Simple not-byte-precise base64 encoder

base64.js

const CODE_A = "A".charCodeAt(0);
const CODE_a = "a".charCodeAt(0);
const CODE_0 = "0".charCodeAt(0);
const CODE_p = "+".charCodeAt(0);
const CODE_s = "/".charCodeAt(0);

// Dear v8, inline this please
const toA = (n) =>
  n === 63
    ? CODE_s
    : n === 62
    ? CODE_p
    : n >= 52
    ? n - 52 + CODE_0
    : n >= 26
    ? n - 26 + CODE_a
    : n + CODE_A;

const SRC_BLOCK_SIZE = 3; // bytes
const DST_BLOCK_SIZE = 4; // bytes

function encode(bytes) {
  const blocks = (bytes.length / SRC_BLOCK_SIZE) | 0;
  const blockBytes = blocks * SRC_BLOCK_SIZE;
  const left = bytes.length - blockBytes;

  const resultSize = blocks * DST_BLOCK_SIZE + (left ? left + 1 : 0);
  const res = new Uint8Array(resultSize);

  let isrc = 0;
  let idst = 0;
  for (
    ;
    isrc < blockBytes;
    isrc += SRC_BLOCK_SIZE, idst += DST_BLOCK_SIZE
  ) {
    const s1 = bytes[isrc + 0];
    const s2 = bytes[isrc + 1];
    const s3 = bytes[isrc + 2];

    res[idst + 0] = toA(s1 >> 2);
    res[idst + 1] = toA(((s1 & 0b00000011) << 4) | (s2 >> 4));
    res[idst + 2] = toA(((s2 & 0b00001111) << 2) | (s3 >> 6));
    res[idst + 3] = toA(s3 & 0b111111);
  }

  switch (left) {
    case 1:
      {
        const s1 = bytes[isrc];

        res[idst + 0] = toA(s1 >> 2);
        res[idst + 1] = toA((s1 & 0b00000011) << 4);
      }
      break;
    case 2:
      {
        const s1 = bytes[isrc];
        const s2 = bytes[isrc + 1];

        res[idst + 0] = toA(s1 >> 2);
        res[idst + 1] = toA(((s1 & 0b00000011) << 4) | (s2 >> 4));
        res[idst + 2] = toA((s2 & 0b00001111) << 2);
      }
      break;
  }

  return res;
}

const toBitString = (p) => (n) => n.toString(2).padStart(p, "0");

// const bytes = new TextEncoder().encode("abcdefghi0");
// console.log(Array.from(bytes).map(toBitString(8)).join(""));
// console.log(Array.from(map8to6(bytes)).map(toBitString(6)).join(""));
// console.log(
//   Array.from(encode(bytes))
//     .map((c) => String.fromCharCode(c))
//     .join("")
// );

// average full res photo
const size = 2_000_000;
const data = new Uint8Array(size);
let rng = 123;
for (let i = 0; i <= size; i++) {
  rng = (rng * 110315 + 1752157) & 0x7fffffff;
  data[i] = rng % 256;
}

console.time("warmup");
for (let i = 0; i <= 100; i++) {
  encode(data);
}
console.timeEnd("warmup");
// gives 701.285ms on 2012 MacBook Air

console.time("1000x");
for (let i = 0; i <= 1000; i++) {
  encode(data);
}
console.timeEnd("1000x");
// gives 6.700s on 2012 MacBook Air

console.log("done");