Stop Twiddling My Bits

Googling for how to compute checksums with Java might return insanity. Quick. What does this function do?

// Java
static String twiddleDee(byte[] data) {
  StringBuffer buf = new StringBuffer();
  for (int i = 0; i < data.length; i++) {
    int halfbyte = (data[i] >>> 4) & 0x0F;
    int two_halfs = 0;
    do {
      if ((0 <= halfbyte) && (halfbyte <= 9))
        buf.append((char) ('0' + halfbyte));
        buf.append((char) ('a' + (halfbyte - 10)));
      halfbyte = data[i] & 0x0F;
    } while (two_halfs++ < 1);
  return buf.toString();

Compute a SHA-1 and output a hex string. This is my first public service code donation:

// Java
public static String sha1(byte[] itsAllBitsAfterAll) {
  MessageDigest digester = newSha1Digester();
  return bytesAsHex(digester.digest());

// This might make a good future post about senseless
// factories
private static MessageDigest newSha1Digester() {
  try {
    return MessageDigest.getInstance("SHA-1");
  } catch (NoSuchAlgorithmException e) {
    throw new RuntimeException(
        "How many times must exceptions be thrown?", e);

static String bytesAsHex(byte[] bytes) {
  Formatter result = new Formatter();
  for (byte nextByte : bytes) {
    result.format("%02x", nextByte);
  return result.toString();

Like the countless optical illusions where the lines turn out to really be straight or the colors are actually the same, the first snippet matches the “bytesAsHex” method. The first is twenty times faster, but the second is twenty times clearer.

Always write the second. If you really need to squeeze those few extra milliseconds out of your code, use a library. If you think you can improve the library, use something open source, write it better, benchmark, and contribute.

Update, seven years later: In the intervening time, I’ve become somewhat more comfortable with bitwise operations and much more wary of dependencies. Today, I would not include a library only to optimize a little bit of string formatting.

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