sha256.js

/**
 * Given a sting s, return the SHA-256 digest of s
 * (encoded as a UTF-16-LE byte array) in hex form.
 * @param {string} s
 * @returns {string}
 * 
 * python -c 'import hashlib; print(hashlib.sha256("a🐦".encode("utf-16-le")).hexdigest())'
 * 49f1ecba591ec4ae7a049570bd21b97dc77d42659f3fbe70a34a8f02ebacee17
 * 
 * python -c 'import hashlib; print(hashlib.sha256("$€𐐷𤭢".encode("utf-16-le")).hexdigest())'
 * 38f4744c36668c13a88e34c4c270292615d55a07fa5d4af9a8fc2364c34273be
 */
async function sha256(s) {
  return Array.prototype.map.call(
    new Uint8Array(
      await window.crypto.subtle.digest(
        "sha-256",
        new Uint8Array([...s]
          .map(x => x.codePointAt())
          .map(y => y > 0xffff ? [((y - 0x10000) >> 10) | 0xD800, ((y - 0x10000) & 0x3ff) | 0xDC00] : y)
          .flat().map(y => [y & 255, y >> 8]).flat())
      )
    ),
    x => x.toString(16).padStart(2, "0")
  ).join('');
}