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Subject: Re: Math in Rockbox

Re: Math in Rockbox

From: BlueChip <cs_bluechip_at_webtribe.net>
Date: 2005-07-06

what values of 'x' ? all positive? include 0?

if (x<0) return(0); // cannot handle negative values of x
if (x>9) return(-1); // result will be > (2^32)-1
unsigned long r = 1;
while (x>0) r = (r<<4)+(r<<1); // r*8 + r*2
return (r);

given you only have 9 possible return values, a lookup table would be
faster and not THAT much more cost in code-space.

At 16:53 06/07/2005, you wrote:
>well... If I just think of a q&d solution:
>
>long exp10(int x) {
> long result = 10;
> while (--x) {
> result = result * 10;
> }
> return result;
>}
>
>I haven't tested this, but it can be something like this. (And I'm sure
>it's slow as hell for large values of x)
>
>Tomas
>
>lear@algonet.se wrote:
>
> >Hi,
> >
> >For an upcoming patch, I need some fixed- or floating-point math. Does
> anyone have
> >a good fixed-point implementation of pow(10, x) (or exp10(x)), or some
> hints
> >on how to implement it? It doesn't need to be particularly fast; small size
> >is to be preferred actually.
> >
> >Or would it be acceptable to bring in the compiler's floating point
> stuff for
> >this? The idea is to not put it in a plugin or a codec (though I could put
> >it in the codec lib, it isn't really the natural place for it :)...
> >
> > Magnus
> >
> >_______________________________________________
> >http://cool.haxx.se/mailman/listinfo/rockbox
> >
> >
>
>_______________________________________________
>http://cool.haxx.se/mailman/listinfo/rockbox

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Received on Wed Jul 6 20:53:43 2005


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