123456789101112131415161718192021222324252627282930313233343536373839404142434445464748
const FIRST_BIT = 0x80;const FIRST_TWO_BITS = 0xc0;const FIRST_THREE_BITS = 0xe0;const FIRST_FOUR_BITS = 0xf0;const FIRST_FIVE_BITS = 0xf8;
const TWO_BIT_CHAR = 0xc0;const THREE_BIT_CHAR = 0xe0;const FOUR_BIT_CHAR = 0xf0;const CONTINUING_CHAR = 0x80;
/** * Determines if the passed in bytes are valid utf8 * @param bytes - An array of 8-bit bytes. Must be indexable and have length property * @param start - The index to start validating * @param end - The index to end validating */export function validateUtf8(  bytes: { [index: number]: number },  start: number,  end: number,): boolean {  let continuation = 0;
  for (let i = start; i < end; i += 1) {    const byte = bytes[i];
    if (continuation) {      if ((byte & FIRST_TWO_BITS) !== CONTINUING_CHAR) {        return false;      }      continuation -= 1;    } else if (byte & FIRST_BIT) {      if ((byte & FIRST_THREE_BITS) === TWO_BIT_CHAR) {        continuation = 1;      } else if ((byte & FIRST_FOUR_BITS) === THREE_BIT_CHAR) {        continuation = 2;      } else if ((byte & FIRST_FIVE_BITS) === FOUR_BIT_CHAR) {        continuation = 3;      } else {        return false;      }    }  }
  return !continuation;}