to-signed-int-byte-array.js

/**
 * Converts a string to a byte array using the UTF-8 encoding scheme, ensuring each byte fits in the signed range.
 * In GraalJS, you cannot call java.lang.String#getBytes(). Use this instead.
 * For details, see https://github.com/oracle/graaljs/issues/708
 * @param {string} str - The input string to be converted.
 *
 * @returns {number[]} - The byte array representation of the input string.
 *
 * @throws {Error} - If an error occurs during the conversion.
 *
 * @example
 * const inputString = 'exampleText';
 * const byteArr = toSignedIntByteArray(inputString);
 * console.log(byteArr); // Output: [101, 120, 97, 109, 112, 108, 101, 84, 101, 120, 116]
 */
function toSignedIntByteArray(str) {
    let byteArray = [];

    for (const char of str) {
      let codePoint = char.codePointAt(0); // Get the Unicode code point of the character

      if (codePoint <= 0x7F) {
          // 1 byte for code points in the range [0, 127]
          byteArray.push(codePoint > 127 ? codePoint - 256 : codePoint);
      } else if (codePoint <= 0x7FF) {

          // 2 bytes for code points in the range [128, 2047]
          byteArray.push(0xC0 | (codePoint >> 6));
          byteArray.push(0x80 | (codePoint & 0x3F));
      } else if (codePoint <= 0xFFFF) {
          // 3 bytes for code points in the range [2048, 65535]
          byteArray.push(0xE0 | (codePoint >> 12));
          byteArray.push(0x80 | ((codePoint >> 6) & 0x3F));
          byteArray.push(0x80 | (codePoint & 0x3F));
      } else if (codePoint <= 0x10FFFF) {
          // 4 bytes for code points in the range [65536, 1114111]
          byteArray.push(0xF0 | (codePoint >> 18));
          byteArray.push(0x80 | ((codePoint >> 12) & 0x3F));
          byteArray.push(0x80 | ((codePoint >> 6) & 0x3F));
          byteArray.push(0x80 | (codePoint & 0x3F));
      }
    }
    return byteArray.map(byte => (byte > 127 ? byte - 256 : byte)); // Ensure each byte fits in signed range
}

ECMAScriptで、node.jsのAPIやTextEncoderに頼らず、文字列をbyte配列に変換する。

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