aes70/src/utf8.js

A controller library for the AES70 protocol.

const E = new TextEncoder();
const D = new TextDecoder();

/**
 * Decodes a UTF8 encoded string from a Uint8Array.
 */
export function buffer_to_utf8(a8) {
  return D.decode(a8);
}

/**
 * Encodes a string as UTF8.
 *
 * @returns {Uint8Array}
 */
export function utf8_to_buffer(str) {
  return E.encode(str);
}

/**
 * Returns the number of bytes of the UTF8 encoding of
 * a string.
 */
export function utf8_encoded_length(str) {
  return utf8_to_buffer(str).byteLength;
}

/**
 * Counts the number of bytes occupied by a UTF8 encoded string
 * of given number of codepoints.
 *
 * @param {DataView} buf - The input buffer.
 * @param {number} pos - The buffer position.
 * @param {number} codepoints - The number of unicode codepoints in the buffer.
 * @returns {number} The number of bytes.
 */
export function utf8_codepoint_length(buf, pos, codepoints) {
  const start = pos;

  /* From table 3-6 in the Unicode standard 4.0: Well-Formed UTF-8
   * Byte Sequences
   *
   *  Code Points   1st Byte  2nd Byte  3rd Byte  4th Byte
   * 000000-00007f   00-7f
   * 000080-0007ff   c2-df     80-bf
   * 000800-000fff    e0       a0-bf     80-bf
   * 001000-00cfff   e1-ec     80-bf     80-bf
   * 00d000-00d7ff    ed       80-9f     80-bf
   * 00e000-00ffff   ee-ef     80-bf     80-bf
   * 010000-03ffff    f0       90-bf     80-bf     80-bf
   * 040000-0fffff   f1-f3     80-bf     80-bf     80-bf
   * 100000-10ffff    f4       80-8f     80-bf     80-bf
   */

  while (codepoints--) {
    const c = buf.getUint8(pos);
    pos++;
    if (c <= 0x7f) continue;
    if (c < 0xc2) throw new Error('Invalid UTF8 sequence.');
    pos++;
    if (c <= 0xdf) continue;
    pos++;
    if (c <= 0xef) continue;
    pos++;
    if (c <= 0xf4) continue;

    throw new Error('Invalid UTF8 sequence.');
  }

  return pos - start;
}

export function count_codepoints(s) {
  let n = 0;

  for (let i = 0; i < s.length; i++, n++) {
    const c = s.charCodeAt(i);

    if (c >= 0xd800 && c <= 0xdbff) {
      i++;

      const c = s.charCodeAt(i);

      if (c < 0xdc00 || c > 0xdfff)
        throw new TypeError('Expected valid unicode string.');
    }
  }

  return n;
}