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Make logcount act like CL on negative arg

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Behaving like Common Lisp is less likely to lead to surprises,
as it yields the same answers on 32- vs 64-bit machines.
* doc/lispref/numbers.texi (Bitwise Operations):
Document behavior on negative integers.
* src/data.c (Flogcount):
Behave like Common Lisp for negative arguments.
* test/src/data-tests.el (data-tests-popcnt)
(data-tests-logcount): Test negative args too.
This commit is contained in:
Paul Eggert 2017-09-30 15:36:52 -07:00
parent d247e1d30a
commit 8136df6a8c
3 changed files with 13 additions and 4 deletions
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7
doc/lispref/numbers.texi
View file

@ -1113,9 +1113,14 @@ bit is one in the result if, and only if, the @var{n}th bit is zero in
@defun logcount integer
This function returns the @dfn{Hamming weight} of @var{integer}: the
number of ones in the binary representation of @var{integer}.
If @var{integer} is negative, it returns the number of zero bits in
its two's complement binary representation. The result is always
nonnegative.
@example
(logcount 42) ; 42 = #b101010
(logcount 43) ; 43 = #b101011
@result{} 4
(logcount -43) ; -43 = #b111...1010101
@result{} 3
@end example
@end defun

6
src/data.c
View file

@ -3071,11 +3071,13 @@ usage: (logxor &rest INTS-OR-MARKERS) */)
DEFUN ("logcount", Flogcount, Slogcount, 1, 1, 0,
doc: /* Return population count of VALUE.
If VALUE is negative, the count is of its two's complement representation. */)
This is the number of one bits in the two's complement representation
of VALUE. If VALUE is negative, return the number of zero bits in the
representation. */)
(Lisp_Object value)
{
CHECK_NUMBER (value);
EMACS_UINT v = XUINT (value);
EMACS_INT v = XINT (value) < 0 ? -1 - XINT (value) : XINT (value);
return make_number (EMACS_UINT_WIDTH <= UINT_WIDTH
? count_one_bits (v)
: EMACS_UINT_WIDTH <= ULONG_WIDTH

4
test/src/data-tests.el
View file

@ -109,12 +109,14 @@
(defun data-tests-popcnt (byte)
"Calculate the Hamming weight of BYTE."
(if (< byte 0)
(setq byte (lognot byte)))
(setq byte (- byte (logand (lsh byte -1) #x55555555)))
(setq byte (+ (logand byte #x33333333) (logand (lsh byte -2) #x33333333)))
(lsh (* (logand (+ byte (lsh byte -4)) #x0f0f0f0f) #x01010101) -24))
(ert-deftest data-tests-logcount ()
(should (cl-loop for n in (number-sequence 0 255)
(should (cl-loop for n in (number-sequence -255 255)
always (= (logcount n) (data-tests-popcnt n))))
;; https://oeis.org/A000120
(should (= 11 (logcount 9727)))