continuation stream for websocket log stream.js

/*
 * idea: subscribe to websocket log
 * but get prior history via HTTP.
 * merge history and maintain sequence.
 * upsert fns close over a Vue ref.
 */
const ref = x => ({value: x});  // mock Vue

let items = ref([]);

const lower_bound = (arr, item, cmp) => {
  /* c++ std::lower_bound equivalent,
   * but w/ three-way comparison fn */
  let begin = 0, end = arr.length;
  while (begin < end) {
    let mid = (begin + end) >>> 1;
    if (cmp(arr[mid], item) < 0) {
      begin = mid + 1; }
    else {
      end = mid; } }
  return begin; };

const asc = (a, b) => {
  /* three-way compare arr[0] */
  if (a == undefined) return -1;
  let a0 = a[0], b0 = b[0];
  if (a0 < b0) return -1;
  else if (a0 > b0) return 1;
  return 0; };

const upsert_one = (it) => {
  let arr = items.value;
  if (asc(arr[arr.length-1], it) < 0) {
    // most-likely: append to end
    arr.push(it); }
  else {
    let idx = lower_bound(arr, it, asc);
    arr.splice(idx, asc(arr[idx], it)===0 ? 1 : 0, it); }
  items.value = arr; };

const upsert_base = (pack) => {
  /* upsert a sequence of items into
   * the map according to index */
  let arr = items.value;
  for (let i=0; i<pack.length; i++) {
    let it = [i, pack[i]];
    let idx = lower_bound(arr, it, asc);
    arr.splice(idx, asc(arr[idx], it)===0 ? 1 : 0, it); }
  items.value = arr; };

let foo = [2,3,1,5,6,6,6,6,0,5,1,3,1,5,6,0];

// connect websocket first
upsert_one([7, foo[7]]);  // new event
upsert_one([8, foo[8]]);  // "
upsert_one([9, foo[9]]);  // "

// get history via HTTP
upsert_base(foo.slice(0, 10));

// websocket continues over time
for (let i=10; i<foo.length; i++) {
  upsert_one([i, foo[i]]); }

// verify sequence
let good = true;
for (let i=0; i<foo.length; i++) {
  good = good && items.value[i][1] == foo[i]; }

console.log(`length ${items.value.length === foo.length ? 'OK' : 'ERROR'}`);
console.log(`data   ${good ? 'OK' : 'ERROR'}`);