gmail `compose` query parameter encoder/decoder

gmail-compose-encoder.js

const fullAlphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
const restrictedAlphabet = 'BCDFGHJKLMNPQRSTVWXZbcdfghjklmnpqrstvwxz';
const threadPrefix = 'thread-';
const messagePrefix = 'msg-';

const isWhitespace = str => /^[\s\xa0]*$/.test(str);
const isInvalidString = str => str ? (str.indexOf(threadPrefix) !== -1 || str.indexOf(messagePrefix) !== -1) : false;

const encode = function(str) {
  if (isWhitespace(str)) return str;
  str = str.replace(threadPrefix, '');
  return transliterate(btoa(str).replace(/=/g, ''), fullAlphabet, restrictedAlphabet);
};

const decode = function(str) {
  if (isInvalidString(str) || !/^[BCDFGHJKLMNPQRSTVWXZbcdfghjklmnpqrstvwxz]*$/.test(str)) return str;
  try {
    const transliterated = atob(transliterate(str, restrictedAlphabet, fullAlphabet))
    return transliterated.indexOf(threadPrefix) === -1 ? threadPrefix + transliterated : transliterated;
  } catch (err) {
    return str;
  }
};

const transliterate = function(subject, inputAlphabet, outputAlphabet) {
  if (!outputAlphabet) throw Error('rd');

  let i, j;
  const inputAlphabetSize = inputAlphabet.length;
  const outputAlphabetSize = outputAlphabet.length;

  let isEqual = true;
  for (i = 0; i < subject.length; i++)
    if (subject.charAt(i) != inputAlphabet.charAt(0)) {
      isEqual = false;
      break;
    }
  if (isEqual) return outputAlphabet.charAt(0);

  const inputAlphabetMap = {};
  for (i = 0; i < inputAlphabetSize; i++) inputAlphabetMap[inputAlphabet.charAt(i)] = i;

  const inputIndices = [];
  for (i = subject.length - 1; i >= 0; i--) {
    const char = subject.charAt(i);
    if (typeof inputAlphabetMap[char] === 'undefined') throw Error("sd`" + subject + "`" + inputAlphabet + "`" + char);
    inputIndices.push(inputAlphabetMap[char]);
  }

  const outputIndices = [];
  for (i = inputIndices.length - 1; i >= 0; i--) {
    let offset = 0;
    for (j = 0; j < outputIndices.length; j++) {
      let index = outputIndices[j] * inputAlphabetSize + offset;
      if (index >= outputAlphabetSize) {
        const remainder = index % outputAlphabetSize;
        offset = (index - remainder) / outputAlphabetSize;
        index = remainder;
      } else {
        offset = 0;
      }
      outputIndices[j] = index;
    }
    while (offset) {
      const remainder = offset % outputAlphabetSize;
      outputIndices.push(remainder);
      offset = (offset - remainder) / outputAlphabetSize;
    }

    offset = inputIndices[i];
    for (j = 0; offset; j++) {
      if (j >= outputIndices.length) outputIndices.push(0);
      let index = outputIndices[j] + offset;
      if (index >= outputAlphabetSize) {
        const remainder = index % outputAlphabetSize;
        offset = (index - remainder) / outputAlphabetSize;
        index = remainder;
      } else {
        offset = 0;
      }
      outputIndices[j] = index;
    }
  }

  const outputBuffer = [];
  for (i = outputIndices.length - 1; i >= 0; i--) {
    const index = outputIndices[i];
    if (index >= outputAlphabet.length || index < 0) throw Error("td`" + outputIndices + "`" + index);
    outputBuffer.push(outputAlphabet.charAt(index));
  }
  return outputBuffer.join('');
};

Related - https://stackoverflow.com/questions/50800330/gmail-api-does-not-support-new-thread-ids