On Tuesday, 3 October 2017 at 01:42:44 UTC, Andrei Alexandrescu
wrote:
A possible solution is to implement mul128 using primitives for
32- and 64-bit numbers. A possible signature would be:
void mul128(ulong a, ulong b, out ulong lo, out ulong hi);
The technique is to do the multiplication "by hand" as if you
had two numbers having two digits each. You obtain a 4-digit
number.
Consider the two-digit decimal numbers ab and cd, for example
43 and 65. Then the multiplication "by hand" is:
(10a + b) * (10c + d) = 100ac + 10(ad + bc) + bd
Since a, b, c, d are single digits it follows we only need
1-digit multiplication and addition with carry. Now of course
our base is not 10 but 2^^32, so we multiply two numbers, each
having two 32-bit "digits" and we get 128 bits worth of result.
mul128() would be a generally useful function to put in
druntime. I think someone on the general forum has implemented
it already, you may want to ask.
Andrei
Thanks for the suggestion!
I think I have also found a way to implement 'mulmod' without'
mul128'.
Is overflow well defined for unsigned types in D?
For example, (MAX_UINT + 1) equals 0 for unsigned types?
I will also come back with a 'mul128' implementation, since there
is none available for the moment in the standard library.
Thanks,
Alex
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