between and among: worth Phobosization?

[Marco Leise]

Am Mon, 16 Dec 2013 15:04:09 -0800
schrieb Andrei Alexandrescu <SeeWebsiteForEmail at erdani.org>:

> On 12/16/13 2:55 PM, "Ola Fosheim Grøstad" 
> <ola.fosheim.grostad+dlang at gmail.com>" wrote:
> > On Monday, 16 December 2013 at 21:45:40 UTC, Walter Bright wrote:
> >>> uint among(T, Us...)(T v, Us vals)
> >>> {
> >>>     foreach (i, U; Us)
> >>>     {
> >>>         if (v == vals[i]) return i + 1;
> >>>     }
> >>>     return 0;
> >>> }
> >>
> >> This has O(n) behavior, which might be unexpected for the user.
> >
> > I would expect one table-lookup for this if vals contains strings, not N
> > ifs. If you look at the example, most of them could be done with perfect
> > hashing on a single character.
> >
> > Is it possible for the compiler/template system to turn this into a
> > switch/dictionary? Or is there something in the language/compiler that
> > makes that impossible?
> 
> It's a good idea, but unfortunately we don't have partial evaluation. 
> We'd need to move whatever compile-time work is to be done in the 
> template arguments area, i.e.
> 
> value.among!("foo", "bar", "baz")
> 
> as opposed to
> 
> value.among("foo", "bar", "baz")
> 
> Now that I think of it we can easily support both forms.
> 
> 
> 
> Andrei

This works well in general. I did the same for writing bits
to a stream:

writeBits()(uint bits, uint value)
writeBits(uint bits)(uint value)

(Now with 2.064 the empty template parenthesis can be omitted.)
Knowing that the bit count is 9 or less, you can optimize for a
case that only writes to two bytes. The one bit case doesn't
even need branching at all.

-- 
Marco