// Run the SHIPPED player functions against a fake SourceBuffer.
//
// The point is that nothing here is a paraphrase of app.js: `bufferedAhead`,
// `evictBehind`, `flushQueue` and `pump` are lifted out of the file as text and
// executed. A model of a fix, written by whoever wrote the fix, agrees with it
// by construction — which is how a passing test sat next to a player that still
// hung. What is modelled here is the *browser*: a SourceBuffer with a ceiling,
// and `updateend` firing for removals as well as appends.
//
// Usage: node mse_harness.mjs <path to app.js> <json config>
import { readFileSync } from 'fs';

const app = readFileSync(process.argv[2], 'utf8');
const cfg = JSON.parse(process.argv[3] || '{}');

const {
  playing = false,          // does the viewer actually press play
  capMB = 100,              // where the browser refuses the append
  fileMB = 493.5,           // the film, from a real upload
  durationS = 3936,
  netMBs = 35,              // measured node throughput
  wallS = 600,
} = cfg;

const grab = (name) => {
  const start = app.indexOf(`const ${name} = useCallback(`);
  if (start < 0) throw new Error(`${name} not found in app.js`);
  const deps = app.indexOf('\n  }, [', start);
  const end = app.indexOf(');', deps) + 2;
  return app.slice(start, end);
};

const useCallback = (fn) => fn;
// Sorted by where they appear in app.js, not by the order this list happens to
// be written in. Extracting into an order of our own would quietly repair a
// hook declared before its own dependency — a real fault, which reached
// production once, and which the harness is otherwise well placed to catch.
const src = ['currentRange', 'bufferedAhead', 'aheadLimit', 'evictBehind',
             'flushQueue', 'pump']
  .sort((a, b) => app.indexOf(`const ${a} = useCallback(`)
                - app.indexOf(`const ${b} = useCallback(`))
  .map(grab).join('\n');

const SEG = 256 * 1024;
const CAP = capMB * 1048576;
const BITRATE = fileMB * 1048576 / durationS;

// Read from app.js too, so a change to the constants is a change to the test.
// Arithmetic is allowed because some of them are written as one (`48 * 1024 *
// 1024`), and refusing anything that is not arithmetic keeps this a reader
// rather than an evaluator of whatever happens to be on the line.
const constOf = (name) => {
  const m = app.match(new RegExp(`const ${name} = ([^;]+);`));
  if (!m) throw new Error(`${name} not found`);
  if (!/^[\d\s.*/+-]+$/.test(m[1])) throw new Error(`${name} is not a number`);
  return Number(new Function(`return (${m[1]});`)());
};
const BUFFER_BEHIND_S = constOf('BUFFER_BEHIND_S');
const STREAM_WINDOW = constOf('STREAM_WINDOW');
const BUFFER_AHEAD_S = constOf('BUFFER_AHEAD_S');
const BUFFER_AHEAD_STEP_BYTES = constOf('BUFFER_AHEAD_STEP_BYTES');
const BUFFER_AHEAD_MAX_BYTES = constOf('BUFFER_AHEAD_MAX_BYTES');
const BUFFER_AHEAD_MAX_S = constOf('BUFFER_AHEAD_MAX_S');
const QUEUE_HIGH_WATER = constOf('QUEUE_HIGH_WATER');
const CREDIT_KEEPALIVE_MS = constOf('CREDIT_KEEPALIVE_MS');

let bytes = 0, ranges = [], appended = 0, removes = 0;
let granted = 0, keepalives = 0, sent = 0, credit = 0, quotaRefusals = 0;

const sb = {
  updating: false,
  get buffered() {
    return {
      get length() { return ranges.length; },
      start: (i) => ranges[i][0],
      end: (i) => ranges[i][1],
    };
  },
  appendBuffer(chunk) {
    if (bytes + chunk.byteLength > CAP) {
      quotaRefusals++;
      const e = new Error('quota'); e.name = 'QuotaExceededError'; throw e;
    }
    const at = ranges.length ? ranges[ranges.length - 1][1] : 0;
    const end = at + chunk.byteLength / BITRATE;
    // A real SourceBuffer coalesces contiguous ranges: `buffered` reports the
    // spans of media it holds, not the appends that built them. Pushing one
    // range per segment made every range a couple of seconds long, which is
    // invisible to code that reads `end(length - 1)` and fatal to code that
    // looks for the range around the playhead.
    if (ranges.length && Math.abs(ranges[ranges.length - 1][1] - at) < 0.001) {
      ranges[ranges.length - 1][1] = end;
    } else {
      ranges.push([at, end]);
    }
    bytes += chunk.byteLength;
    appended++;
  },
  remove(a, b) {
    removes++;
    // remove(a, b) takes a span out of whatever it overlaps, trimming a range
    // rather than only dropping whole ones — otherwise a coalesced range is
    // never evicted at all and the buffer grows without limit.
    let dropped = 0;
    const kept = [];
    for (const [s, e] of ranges) {
      if (e <= a || s >= b) { kept.push([s, e]); continue; }
      if (s < a) kept.push([s, a]);
      if (e > b) kept.push([b, e]);
      dropped += (Math.min(e, b) - Math.max(s, a)) * BITRATE;
    }
    ranges = kept;
    bytes -= dropped;
    // A real remove() is asynchronous and fires updateend when it lands. That
    // event is indistinguishable from an append's unless the player kept track.
    pendingRemoveEvents++;
  },
};
let pendingRemoveEvents = 0;

// `paused` is part of the element the player reads, not decoration: the
// read-ahead budget only grows while the film is running, and a fake video
// that never reports being paused would exercise the growing branch in a run
// whose whole point is that nobody pressed play.
const video = { currentTime: 0, paused: !playing };
const sbRef = { current: sb }, videoRef = { current: video };
const msRef = { current: { readyState: 'open', endOfStream() {} } };
const queueRef = { current: [] };
const appendingRef = { current: false }, endedRef = { current: false };
const outstandingRef = { current: 0 }, lastPokeRef = { current: 0 };
const quotaRef = { current: 0 }, stalledRef = { current: false };
const awaitingInitRef = { current: false };
// The film's own average bitrate, as `stream_init` gives the player, and the
// budget state the read-ahead walks up from. Both start where the component
// starts them.
const bitrateRef = { current: BITRATE };
const aheadBytesRef = { current: 0 };
const aheadCapRef = { current: BUFFER_AHEAD_MAX_BYTES };
const quotaHoldRef = { current: false };
const transportRef = {
  current: {
    connected: true,
    grantStreamCredit(n) {
      if (n === 0) { keepalives++; return; }
      granted += n; credit += n;
    },
  },
};

const fns = new Function(
  'sbRef,videoRef,msRef,queueRef,appendingRef,endedRef,outstandingRef,' +
  'lastPokeRef,transportRef,BUFFER_BEHIND_S,BUFFER_AHEAD_S,QUEUE_HIGH_WATER,' +
  'CREDIT_KEEPALIVE_MS,STREAM_WINDOW,quotaRef,stalledRef,awaitingInitRef,' +
  'bitrateRef,aheadBytesRef,aheadCapRef,quotaHoldRef,' +
  'BUFFER_AHEAD_STEP_BYTES,' +
  'BUFFER_AHEAD_MAX_BYTES,BUFFER_AHEAD_MAX_S,console,useCallback',
  src + '\n return {currentRange, bufferedAhead, aheadLimit, evictBehind,'
      + ' flushQueue, pump};'
)(sbRef, videoRef, msRef, queueRef, appendingRef, endedRef, outstandingRef,
  lastPokeRef, transportRef, BUFFER_BEHIND_S, BUFFER_AHEAD_S, QUEUE_HIGH_WATER,
  CREDIT_KEEPALIVE_MS, STREAM_WINDOW, quotaRef, stalledRef, awaitingInitRef,
  bitrateRef, aheadBytesRef, aheadCapRef, quotaHoldRef, BUFFER_AHEAD_STEP_BYTES,
  BUFFER_AHEAD_MAX_BYTES, BUFFER_AHEAD_MAX_S, console, useCallback);

// The player's own `updateend` listener, transcribed — the one part of the
// component that is a listener rather than a callback, and the place the
// remove/append distinction lives.
const updateend = () => {
  appendingRef.current = false;
  fns.pump();
};

credit = STREAM_WINDOW;
outstandingRef.current = STREAM_WINDOW;
let wall = 0;
const TICK = 0.05;
// Bytes the link has carried but not yet spent on a whole segment. This used
// to be re-created inside the loop and thrown away at the end of every tick,
// so a link slower than one segment per tick — 5 MB/s at this resolution,
// which is every mobile network there is — delivered *nothing at all* and the
// run reported a player that had simply never been fed. Anything below about
// forty megabits was unmeasurable here, which is most of the cases worth
// measuring. Carried over instead, and capped at one window's worth, because
// a link cannot bank a burst larger than what the node may have in flight.
let netBudget = 0;
while (wall < wallS) {
  wall += TICK;
  if (playing) {
    const end = ranges.length ? ranges[ranges.length - 1][1] : 0;
    video.currentTime = Math.min(video.currentTime + TICK, end);
  }
  fns.pump();                                    // the 1 s timer and timeupdate
  while (pendingRemoveEvents > 0) { pendingRemoveEvents--; updateend(); }

  netBudget = Math.min(netBudget + netMBs * 1048576 * TICK, SEG * STREAM_WINDOW);
  while (credit > 0 && netBudget >= SEG) {
    credit--; netBudget -= SEG; sent++;
    outstandingRef.current = Math.max(0, outstandingRef.current - 1);
    queueRef.current.push({ byteLength: SEG });
    fns.flushQueue();
    if (appendingRef.current) updateend();
    while (pendingRemoveEvents > 0) { pendingRemoveEvents--; updateend(); }
  }
}

console.log(JSON.stringify({
  sentMB: +(sent * SEG / 1048576).toFixed(1),
  heldInBufferMB: +(bytes / 1048576).toFixed(1),
  queueDepth: queueRef.current.length,
  bufferedAheadS: +fns.bufferedAhead().toFixed(1),
  aheadLimitS: +fns.aheadLimit().toFixed(1),
  budgetMB: +(aheadBytesRef.current / 1048576).toFixed(1),
  budgetCapMB: +(aheadCapRef.current / 1048576).toFixed(1),
  bitrateMbits: +(BITRATE * 8 / 1e6).toFixed(2),
  watchedS: +video.currentTime.toFixed(1),
  grants: granted,
  keepalives,
  removes,
  quotaRefusals: quotaRef.current,
  hitCeiling: bytes >= CAP * 0.99,
}));