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

[dnspies]

On Monday, 24 March 2014 at 08:06:53 UTC, dnspies wrote:
On Sunday, 23 March 2014 at 21:23:18 UTC, Andrei Alexandrescu 
wrote:
Here's a baseline: http://goo.gl/91vIGc. Destroy!
Andrei

Here's mine (the loop gets unrolled):

dchar front(const char[] s) {
  assert(s.length > 0);
  byte c = s[0];
  dchar res = cast(ubyte)c;
  if(c >= 0) {
    return res;
  }
  c <<= 1;
  assert(c < 0);
  for(int i = 1; i < 4; i++) {
    assert(i < s.length);
    ubyte d = s[i];
    assert((d >> 6) == 0b10);
    res = (res << 8) | d;
    c <<= 1;
    if(c >= 0)
      return res;
  }
  assert(false);
}

Sorry, I misunderstood.  We only want the x's in the output.  
Here's my revised solution

http://goo.gl/PL729J

dchar front(const char[] s) {
  assert(s.length > 0);
  byte c = s[0];
  dchar res = cast(ubyte)c;
  if(c >= 0)
    return res;
  dchar cover = 0b0100_0000;
  c <<= 1;
  assert(c < 0);
  for(int i = 1; i < 4; i++) {
    assert(i < s.length);
    ubyte d = s[i];
    assert((d >> 6) == 0b10);
    cover <<= 5;
    res = ((res << 6) & (cover - 1)) | (d & 0b0011_1111);
    c <<= 1;
    if(c >= 0)
      return res;
  }
  assert(false);
}