Challenge: write a really really small front() for UTF8

[Michel Fortin]

On 2014-03-24 05:09:15 +0000, Andrei Alexandrescu 
<SeeWebsiteForEmail at erdani.org> said:

> On 3/23/14, 9:49 PM, Michel Fortin wrote:
>> 
> http://goo.gl/y9EVFr
> 
> Andrei

As I said earlier, that front2 version is broken. It's missing a break. 
Adding the break makes it non-interesting from an instruction count 
point of view.

Also, there's still an issue in my code above (found by safetyOff): the 
first "if" for ASCII should use 0b1000_0000, not 0b1000000 (missing one 
zero). Here's the corrected (again) version of front1:

dchar front1(char[] s)
{
  if (s[0] < 0b1000_0000)
    return s[0]; // ASCII

  // pattern     indicator  tailLength
  // 0b1100xxxx  0b00 (0)   1
  // 0b1101xxxx  0b01 (1)   1 == indicator
  // 0b1110xxxx  0b10 (2)   2 == indicator
  // 0b1111xxxx  0b11 (3)   3 == indicator
  // note: undefined result for illegal 0b11111xxx case

  auto indicator = (s[0] >> 4) & 0b11;
  auto tailLength = indicator ? indicator : 1;

  dchar result = s[0] & (0b0011_1111 >> tailLength);
  foreach (i; 0..tailLength)
      result = (result << 6) | (s[1+i] & 0b0011_1111);
  return result;
}

And I'm going to suggest a variant of the above with one less branch 
(but three more instructions): look at how tailLenght is computed by 
or'ing with the negative of bit 2 of indicator. I suspect it'll be 
faster with non-ASCII input, unless it gets inlined less.

dchar front2(char[] s)
{
  if (s[0] < 0b1000_0000)
    return s[0]; // ASCII

  // pattern     indicator  tailLength
  // 0b1100xxxx  0b00 (0)   1
  // 0b1101xxxx  0b01 (1)   1
  // 0b1110xxxx  0b10 (2)   2
  // 0b1111xxxx  0b11 (3)   3
  // note: undefined result for illegal 0b11111xxx case

  auto indicator = ((s[0] >> 4) & 0b11);
  auto tailLength = indicator | ((~indicator >> 1) & 0b1);

  dchar result = s[0] & (0b0011_1111 >> tailLength);
  foreach (i; 0..tailLength)
      result = (result << 6) | (s[1+i] & 0b0011_1111);
  return result;
}

-- 
Michel Fortin
michel.fortin at michelf.ca
http://michelf.ca