Generate Zstandard table representation from frequencies

index.js

// Will be used in fzstd if the default distributions ever update
// Not fast because it's meant to be run once for precomputation

// accuracy log
const al = 5;
// distribution
const dist = [1, 1, 1, 1, 1, 1, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1,
  1, 1, 1, 1, 1, 1, 1, 1,-1,-1,-1,-1,-1];

const out = new Uint8Array(1000);
let pos = 4;
out[0] = al - 5;
const msb = (val) => {
  let bits = 0;
  for (; (1 << bits) <= val; ++bits);
  return bits - 1;
}
let probs = 1 << al;
for (const val of dist) {
  const ev = (val + 1) << (pos & 7);
  out[pos >> 3] |= ev;
  out[(pos >> 3) + 1] |= ev >> 8;
  const bits = msb(probs + 1) + 1;
  const msv = (1 << bits) - 2 - probs;
  pos += bits - ((val + 1) < msv);
  probs -= Math.abs(val);
}
console.log(out, probs, pos);