Re: Adapting switch(x){case(y):...} with hashing

[Andy Walker <jawalker at stl dot quik dot com>]

I will look into gperf.  Thanks.

    Andy

Geoff Keating wrote:

> dewar@gnat.com (Robert Dewar) writes:
>
> > <<One of the obscure to-do list entries had an interesting request.
> > Currently, g++ uses a binary-tree / jump-table solution to handle
> > "switch"-"case" processing.  The request was for hashing to be added as
> > an alternative when it would be appropriate (the note suggested sparse
> > sets).  Obviously, the problem is more complex than it appears, and
> > binary searches have their strengths.  Also, memory requirements may be
> > critically fatal.
> > >>
> >
> > I don't see why there should be any conceptual problem in implementing
> > this approach. All BCPL compilers had this capability for example.
>
> There are many things you can do in this area.
>
> For instance, often people have an enum:
>
> enum { FOO_0, FOO_1, FOO_2, FOO_3, FOO_4, FOO_5, FOO_6 };
>
> and write
>
> if (x == FOO_1 || x == FOO_2 || x == FOO_4 || x == FOO_5 || x == FOO_6)
>
> or have a switch statement:
>
> switch (x)
> {
>   case FOO_1: case FOO_2: case FOO_4: case FOO_5: case FOO_6:
>     ...
>   case FOO_0: case FOO_3:
>     ...
> }
>
> and this could be turned into, for instance
>
> if (x & ~7)
> {
>   int t = 1 << x;
>   if (t & 0x76)
>     ...
>   else if (t & 0x09)
>     ...
> }
>
> For hashing, you'd probably want to try to find a perfect hash, like gperf,
> when possible.
>
> --
> - Geoffrey Keating <geoffk@geoffk.org>