simplified.ts

function inRange(a: number, min: number, max: number) {
  return min <= a && a <= max;
}

class UTF8Encoder {
  bytes = new Uint8Array(4).fill(0)
  len = 0;

  toBytes(codePoint: number):void {

    if (inRange(codePoint, 0x00, 0x7f)) {
      this.len = 1;
      this.bytes[0] = codePoint;
      return;
    }

    let offset: number = 0;
    if (inRange(codePoint, 0x0080, 0x07ff)) {
      this.len = 2;
      offset = 0xc0;
    } else if (inRange(codePoint, 0x0800, 0xffff)) {
      this.len = 3;
      offset = 0xe0;
    } else if (inRange(codePoint, 0x10000, 0x10ffff)) {
      this.len = 4;
      offset = 0xf0;
    } else {
      throw TypeError(`Code point out of range: \\x${codePoint.toString(16)}`);
    }

    let count = this.len - 1;

    this.bytes[0] = (codePoint >> (6 * count)) + offset;

    while (count > 0) {
      const temp = codePoint >> (6 * (count - 1));
      this.bytes[this.len - count] = 0x80 | (temp & 0x3f)
      count--;
    }
  }
}

class CPIterator {
  constructor(private s: string) {}
  currentValue = {value: 0, done: false}
  offset = 0
  next() {
    if (this.offset < this.s.length) {
      let cP = this.currentValue.value = this.s.codePointAt(this.offset)!;
      if (cP >= 0x10000) { this.offset += 2 }
      else this.offset += 1;
      return this.currentValue;
    } else {
      this.currentValue.done = true;
      return this.currentValue;
    }
  }
}

class CPIteratorWrapper {
  constructor(private iter: CPIterator) {}
  [Symbol.iterator]() { return this.iter; }
}

function codepointIterator(s: string) {
  let cpIterator = new CPIterator(s);
  return new CPIteratorWrapper(cpIterator);
}

function autoGrow(array: Uint8Array) {
  let replacement = new Uint8Array(4 + (array.length * 1.5) | 0);
    replacement.set(array);
  return replacement;
}

function copyTo(array:Uint8Array, offset: number, bytes: Uint8Array, len: number) {

  for (let byte = 0; byte < len; ++byte) {
    array[offset + byte] = bytes[byte];
  }
}

export class TextEncoder {
  /** Returns "utf-8". */
  readonly encoding = "utf-8";
  /** Returns the result of running UTF-8's encoder. */
  encode(input = ""): Uint8Array {
    const encoder = new UTF8Encoder();

    // fast path: assume we only need input.length bytes
    // works if no characters are outside ascii
    let array = new Uint8Array(input.length);

    let offset = 0;
    for (let cP of codepointIterator(input)) {
      encoder.toBytes(cP);
      if (offset + encoder.len > array.length) {
        array = autoGrow(array);
      }
      copyTo(array, offset, encoder.bytes, encoder.len);
      offset += encoder.len;
    }

    return array.slice(0, offset);
  }
}