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digitalmars.D.bugs - [Issue 7013] New: Mutable interface for BigInts

reply d-bugmail puremagic.com writes:
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



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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Nov 25 2011
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7013


hsteoh quickfur.ath.cx changed:

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



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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Jul 09 2013
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7013






 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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Jul 09 2013
prev sibling next sibling parent d-bugmail puremagic.com writes:
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 When you perform x+=y; the data inside x probably doesn't change.


I meant the original x.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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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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prev sibling parent d-bugmail puremagic.com writes:
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 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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