/**
 * Converts a JS string from its native UCS-2/UTF-16 representation into a
 * Uint8Array of the bytes used to represent the equivalent characters in UTF-8.
 *
 * Cribbed from the `goog.crypt.stringToUtf8ByteArray` function in the Google
 * Closure library, though updated to use typed arrays.
 */
export const fromUtf8 = (input: string): Uint8Array => {
  const bytes: Array<number> = [];
  for (let i = 0, len = input.length; i < len; i++) {
    const value = input.charCodeAt(i);
    if (value < 0x80) {
      bytes.push(value);
    } else if (value < 0x800) {
      bytes.push((value >> 6) | 0b11000000, (value & 0b111111) | 0b10000000);
    } else if (i + 1 < input.length && (value & 0xfc00) === 0xd800 && (input.charCodeAt(i + 1) & 0xfc00) === 0xdc00) {
      const surrogatePair = 0x10000 + ((value & 0b1111111111) << 10) + (input.charCodeAt(++i) & 0b1111111111);
      bytes.push(
        (surrogatePair >> 18) | 0b11110000,
        ((surrogatePair >> 12) & 0b111111) | 0b10000000,
        ((surrogatePair >> 6) & 0b111111) | 0b10000000,
        (surrogatePair & 0b111111) | 0b10000000
      );
    } else {
      bytes.push((value >> 12) | 0b11100000, ((value >> 6) & 0b111111) | 0b10000000, (value & 0b111111) | 0b10000000);
    }
  }

  return Uint8Array.from(bytes);
};

/**
 * Converts a typed array of bytes containing UTF-8 data into a native JS
 * string.
 *
 * Partly cribbed from the `goog.crypt.utf8ByteArrayToString` function in the
 * Google Closure library, though updated to use typed arrays and to better
 * handle astral plane code points.
 */
export const toUtf8 = (input: Uint8Array): string => {
  let decoded = "";
  for (let i = 0, len = input.length; i < len; i++) {
    const byte = input[i];
    if (byte < 0x80) {
      decoded += String.fromCharCode(byte);
    } else if (0b11000000 <= byte && byte < 0b11100000) {
      const nextByte = input[++i];
      decoded += String.fromCharCode(((byte & 0b11111) << 6) | (nextByte & 0b111111));
    } else if (0b11110000 <= byte && byte < 0b101101101) {
      const surrogatePair = [byte, input[++i], input[++i], input[++i]];
      const encoded = "%" + surrogatePair.map((byteValue) => byteValue.toString(16)).join("%");
      decoded += decodeURIComponent(encoded);
    } else {
      decoded += String.fromCharCode(
        ((byte & 0b1111) << 12) | ((input[++i] & 0b111111) << 6) | (input[++i] & 0b111111)
      );
    }
  }

  return decoded;
};