On 6/8/05, David Abrahams <dave@boost-consulting.com> wrote:
Peder Holt <peder.holt@gmail.com> writes:
Is there any interest in a compile-time meta-version of double?
Syntax:
//Definition typedef META_DOUBLE(3.1415926535897932384626433) pi; //Or META_DOUBLE(3.1415926535897932384626433) pi_;
Whoa, that's nice syntax!
Thanks :)
I'm still trying to guess how you do it. Does the PP treat 3.14 as three tokens?
Currently, I use a naive approach, that does not handle rounding errors correctly for exponents. It goes something like this: #define REMOVE_EXP256(number) number*(number>1e256?1e-256:1.) #define REMOVE_EXP128(number) \ REMOVE_EXP256(number)*(REMOVE_EXP256(number)>1e128?1e-128:1.) etc etc to #define REMOVE_EXP1(number)\ REMOVE_EXP2(number)*(REMOVE_EXP2(number)>1e1?1e-1:1.) And similar for negative exponents. After removing the exponent (normalising the number) I convert the number to int: #define BOOST_META_NORMALISED_DOUBLE_TO_INTA(value) (value<0?-1:1)*int((value)*1e8) #define BOOST_META_NORMALISED_DOUBLE_TO_INTB(value) (value<0?-1:1)*int(((value)*1e8-int((value)*1e8))*1e8) As mentioned, since double is 2-based, the above approach gives round-off errors since it is 10-based. Replacing the above with a 2-based exponent seems to solve this problem
//Mathematical operations typedef math::meta::add<pi,pi>::type pi2;
//Evaluation: assert(META_DOUBLE_EVAL(pi2)==3.1415926535897932384626433*2);
Really, even with FP rounding errors? I guess multiplying by 2 is fairly safe...
I think so, with the 2-based exponent I currently use.
This implementation uses two ints to represent the decimals, and a short to represent the exponent.
I don't understand why you'd choose int for one and short for the other when they have the same range requirements: both int and short are required to be at least 16 bits and neither short nor int is required to be more than 16 bits. Can you explain that?
Should have used long in stead of int. This has been fixed. Correction from above: I use 64 bits to represent the mantissa + sign and 16 bits to represent the exponent Regards, Peder
-- Dave Abrahams Boost Consulting www.boost-consulting.com
_______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost