (!media) {
      return;
    }
    if (
      !this.loadedmetadata &&
      frag.type == PlaylistLevelType.MAIN &&
      media.buffered.length &&
      this.fragCurrent?.sn === this.fragPrevious?.sn
    ) {
      this.loadedmetadata = true;
      this.seekToStartPos();
    }
    this.tick();
  }
 
  protected seekToStartPos() {}
 
  protected _handleFragmentLoadComplete(fragLoadedEndData: PartsLoadedData) {
    const { transmuxer } = this;
    if (!transmuxer) {
      return;
    }
    const { frag, part, partsLoaded } = fragLoadedEndData;
    // If we did not load parts, or loaded all parts, we have complete (not partial) fragment data
    const complete =
      !partsLoaded ||
      partsLoaded.length === 0 ||
      partsLoaded.some((fragLoaded) => !fragLoaded);
    const chunkMeta = new ChunkMetadata(
      frag.level,
      frag.sn as number,
      frag.stats.chunkCount + 1,
      0,
      part ? part.index : -1,
      !complete
    );
    transmuxer.flush(chunkMeta);
  }
 
  // eslint-disable-next-line @typescript-eslint/no-unused-vars
  protected _handleFragmentLoadProgress(
    frag: PartsLoadedData | FragLoadedData
  ) {}
 
  protected _doFragLoad(
    frag: Fragment,
    level: Level,
    targetBufferTime: number | null = null,
    progressCallback?: FragmentLoadProgressCallback
  ): Promise<PartsLoadedData | FragLoadedData | null> {
    const details = level?.details;
    if (!this.levels || !details) {
      throw new Error(
        `frag load aborted, missing level${details ? '' : ' detail'}s`
      );
    }
    let keyLoadingPromise: Promise<KeyLoadedData | void> | null = null;
    if (frag.encrypted && !frag.decryptdata?.key) {
      this.log(
        `Loading key for ${frag.sn} of [${details.startSN}-${details.endSN}], ${
          this.logPrefix === '[stream-controller]' ? 'level' : 'track'
        } ${frag.level}`
      );
      this.state = State.KEY_LOADING;
      this.fragCurrent = frag;
      keyLoadingPromise = this.keyLoader.load(frag).then((keyLoadedData) => {
        if (!this.fragContextChanged(keyLoadedData.frag)) {
          this.hls.trigger(Events.KEY_LOADED, keyLoadedData);
          return keyLoadedData;
        }
      });
      this.hls.trigger(Events.KEY_LOADING, { frag });
      this.throwIfFragContextChanged('KEY_LOADING');
    } else if (!frag.encrypted && details.encryptedFragments.length) {
      this.keyLoader.loadClear(frag, details.encryptedFragments);
    }
 
    targetBufferTime = Math.max(frag.start, targetBufferTime || 0);
    if (this.config.lowLatencyMode) {
      const partList = details.partList;
      if (partList && progressCallback) {
        if (targetBufferTime > frag.end && details.fragmentHint) {
          frag = details.fragmentHint;
        }
        const partIndex = this.getNextPart(partList, frag, targetBufferTime);
        if (partIndex > -1) {
          const part = partList[partIndex];
          this.log(
            `Loading part sn: ${frag.sn} p: ${part.index} cc: ${
              frag.cc
            } of playlist [${details.startSN}-${
              details.endSN
            }] parts [0-${partIndex}-${partList.length - 1}] ${
              this.logPrefix === '[stream-controller]' ? 'level' : 'track'
            }: ${frag.level}, target: ${parseFloat(
              targetBufferTime.toFixed(3)
            )}`
          );
          this.nextLoadPosition = part.start + part.duration;
          this.state = State.FRAG_LOADING;
          this.hls.trigger(Events.FRAG_LOADING, {
            frag,
            part,
            targetBufferTime,
          });
          this.throwIfFragContextChanged('FRAG_LOADING parts');
          if (keyLoadingPromise) {
            return keyLoadingPromise
              .then((keyLoadedData) => {
                if (
                  !keyLoadedData ||
                  this.fragContextChanged(keyLoadedData.frag)
                ) {
                  return null;
                }
                return this.doFragPartsLoad(
                  frag,
                  part,
                  level,
                  progressCallback
                );
              })
              .catch((error) => this.handleFragLoadError(error));
          }
 
          return this.doFragPartsLoad(
            frag,
            part,
            level,
            progressCallback
          ).catch((error: LoadError) => this.handleFragLoadError(error));
        } else if (
          !frag.url ||
          this.loadedEndOfParts(partList, targetBufferTime)
        ) {
          // Fragment hint has no parts
          return Promise.resolve(null);
        }
      }
    }
 
    this.log(
      `Loading fragment ${frag.sn} cc: ${frag.cc} ${
        details ? 'of [' + details.startSN + '-' + details.endSN + '] ' : ''
      }${this.logPrefix === '[stream-controller]' ? 'level' : 'track'}: ${
        frag.level
      }, target: ${parseFloat(targetBufferTime.toFixed(3))}`
    );
    // Don't update nextLoadPosition for fragments which are not buffered
    if (Number.isFinite(frag.sn as number) && !this.bitrateTest) {
      this.nextLoadPosition = frag.start + frag.duration;
    }
    this.state = State.FRAG_LOADING;
    this.hls.trigger(Events.FRAG_LOADING, { frag, targetBufferTime });
    this.throwIfFragContextChanged('FRAG_LOADING');
 
    // Load key before streaming fragment data
    const dataOnProgress = this.config.progressive;
    if (dataOnProgress && keyLoadingPromise) {
      return keyLoadingPromise
        .then((keyLoadedData) => {
          if (!keyLoadedData || this.fragContextChanged(keyLoadedData?.frag)) {
            return null;
          }
          return this.fragmentLoader.load(frag, progressCallback);
        })
        .catch((error) => this.handleFragLoadError(error));
    }
 
    // load unencrypted fragment data with progress event,
    // or handle fragment result after key and fragment are finished loading
    return Promise.all([
      this.fragmentLoader.load(
        frag,
        dataOnProgress ? progressCallback : undefined
      ),
      keyLoadingPromise,
    ])
      .then(([fragLoadedData]) => {
        if (!dataOnProgress && fragLoadedData && progressCallback) {
          progressCallback(fragLoadedData);
        }
        return fragLoadedData;
      })
      .catch((error) => this.handleFragLoadError(error));
  }
 
  private throwIfFragContextChanged(context: string): void | never {
    // exit if context changed during event loop
    if (this.fragCurrent === null) {
      throw new Error(`frag load aborted, context changed in ${context}`);
    }
  }
 
  private doFragPartsLoad(
    frag: Fragment,
    fromPart: Part,
    level: Level,
    progressCallback: FragmentLoadProgressCallback
  ): Promise<PartsLoadedData | null> {
    return new Promise(
      (resolve: ResolveFragLoaded, reject: RejectFragLoaded) => {
        const partsLoaded: FragLoadedData[] = [];
        const initialPartList = level.details?.partList;
        const loadPart = (part: Part) => {
          this.fragmentLoader
            .loadPart(frag, part, progressCallback)
            .then((partLoadedData: FragLoadedData) => {
              partsLoaded[part.index] = partLoadedData;
              const loadedPart = partLoadedData.part as Part;
              this.hls.trigger(Events.FRAG_LOADED, partLoadedData);
              const nextPart =
                getPartWith(level, frag.sn as number, part.index + 1) ||
                findPart(initialPartList, frag.sn as number, part.index + 1);
              if (nextPart) {
                loadPart(nextPart);
              } else {
                return resolve({
                  frag,
                  part: loadedPart,
                  partsLoaded,
                });
              }
            })
            .catch(reject);
        };
        loadPart(fromPart);
      }
    );
  }
 
  private handleFragLoadError(error: LoadError | Error) {
    if ('data' in error) {
      const data = error.data;
      if (error.data && data.details === ErrorDetails.INTERNAL_ABORTED) {
        this.handleFragLoadAborted(data.frag, data.part);
      } else {
        this.hls.trigger(Events.ERROR, data as ErrorData);
      }
    } else {
      this.hls.trigger(Events.ERROR, {
        type: ErrorTypes.OTHER_ERROR,
        details: ErrorDetails.INTERNAL_EXCEPTION,
        err: error,
        fatal: true,
      });
    }
    return null;
  }
 
  protected _handleTransmuxerFlush(chunkMeta: ChunkMetadata) {
    const context = this.getCurrentContext(chunkMeta);
    if (!context || this.state !== State.PARSING) {
      if (
        !this.fragCurrent &&
        this.state !== State.STOPPED &&
        this.state !== State.ERROR
      ) {
        this.state = State.IDLE;
      }
      return;
    }
    const { frag, part, level } = context;
    const now = self.performance.now();
    frag.stats.parsing.end = now;
    if (part) {
      part.stats.parsing.end = now;
    }
    this.updateLevelTiming(frag, part, level, chunkMeta.partial);
  }
 
  protected getCurrentContext(
    chunkMeta: ChunkMetadata
  ): { frag: Fragment; part: Part | null; level: Level } | null {
    const { levels } = this;
    const { level: levelIndex, sn, part: partIndex } = chunkMeta;
    if (!levels || !levels[levelIndex]) {
      this.warn(
        `Levels object was unset while buffering fragment ${sn} of level ${levelIndex}. The current chunk will not be buffered.`
      );
      return null;
    }
    const level = levels[levelIndex];
    const part = partIndex > -1 ? getPartWith(level, sn, partIndex) : null;
    const frag = part
      ? part.fragment
      : getFragmentWithSN(level, sn, this.fragCurrent);
    if (!frag) {
      return null;
    }
    return { frag, part, level };
  }
 
  protected bufferFragmentData(
    data: RemuxedTrack,
    frag: Fragment,
    part: Part | null,
    chunkMeta: ChunkMetadata
  ) {
    if (!data || this.state !== State.PARSING) {
      return;
    }
 
    const { data1, data2 } = data;
    let buffer = data1;
    if (data1 && data2) {
      // Combine the moof + mdat so that we buffer with a single append
      buffer = appendUint8Array(data1, data2);
    }
 
    if (!buffer || !buffer.length) {
      return;
    }
 
    const segment: BufferAppendingData = {
      type: data.type,
      frag,
      part,
      chunkMeta,
      parent: frag.type,
      data: buffer,
    };
    this.hls.trigger(Events.BUFFER_APPENDING, segment);
 
    if (data.dropped && data.independent && !part) {
      // Clear buffer so that we reload previous segments sequentially if required
      this.flushBufferGap(frag);
    }
  }
 
  protected flushBufferGap(frag: Fragment) {
    const media = this.media;
    if (!media) {
      return;
    }
    // If currentTime is not buffered, clear the back buffer so that we can backtrack as much as needed
    if (!BufferHelper.isBuffered(media, media.currentTime)) {
      this.flushMainBuffer(0, frag.start);
      return;
    }
    // Remove back-buffer without interrupting playback to allow back tracking
    const currentTime = media.currentTime;
    const bufferInfo = BufferHelper.bufferInfo(media, currentTime, 0);
    const fragDuration = frag.duration;
    const segmentFraction = Math.min(
      this.config.maxFragLookUpTolerance * 2,
      fragDuration * 0.25
    );
    const start = Math.max(
      Math.min(frag.start - segmentFraction, bufferInfo.end - segmentFraction),
      currentTime + segmentFraction
    );
    if (frag.start - start > segmentFraction) {
      this.flushMainBuffer(start, frag.start);
    }
  }
 
  protected getFwdBufferInfo(
    bufferable: Bufferable | null,
    type: PlaylistLevelType
  ): BufferInfo | null {
    const { config } = this;
    const pos = this.getLoadPosition();
    if (!Number.isFinite(pos)) {
      return null;
    }
    const bufferInfo = BufferHelper.bufferInfo(
      bufferable,
      pos,
      config.maxBufferHole
    );
    // Workaround flaw in getting forward buffer when maxBufferHole is smaller than gap at current pos
    if (bufferInfo.len === 0 && bufferInfo.nextStart !== undefined) {
      const bufferedFragAtPos = this.fragmentTracker.getBufferedFrag(pos, type);
      if (bufferedFragAtPos && bufferInfo.nextStart < bufferedFragAtPos.end) {
        return BufferHelper.bufferInfo(
          bufferable,
          pos,
          Math.max(bufferInfo.nextStart, config.maxBufferHole)
        );
      }
    }
    return bufferInfo;
  }
 
  protected getMaxBufferLength(levelBitrate?: number): number {
    const { config } = this;
    let maxBufLen;
    if (levelBitrate) {
      maxBufLen = Math.max(
        (8 * config.maxBufferSize) / levelBitrate,
        config.maxBufferLength
      );
    } else {
      maxBufLen = config.maxBufferLength;
    }
    return Math.min(maxBufLen, config.maxMaxBufferLength);
  }
 
  protected reduceMaxBufferLength(threshold?: number) {
    const config = this.config;
    const minLength = threshold || config.maxBufferLength;
    if (config.maxMaxBufferLength >= minLength) {
      // reduce max buffer length as it might be too high. we do this to avoid loop flushing ...
      config.maxMaxBufferLength /= 2;
      this.warn(`Reduce max buffer length to ${config.maxMaxBufferLength}s`);
      return true;
    }
    return false;
  }
 
  protected getNextFragment(
    pos: number,
    levelDetails: LevelDetails
  ): Fragment | null {
    const fragments = levelDetails.fragments;
    const fragLen = fragments.length;
 
    if (!fragLen) {
      return null;
    }
 
    // find fragment index, contiguous with end of buffer position
    const { config } = this;
    const start = fragments[0].start;
    let frag;
 
    if (levelDetails.live) {
      const initialLiveManifestSize = config.initialLiveManifestSize;
      if (fragLen < initialLiveManifestSize) {
        this.warn(
          `Not enough fragments to start playback (have: ${fragLen}, need: ${initialLiveManifestSize})`
        );
        return null;
      }
      // The real fragment start times for a live stream are only known after the PTS range for that level is known.
      // In order to discover the range, we load the best matching fragment for that level and demux it.
      // Do not load using live logic if the starting frag is requested - we want to use getFragmentAtPosition() so that
      // we get the fragment matching that start time
      if (
        !levelDetails.PTSKnown &&
        !this.startFragRequested &&
        this.startPosition === -1
      ) {
        frag = this.getInitialLiveFragment(levelDetails, fragments);
        this.startPosition = frag
          ? this.hls.liveSyncPosition || frag.start
          : pos;
      }
    } else if (pos <= start) {
      // VoD playlist: if loadPosition before start of playlist, load first fragment
      frag = fragments[0];
    }
 
    // If we haven't run into any special cases already, just load the fragment most closely matching the requested position
    if (!frag) {
      const end = config.lowLatencyMode
        ? levelDetails.partEnd
        : levelDetails.fragmentEnd;
      frag = this.getFragmentAtPosition(pos, end, levelDetails);
    }
 
    return this.mapToInitFragWhenRequired(frag);
  }
 
  mapToInitFragWhenRequired(frag: Fragment | null): typeof frag {
    // If an initSegment is present, it must be buffered first
    if (frag?.initSegment && !frag?.initSegment.data && !this.bitrateTest) {
      return frag.initSegment;
    }
 
    return frag;
  }
 
  getNextPart(
    partList: Part[],
    frag: Fragment,
    targetBufferTime: number
  ): number {
    let nextPart = -1;
    let contiguous = false;
    let independentAttrOmitted = true;
    for (let i = 0, len = partList.length; i < len; i++) {
      const part = partList[i];
      independentAttrOmitted = independentAttrOmitted && !part.independent;
      if (nextPart > -1 && targetBufferTime < part.start) {
        break;
      }
      const loaded = part.loaded;
      if (loaded) {
        nextPart = -1;
      } else if (
        (contiguous || part.independent || independentAttrOmitted) &&
        part.fragment === frag
      ) {
        nextPart = i;
      }
      contiguous = loaded;
    }
    return nextPart;
  }
 
  private loadedEndOfParts(
    partList: Part[],
    targetBufferTime: number
  ): boolean {
    const lastPart = partList[partList.length - 1];
    return lastPart && targetBufferTime > lastPart.start && lastPart.loaded;
  }
 
  /*
   This method is used find the best matching first fragment for a live playlist. This fragment is used to calculate the
   "sliding" of the playlist, which is its offset from the start of playback. After sliding we can compute the real
   start and end times for each fragment in the playlist (after which this method will not need to be called).
  */
  protected getInitialLiveFragment(
    levelDetails: LevelDetails,
    fragments: Array<Fragment>
  ): Fragment | null {
    const fragPrevious = this.fragPrevious;
    let frag: Fragment | null = null;
    if (fragPrevious) {
      if (levelDetails.hasProgramDateTime) {
        // Prefer using PDT, because it can be accurate enough to choose the correct fragment without knowing the level sliding
        this.log(
          `Live playlist, switching playlist, load frag with same PDT: ${fragPrevious.programDateTime}`
        );
        frag = findFragmentByPDT(
          fragments,
          fragPrevious.endProgramDateTime,
          this.config.maxFragLookUpTolerance
        );
      }
      if (!frag) {
        // SN does not need to be accurate between renditions, but depending on the packaging it may be so.
        const targetSN = (fragPrevious.sn as number) + 1;
        if (
          targetSN >= levelDetails.startSN &&
          targetSN <= levelDetails.endSN
        ) {
          const fragNext = fragments[targetSN - levelDetails.startSN];
          // Ensure that we're staying within the continuity range, since PTS resets upon a new range
          if (fragPrevious.cc === fragNext.cc) {
            frag = fragNext;
            this.log(
              `Live playlist, switching playlist, load frag with next SN: ${
                frag!.sn
              }`
            );
          }
        }
        // It's important to stay within the continuity range if available; otherwise the fragments in the playlist
        // will have the wrong start times
        if (!frag) {
          frag = findFragWithCC(fragments, fragPrevious.cc);
          if (frag) {
            this.log(
              `Live playlist, switching playlist, load frag with same CC: ${frag.sn}`
            );
          }
        }
      }
    } else {
      // Find a new start fragment when fragPrevious is null
      const liveStart = this.hls.liveSyncPosition;
      if (liveStart !== null) {
        frag = this.getFragmentAtPosition(
          liveStart,
          this.bitrateTest ? levelDetails.fragmentEnd : levelDetails.edge,
          levelDetails
        );
      }
    }
 
    return frag;
  }
 
  /*
  This method finds the best matching fragment given the provided position.
   */
  protected getFragmentAtPosition(
    bufferEnd: number,
    end: number,
    levelDetails: LevelDetails
  ): Fragment | null {
    const { config } = this;
    let { fragPrevious } = this;
    let { fragments, endSN } = levelDetails;
    const { fragmentHint } = levelDetails;
    const tolerance = config.maxFragLookUpTolerance;
 
    const loadingParts = !!(
      config.lowLatencyMode &&
      levelDetails.partList &&
      fragmentHint
    );
    if (loadingParts && fragmentHint && !this.bitrateTest) {
      // Include incomplete fragment with parts at end
      fragments = fragments.concat(fragmentHint);
      endSN = fragmentHint.sn as number;
    }
 
    let frag;
    if (bufferEnd < end) {
      const lookupTolerance = bufferEnd > end - tolerance ? 0 : tolerance;
      // Remove the tolerance if it would put the bufferEnd past the actual end of stream
      // Uses buffer and sequence number to calculate switch segment (required if using EXT-X-DISCONTINUITY-SEQUENCE)
      frag = findFragmentByPTS(
        fragPrevious,
        fragments,
        bufferEnd,
        lookupTolerance
      );
    } else {
      // reach end of playlist
      frag = fragments[fragments.length - 1];
    }
 
    if (frag) {
      const curSNIdx = frag.sn - levelDetails.startSN;
      // Move fragPrevious forward to support forcing the next fragment to load
      // when the buffer catches up to a previously buffered range.
      if (this.fragmentTracker.getState(frag) === FragmentState.OK) {
        fragPrevious = frag;
      }
      if (fragPrevious && frag.sn === fragPrevious.sn && !loadingParts) {
        // Force the next fragment to load if the previous one was already selected. This can occasionally happen with
        // non-uniform fragment durations
        const sameLevel = fragPrevious && frag.level === fragPrevious.level;
        if (sameLevel) {
          const nextFrag = fragments[curSNIdx + 1];
          if (
            frag.sn < endSN &&
            this.fragmentTracker.getState(nextFrag) !== FragmentState.OK
          ) {
            this.log(
              `SN ${frag.sn} just loaded, load next one: ${nextFrag.sn}`
            );
            frag = nextFrag;
          } else {
            frag = null;
          }
        }
      }
    }
    return frag;
  }
 
  protected synchronizeToLiveEdge(levelDetails: LevelDetails) {
    const { config, media } = this;
    if (!media) {
      return;
    }
    const liveSyncPosition = this.hls.liveSyncPosition;
    const currentTime = media.currentTime;
    const start = levelDetails.fragments[0].start;
    const end = levelDetails.edge;
    const withinSlidingWindow =
      currentTime >= start - config.maxFragLookUpTolerance &&
      currentTime <= end;
    // Continue if we can seek forward to sync position or if current time is outside of sliding window
    if (
      liveSyncPosition !== null &&
      media.duration > liveSyncPosition &&
      (currentTime < liveSyncPosition || !withinSlidingWindow)
    ) {
      // Continue if buffer is starving or if current time is behind max latency
      const maxLatency =
        config.liveMaxLatencyDuration !== undefined
          ? config.liveMaxLatencyDuration
          : config.liveMaxLatencyDurationCount * levelDetails.targetduration;
      if (
        (!withinSlidingWindow && media.readyState < 4) ||
        currentTime < end - maxLatency
      ) {
        if (!this.loadedmetadata) {
          this.nextLoadPosition = liveSyncPosition;
        }
        // Only seek if ready and there is not a significant forward buffer available for playback
        if (media.readyState) {
          this.warn(
            `Playback: ${currentTime.toFixed(
              3
            )} is located too far from the end of live sliding playlist: ${end}, reset currentTime to : ${liveSyncPosition.toFixed(
              3
            )}`
          );
          media.currentTime = liveSyncPosition;
        }
      }
    }
  }
 
  protected alignPlaylists(
    details: LevelDetails,
    previousDetails?: LevelDetails
  ): number {
    const { levels, levelLastLoaded, fragPrevious } = this;
    const lastLevel: Level | null =
      levelLastLoaded !== null ? levels![levelLastLoaded] : null;
 
    // FIXME: If not for `shouldAlignOnDiscontinuities` requiring fragPrevious.cc,
    //  this could all go in level-helper mergeDetails()
    const length = details.fragments.length;
    if (!length) {
      this.warn(`No fragments in live playlist`);
      return 0;
    }
    const slidingStart = details.fragments[0].start;
    const firstLevelLoad = !previousDetails;
    const aligned = details.alignedSliding && Number.isFinite(slidingStart);
    if (firstLevelLoad || (!aligned && !slidingStart)) {
      alignStream(fragPrevious, lastLevel, details);
      const alignedSlidingStart = details.fragments[0].start;
      this.log(
        `Live playlist sliding: ${alignedSlidingStart.toFixed(2)} start-sn: ${
          previousDetails ? previousDetails.startSN : 'na'
        }->${details.startSN} prev-sn: ${
          fragPrevious ? fragPrevious.sn : 'na'
        } fragments: ${length}`
      );
      return alignedSlidingStart;
    }
    return slidingStart;
  }
 
  protected waitForCdnTuneIn(details: LevelDetails) {
    // Wait for Low-Latency CDN Tune-in to get an updated playlist
    const advancePartLimit = 3;
    return (
      details.live &&
      details.canBlockReload &&
      details.partTarget &&
      details.tuneInGoal >
        Math.max(details.partHoldBack, details.partTarget * advancePartLimit)
    );
  }
 
  protected setStartPosition(details: LevelDetails, sliding: number) {
    // compute start position if set to -1. use it straight away if value is defined
    let startPosition = this.startPosition;
    if (startPosition < sliding) {
      startPosition = -1;
    }
    if (startPosition === -1 || this.lastCurrentTime === -1) {
      // first, check if start time offset has been set in playlist, if yes, use this value
      const startTimeOffset = details.startTimeOffset!;
      if (Number.isFinite(startTimeOffset)) {
        startPosition = sliding + startTimeOffset;
        if (startTimeOffset < 0) {
          startPosition += details.totalduration;
        }
        startPosition = Math.min(
          Math.max(sliding, startPosition),
          sliding + details.totalduration
        );
        this.log(
          `Start time offset ${startTimeOffset} found in playlist, adjust startPosition to ${startPosition}`
        );
        this.startPosition = startPosition;
      } else if (details.live) {
        // Leave this.startPosition at -1, so that we can use `getInitialLiveFragment` logic when startPosition has
        // not been specified via the config or an as an argument to startLoad (#3736).
        startPosition = this.hls.liveSyncPosition || sliding;
      } else {
        this.startPosition = startPosition = 0;
      }
      this.lastCurrentTime = startPosition;
    }
    this.nextLoadPosition = startPosition;
  }
 
  protected getLoadPosition(): number {
    const { media } = this;
    // if we have not yet loaded any fragment, start loading from start position
    let pos = 0;
    if (this.loadedmetadata && media) {
      pos = media.currentTime;
    } else if (this.nextLoadPosition) {
      pos = this.nextLoadPosition;
    }
 
    return pos;
  }
 
  private handleFragLoadAborted(frag: Fragment, part: Part | undefined) {
    if (this.transmuxer && frag.sn !== 'initSegment' && frag.stats.aborted) {
      this.warn(
        `Fragment ${frag.sn}${part ? ' part' + part.index : ''} of level ${
          frag.level
        } was aborted`
      );
      this.resetFragmentLoading(frag);
    }
  }
 
  protected resetFragmentLoading(frag: Fragment) {
    if (
      !this.fragCurrent ||
      (!this.fragContextChanged(frag) &&
        this.state !== State.FRAG_LOADING_WAITING_RETRY)
    ) {
      this.state = State.IDLE;
    }
  }
 
  protected onFragmentOrKeyLoadError(
    filterType: PlaylistLevelType,
    data: ErrorData
  ) {
    if (data.fatal) {
      this.stopLoad();
      this.state = State.ERROR;
      return;
    }
    const config = this.config;
    if (data.chunkMeta) {
      // Parsing Error: no retries
      const context = this.getCurrentContext(data.chunkMeta);
      if (context) {
        data.frag = context.frag;
        data.levelRetry = true;
        this.fragLoadError = config.fragLoadingMaxRetry;
      }
    }
    const frag = data.frag;
    // Handle frag error related to caller's filterType
    if (!frag || frag.type !== filterType) {
      return;
    }
    const fragCurrent = this.fragCurrent;
    console.assert(
      fragCurrent &&
        frag.sn === fragCurrent.sn &&
        frag.level === fragCurrent.level &&
        frag.urlId === fragCurrent.urlId,
      'Frag load error must match current frag to retry'
    );
    // keep retrying until the limit will be reached
    if (this.fragLoadError + 1 <= config.fragLoadingMaxRetry) {
      if (!this.loadedmetadata) {
        this.startFragRequested = false;
        this.nextLoadPosition = this.startPosition;
      }
      // exponential backoff capped to config.fragLoadingMaxRetryTimeout
      const delay = Math.min(
        Math.pow(2, this.fragLoadError) * config.fragLoadingRetryDelay,
        config.fragLoadingMaxRetryTimeout
      );
      this.warn(
        `Fragment ${frag.sn} of ${filterType} ${frag.level} failed to load, retrying in ${delay}ms`
      );
      this.retryDate = self.performance.now() + delay;
      this.fragLoadError++;
      this.state = State.FRAG_LOADING_WAITING_RETRY;
    } else if (data.levelRetry) {
      if (filterType === PlaylistLevelType.AUDIO) {
        // Reset current fragment since audio track audio is essential and may not have a fail-over track
        this.fragCurrent = null;
      }
      // Fragment errors that result in a level switch or redundant fail-over
      // should reset the stream controller state to idle
      this.fragLoadError = 0;
      this.state = State.IDLE;
    } else {
      logger.error(
        `${data.details} reaches max retry, redispatch as fatal ...`
      );
      // switch error to fatal
      data.fatal = true;
      this.hls.stopLoad();
      this.state = State.ERROR;
    }
  }
 
  protected afterBufferFlushed(
    media: Bufferable,
    bufferType: SourceBufferName,
    playlistType: PlaylistLevelType
  ) {
    if (!media) {
      return;
    }
    // After successful buffer flushing, filter flushed fragments from bufferedFrags use mediaBuffered instead of media
    // (so that we will check against video.buffered ranges in case of alt audio track)
    const bufferedTimeRanges = BufferHelper.getBuffered(media);
    this.fragmentTracker.detectEvictedFragments(
      bufferType,
      bufferedTimeRanges,
      playlistType
    );
    if (this.state === State.ENDED) {
      this.resetLoadingState();
    }
  }
 
  protected resetLoadingState() {
    this.log('Reset loading state');
    this.fragCurrent = null;
    this.fragPrevious = null;
    this.state = State.IDLE;
  }
 
  protected resetStartWhenNotLoaded(level: number): void {
    // if loadedmetadata is not set, it means that first frag request failed
    // in that case, reset startFragRequested flag
    if (!this.loadedmetadata) {
      this.startFragRequested = false;
      const details = this.levels ? this.levels[level].details : null;
      if (details?.live) {
        // Update the start position and return to IDLE to recover live start
        this.startPosition = -1;
        this.setStartPosition(details, 0);
        this.resetLoadingState();
      } else {
        this.nextLoadPosition = this.startPosition;
      }
    }
  }
 
  private updateLevelTiming(
    frag: Fragment,
    part: Part | null,
    level: Level,
    partial: boolean
  ) {
    const details = level.details as LevelDetails;
    console.assert(!!details, 'level.details must be defined');
    const parsed = Object.keys(frag.elementaryStreams).reduce(
      (result, type) => {
        const info = frag.elementaryStreams[type];
        if (info) {
          const parsedDuration = info.endPTS - info.startPTS;
          if (parsedDuration <= 0) {
            // Destroy the transmuxer after it's next time offset failed to advance because duration was <= 0.
            // The new transmuxer will be configured with a time offset matching the next fragment start,
            // preventing the timeline from shifting.
            this.warn(
              `Could not parse fragment ${frag.sn} ${type} duration reliably (${parsedDuration})`
            );
            return result || false;
          }
          const drift = partial
            ? 0
            : updateFragPTSDTS(
                details,
                frag,
                info.startPTS,
                info.endPTS,
                info.startDTS,
                info.endDTS
              );
          this.hls.trigger(Events.LEVEL_PTS_UPDATED, {
            details,
            level,
            drift,
            type,
            frag,
            start: info.startPTS,
            end: info.endPTS,
          });
          return true;
        }
        return result;
      },
      false
    );
    if (!parsed) {
      this.warn(
        `Found no media in fragment ${frag.sn} of level ${level.id} resetting transmuxer to fallback to playlist timing`
      );
      this.resetTransmuxer();
    }
    this.state = State.PARSED;
    this.hls.trigger(Events.FRAG_PARSED, { frag, part });
  }
 
  protected resetTransmuxer() {
    if (this.transmuxer) {
      this.transmuxer.destroy();
      this.transmuxer = null;
    }
  }
 
  set state(nextState) {
    const previousState = this._state;
    if (previousState !== nextState) {
      this._state = nextState;
      this.log(`${previousState}->${nextState}`);
    }
  }
 
  get state() {
    return this._state;
  }
}