Add population count (number or 1 bits) function to int or int32-int64

[DartBot]

This issue was originally filed by @ahmetaa

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This is a minor request. There are cases we need to count 1 bits in an integer. Something like:
http://docs.oracle.com/javase/7/docs/api/java/lang/Integer.html#bitCount(int)

[lrhn]

Removed Type-Defect label.
Added Type-Enhancement, Area-Library, Triaged labels.

[lrhn]

I don't think it will make sense in general for int. As an arbitrary precision two's complement number, it doesn't have a finite bit representation (negative numbers conceptually have an infinite number of '1' bits, which can be seen in bitwise operations).

If there was a class, say Int32, for some finite representation integers, the bitCount operation would make sense on that. I don't really want to add operations on int that act on a limited number of bits, so int.countBits32() doesn't seem right.

That's just speculations, though. We'll think about it, but no promises.

[rakudrama]

I think bitCount would be fine if it returned a special value for negative numbers.
I suggest null or -1 so the return type could be int, but double.Infinity would be ok.
Or it could throw an exception, like integer % by zero on the VM.

I don't think we should force people to use some Int32 class when, for some reasonable fraction of programs, in practice all the values are non-negative.

You can write (x & 0xFFFFFFFF).bitCount to always be sure of an int in the range 0..32, and that idiom is easily adapted to bit fields and numbers of bits that are unlikely to have a class, e.g. 17.

[rakudrama]

Or like logcount in common lisp, count the zeroes in negative numbers.

[DartBot]

This comment was originally written by @mdakin

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I think expecting programmers to use (x & 0xFFFFFFFF) would be error prone.

How about providing .asUnsigned(n) even maybe .asUnsigned32() and .asUnsigned64() etc. methods? (as mentioned in http://code.google.com/p/dart/issues/detail?id=6486)

Or adding all good variations like bitCount(), bitCount32() and bitCount64() to int?

Another option would be having a class like Bits and move all methods there. But this one is not good.

[floitschG]

Issue #10212 has been merged into this issue.

[floitschG]

Under consideration, but not in the near future.

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Added this to the Later milestone.

[kasperl]

Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

[kasperl]

Removed Oldschool-Milestone-Later label.

[srawlins]

Is this still desirable, after pkg/math was scrapped? Would this go into int still?