http://d.puremagic.com/issues/show_bug.cgi?id=2973

           Summary: std.math.pow(int, int), etc.
           Product: D
           Version: 2.030
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: dsimcha@yahoo.com


std.math should have a function for efficiently and accurately computing int^int, int ^ uint, etc.  Casting to floating point is not a good solution b/c you lose performance and, in some cases, precision.  Furthermore, this would be dead simple to implement.

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ZHOU Zhenyu <rinick@gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |rinick@gmail.com




--- Comment #1 from ZHOU Zhenyu <rinick@gmail.com>  2009-07-21 01:02:35 PDT ---
(In reply to comment #0)
> std.math should have a function for efficiently and accurately computing int^int, int ^ uint, etc.  Casting to floating point is not a good solution b/c you lose performance and, in some cases, precision.  Furthermore, this would be dead simple to implement.

agree
int^uint and long^uint are useful.

but int^int is not necessary, because:
int a = pow(2,-1); this doesn't make any sense.

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Andrei Alexandrescu <andrei@metalanguage.com> changed:

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                 CC|                            |andrei@metalanguage.com




--- Comment #2 from Andrei Alexandrescu <andrei@metalanguage.com>  2009-07-21 19:19:38 PDT ---
We currently have:

pure nothrow F pow(F)(F x, uint n) if (isFloatingPoint!(F));
pure nothrow F pow(F)(F x, int n) if (isFloatingPoint!(F));
F pow(F)(F x, F y) if (isFloatingPoint!(F));

I think we're in good shape. We could add overloads for raising integrals to unsigned powers but I fear confusion due to wraparound. The magnitudes of exponents for which efficiency could become an issue are also those that would wraparound the result. If no compelling arguments come up, I'll close this soon.

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--- Comment #3 from David Simcha <dsimcha@yahoo.com>  2009-07-21 19:55:36 PDT ---
Couldn't you just stick an assert in there to make sure it doesn't wrap around?
 I haven't tested, but I would think that even in debug mode, this would be
faster than using the float version.

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--- Comment #4 from Andrei Alexandrescu <andrei@metalanguage.com>  2009-07-21 20:15:20 PDT ---
(In reply to comment #3)
> Couldn't you just stick an assert in there to make sure it doesn't wrap around?
>  I haven't tested, but I would think that even in debug mode, this would be
> faster than using the float version.

1. Please test. What powers are you looking at? For most bases and most exponents the result will go off range.

2. I prefer returning a floating point number instead of attempting to return an integer and failing dynamically, all for a doubtful speed benefit.

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--- Comment #5 from ZHOU Zhenyu <rinick@gmail.com>  2009-07-21 23:22:10 PDT ---
(In reply to comment #4)
> What powers are you looking at? For most bases and most exponents the result will go off range.

long a = foo();
long b = a * a * a;

I don't want to call foo() again.
So I have to define one more variable just because pow doesn't accept integer.
(In fact, I have a longPow function and I use it from time to time)

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Don <clugdbug@yahoo.com.au> changed:

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                 CC|                            |clugdbug@yahoo.com.au




--- Comment #6 from Don <clugdbug@yahoo.com.au>  2009-08-11 04:58:06 PDT ---
> Casting to floating point is not a good solution b/c
> you lose performance and, in some cases, precision.

Are you sure?
Performance:
Floating point multiplication is typically the same speed or faster than
integer multiplication on most CPUs from the past 15 years. On most Intel CPUs,
integer multiplication is actually done in the floating point unit.

Precision:
You'll only lose precision if the base is greater than the 1/real.epsilon. For
an 80-bit real, this means precision is lost only when the result is exactly
equal to ulong.max. And the only time that can happen is with pow(ulong.max,
1).
(because ulong.max is divisible by 5, but not by 25).

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Andrei Alexandrescu <andrei@metalanguage.com> changed:

           What    |Removed                     |Added
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             Status|NEW                         |ASSIGNED
         AssignedTo|nobody@puremagic.com        |andrei@metalanguage.com


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David Simcha <dsimcha@yahoo.com> changed:

           What    |Removed                     |Added
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             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |WONTFIX


--- Comment #7 from David Simcha <dsimcha@yahoo.com> 2009-10-18 07:51:24 PDT ---
Since I filed this one and I'm thoroughly convinced now that it's not necessary and noone else seems to want it, I'm resolving it as WONTFIX.

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--- Comment #8 from Andrei Alexandrescu <andrei@metalanguage.com> 2009-10-18 13:06:15 PDT ---
(In reply to comment #7)
> Since I filed this one and I'm thoroughly convinced now that it's not necessary and noone else seems to want it, I'm resolving it as WONTFIX.

Thanks for following through.

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