http://d.puremagic.com/issues/show_bug.cgi?id=7013

           Summary: Mutable interface for BigInts
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2011-11-25 16:52:13 PST ---
When multi-precision numbers become large or quite large, the time spent managing their memory risk becoming an issue. To improve this situation the GNU multiprecision integer library allows for mutable numbers, that allow in-place modification.

There was a discussion about offering a mutable interface for the General
Multiprecision PYthon project (GMPY) numbers too (see "Mutability... but not
for now"):
http://gmpy.sourceforge.net/

So maybe it's useful to add a mutable interface to Phobos bigints. A simple way to do it is to add a "mutable" attribute that supports just few in-place operations like += -= *= /= ^^= &= |= (but not other operations like == that don't need mutability, to keep the situation semantically more clean):

auto x = BigInt("...");
x.mutable += BigInt("...");

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hsteoh@quickfur.ath.cx changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hsteoh@quickfur.ath.cx


--- Comment #1 from hsteoh@quickfur.ath.cx 2013-07-09 08:59:04 PDT ---
In latest git HEAD, the following works:

import std.bigint;
void main() {
        BigInt x = 123;
        BigInt y = 321;
        x += y;
        assert(x == 444);
}

Not sure what's happening under the hood, though. Is it making internal allocations?

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--- Comment #2 from bearophile_hugs@eml.cc 2013-07-09 09:57:35 PDT ---
(In reply to comment #1)
> In latest git HEAD, the following works:
> 
> import std.bigint;
> void main() {
>         BigInt x = 123;
>         BigInt y = 321;
>         x += y;
>         assert(x == 444);
> }
> 
> Not sure what's happening under the hood, though. Is it making internal allocations?

That code works, but it's not about what this enhancement request is about. When you perform x+=y; the data inside x probably doesn't change.

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--- Comment #3 from bearophile_hugs@eml.cc 2013-07-09 09:58:59 PDT ---
(In reply to comment #2)

> When you perform x+=y; the data inside x probably doesn't change.

I meant the original x.

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--- Comment #4 from hsteoh@quickfur.ath.cx 2013-07-09 10:07:34 PDT ---
You're right, I looked at the code for BigInt, every time you do +=, it allocates a new underlying buffer. That's pretty inefficient if you're using these operations in an inner loop.

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--- Comment #5 from bearophile_hugs@eml.cc 2013-07-09 10:24:39 PDT ---
(In reply to comment #4)
> You're right, I looked at the code for BigInt, every time you do +=, it allocates a new underlying buffer. That's pretty inefficient if you're using these operations in an inner loop.

On the other hand I think a mutable integer is not what most people expect, and it can cause some undesired side effects (and bugs).

That's why I suggested to introduce a specific syntax that allows you to manage bigints as mutable buffers where max performance is needed, and keep their behavour clean on default. Generally I think it's better to perform tricky optimizations only on explicit request.

Some possible alternative syntaxes:

x.mutable += y;
x.mulAcc(y);
x.mutate!"+"(y);

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