static int[] hexToInts(S s) {
  if (odd(l(s))) fail("Hex string has odd length: " + quote(shorten(10, s)));
  int n = l(s) / 8;
  int[] ints = new[n];
  for i to n: {
    int value = 0;
    for j to 4: {
      int a = parseHexChar(s.charAt(i*8+j*2));
      int b = parseHexChar(s.charAt(i*8+j*2+1));
      if (a < 0 || b < 0)
        fail("Bad hex byte: " + quote(substring(s, i*2, i*2+2)) + " at " + i*2 + "/" + l(s));
      value |= ((a << 4) | b) << (j*8);
    }
    ints[i] = value;
  }
  ret ints;
}