recursive template at ctfe

[Shammah Chancellor]

On 2013-11-26 21:31:55 +0000, bioinfornatics said:

> On Tuesday, 26 November 2013 at 21:18:29 UTC, Shammah Chancellor wrote:
>> On 2013-11-26 21:00:56 +0000, bioinfornatics said:
>> 
>>> On Tuesday, 26 November 2013 at 20:50:13 UTC, bioinfornatics wrote:
>>>> On Tuesday, 26 November 2013 at 20:29:00 UTC, bearophile wrote:
>>>>> bioinfornatics:
>>>>> 
>>>>>> I wrote some template to compute at compile time how many bits is need 
>>>>>> for a number x. http://www.dpaste.dzfl.pl/99a842fd
>>>>>> 
>>>>>> That works for small number but after i got an error about limit recursion
>>>>> 
>>>>> Instead of template recursion, have you tried regular code run at compile time?
>>>>> 
>>>>> http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogObvious
>>>>> 
>>>>> Bye,
>>>>> bearophile
>>>> 
>>>> Thanks i will take a look
>>> 
>>> this one seem to be interesting 
>>> http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogLookup
>>> 
>>> is like i do but use a table than me i used a recursive way. I go to try it :-)
>> 
>> I think you just have a bug in your template code not terminating.   At 
>> most you should execute 4 times.   I think there's a problem with 
>> casting down from byte to bool.
>> 
>> However, not user if LesserType is what you're after, but this works:
>> 
>> template log2(ulong x)
>> {
>> static if( x >> 8 )
>> {
>> enum log2 = log2!(x >> 8) + 8;
>> }
>> else static if( x >> 1)
>> {
>> enum log2 = log2!(x>>1) + 1;
>> } else
>> {
>> enum log2 = 1;
>> }
>> }
> 
> Thanks your way is much easier to write, works fine :-)

I would suggest turning it into a compile-time-function though as this 
will instantiate a large number of templates if you use is 
repetitively.   Remember that templates are cached by DMD.

int log2(const ulong x)
{
   if( x >> 8 )
  {
    return log2(x >> 8) + 8;
  }
  else if( x >> 1)
  {
    return log2(x>>1) + 1;
  } else
  {
    return 1;
  }
}

void main()
{
  pragma(msg, log2(1UL<<63));
}