## Enhancement Request: Adding population count for Integers to the standard image

Richard A. O'Keefe ok at cs.otago.ac.nz
Thu Dec 9 01:13:20 UTC 2004

```"Matthew Hamrick" <Matthew.Hamrick at gibson.com> wrote
(edited somewhat to take fewer lines and be more ASCII-friendly):

popCount
"Answers the number of bits set in an integer."
| t1 t2 |
t1 := self.
t2 := 0.
[t1 == 0] whileFalse: [t1 := t1 bitAnd: t1 - 1. t2 := t2 + 1].
^ t2

What is this supposed to do when the number is negative?

-1 bitAnd: -2	=> -2
-2 bitAnd: -3	=> -4
-4 bitAnd: -5	=> -8

In Squeak 3.6-5429-full.image, (-1 popCount) eventually terminates because
m := -1 bitShift: 31.
m := m bitAnd: m - 1
sets m to 0, instead of to the correct value (-1 bitShift: 32).
First time I've ever had Squeak give me the wrong answer for an
integer calculation.

Notionally, a Squeak negative integer has infinitely many 1 bits, so I
suggest

popCount
"self >= 0 => answer the number of 1 bits in the binary representation.
self <  0 => answer the number of 0 bits in the binary representation.
Two's complement is assumed.
"
|m n|
(m := self) < 0 ifTrue: [m := -1 - self].
n := 0.
[m == 0] whileFalse: [m := m bitAnd: m-1. n := n+1].
^n

Another alternative would be to make it an error to send #popCount to a
negative integer.

```

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