Re: [boost] [fixed_point] Request for interest in a binary fixed point library

Subject: Re: [boost] [fixed_point] Request for interest in a binary fixed point library
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2012-04-11 19:19:48

Le 12/04/12 00:25, Neal Becker a écrit :
> Vicente J. Botet Escriba wrote:
>
>> Le 11/04/12 14:37, Neal Becker a écrit :
> ...
>>> * fixed point<->   flt should be provided.  fixed.as_double() for example.
>> Should the
>>
>> fixed point ->   flt conversion take care of the rounding policy?
>>
> In my prototype code, every time a right shift is used, rounding policy is
> applied.
>
> rounding policy is a template parameter, with one default (the only one I care
> about).
>
> The default is,
> template<typename base_type=int>
> struct rnd {
>    static base_type apply (base_type x, int frac_bits) {
>      return ((x>>  (frac_bits-1)) + 1)>>  1;
>    }
> };
>
> To shift right by 'frac_bits' places, with rounding, we shift right 1 less
> place, add 1, then shift 1 more place.  When followed by saturation, this is the
> most common rounding operation I use for 2's complement hardware.
I want the library to avoid myself to apply these explicit operations. I 
don't  want to see on my code a shift to apply a round. IMO rounding 
should be applied when the resolution decreases during a conversion and 
the shift must depend on the difference and the rounding policy.
>>> Also,
>>> fixed (double, integer_bits, frac_bits) constructor, or something like it.
>> Could you be more precise, please?
> I want to be able to construct a fixed from a float, specifying the number of
> integer and fractional bits (or equivalent).
>
>
I don't understand. If the fixed_point type conveys the number of 
integer and fractional bits, why do you want to give them as parameters 
of the constructor?

Regards,
Vicente