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digitalmars.D.learn - BigFloat?

reply Geroge Little <dothadeedumdi maildrop.cc> writes:
Is there support for BigFloat in phobos or any other package? I 
was playing around with D and wrote some code that calculates a 
Fibonacci sequence (iterative) with overflow detection that 
upgrades ulong to BigInt. I also wanted to use Binet's formula 
which requires sqrt(5) but it only works up to n=96 or so due to 
floating point precision loss. The code is here:

https://gist.github.com/ggl/38458b57b1eb3945ce447c8bf1d4e458
Apr 10
next sibling parent "H. S. Teoh via Digitalmars-d-learn" <digitalmars-d-learn puremagic.com> writes:
On Mon, Apr 10, 2017 at 06:54:54PM +0000, Geroge Little via Digitalmars-d-learn
wrote:
 Is there support for BigFloat in phobos or any other package? I was
 playing around with D and wrote some code that calculates a Fibonacci
 sequence (iterative) with overflow detection that upgrades ulong to
 BigInt. I also wanted to use Binet's formula which requires sqrt(5)
 but it only works up to n=96 or so due to floating point precision
 loss. The code is here:
 
 https://gist.github.com/ggl/38458b57b1eb3945ce447c8bf1d4e458
There is no BigFloat in phobos, you could try looking at code.dlang.org to see if there's anything that you could use. One way to use sqrt(5) in your calculations might be to use a quadratic rational representation, i.e., as a triple (x,y,z) representing (x + y*√5)/z where x, y, z are integers (can be BigInt). The main observation is that numbers of this form are closed under +, -, *, and / (excluding divison by 0 of course), and thus form a field. Even nicer, is that all calculations are exact (it can be reduced to purely integer manipulations). So you could ostensibly implement a QuadRational type based on BigInt that you can use to freely compute with √5. The key to implementing division is to note that (x + y*√5)*(x - y*√5) = x^2 - 5*y^2, so you can use this fact to eliminate √5 from the denominator so the rest of the computation can be carried out purely using +, -, and *. I.e.: (x1 + y1*√5) ------------ (x2 + y2*√5) (x1 + y1*√5) (x2 - y2*√5) = ------------ * ------------ (x2 + y2*√5) (x2 - y2*√5) (x1 + y1*√5) * (x2 - y2*√5) = --------------------------- x2^2 - 5*y2^2 so you just multiply out the top, which will be of the form p+q*√5, and with the denominator you again have the representation (x + y*√5)/z. You can then reduce the representation by dividing each of x, y, z with gcd(x,y,z). Note that this scheme works with both BigInt and built-in types like long/ulong, but BigInt is recommended because the square terms in the denominator means that performing divisions tend to overflow long/ulong pretty quickly. Some time ago I wanted to implement a QuadRational library that basically does the above, plus a neat algorithm for exact comparison, but didn't finish it because I got a bit too ambitious and wanted to support numbers of the form x + y*√a + z*√b + ... as well. Turns out that was too much because the required representation would be exponential in length in the number of different radicals you wish to support. Perhaps a less ambitious library would be one that supports numbers of the form (x + y*√r)/z, where r is fixed. It would then encompass the complex numbers (by setting r=-1, though it loses orderability in that case), combinations of √5 like you have. This can be pretty useful for implementing exact arithmetic in certain geometrical computations (mainly r=√2 for things containing octagons and r=√5 for things involving pentagons, r=√3 for certain triangular constructions). T -- It is not the employer who pays the wages. Employers only handle the money. It is the customer who pays the wages. -- Henry Ford
Apr 10
prev sibling parent Russel Winder via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
On Mon, 2017-04-10 at 12:17 -0700, H. S. Teoh via Digitalmars-d-learn
wrote:
 [=E2=80=A6]
=20
 There is no BigFloat in phobos, you could try looking at
 code.dlang.org
 to see if there's anything that you could use.
=20
[=E2=80=A6] Isn't the way forward here just to wrap GMP: https://code.dlang.org/packages/gmp-d The question is really whether to deprecate the current BigInt in D. --=20 Russel. =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D Dr Russel Winder t: +44 20 7585 2200 voip: sip:russel.winder ekiga.n= et 41 Buckmaster Road m: +44 7770 465 077 xmpp: russel winder.org.uk London SW11 1EN, UK w: www.russel.org.uk skype: russel_winder
Apr 10