[Openmcl-devel] ccl::log-e and bignum arg
Waldek Hebisch
hebisch at math.uni.wroc.pl
Wed Jun 2 09:56:49 UTC 2010
R. Matthew Emerson wrote:
> I don't understand what CCL::LOG-E is doing when it gets a bignum.
>
> Can anyone clue me in? Any links, references, etc. appreciated.
>
> If anyone
> (defun log-e (x)
> (cond
> ((bignump x)
> (if (minusp x)
;;; reduce to positive arguments
> (complex (log-e (- x)) pi)
> (let* ((base1 3)
> (guess (floor (1- (integer-length x))
> (log base1 2)))
> (guess1 (* guess (log-e base1))))
> (+ guess1 (log-e (/ x (expt base1 guess)))))))
> ...))
>
The code uses formula log(a*b) = log(a) + log(b). The line
with 'complex' takes as 'a' the sign of 'x' so the in the rest
there are only nonnegative numbers. The second part takes
a = 3^guess where guess is choosen so that b is of moderate
magnitude. log(3^guess) = guess*log(3) which above is called
guess1.
The purpose of second step presumably is to avoid overflow or
huge loss of precision converting bignum to floeating point number.
Since b is of moderate magnitude it can be converted to floating
point number without trouble.
Not so nice that the code uses single precision (Common Lisp default)
during intermediate calculations -- AFAICS it gives result with
lower precision that it could have.
--
Waldek Hebisch
hebisch at math.uni.wroc.pl
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