Add logcount (Bug#22689)

* doc/lispref/numbers.texi (Bitwise Operations): Add documentation.
* etc/NEWS: Mention.
* src/data.c (logcount32, logcount64): New functions.
(logcount): New Lisp function.
(syms_of_data): Declare it.
* test/src/data-tests.el (data-tests-popcnt, data-tests-logcount): New
test.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/src/data-tests.el b/test/src/data-tests.el
index 8de8c145d40..d1154cc5c44 100644
--- a/test/src/data-tests.el
+++ b/test/src/data-tests.el
@@ -107,6 +107,19 @@
   (should (isnan (min 1.0 0.0e+NaN)))
   (should (isnan (min 1.0 0.0e+NaN 1.1))))
 
+(defun data-tests-popcnt (byte)
+  "Calculate the Hamming weight of 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)
+                   always (= (logcount n) (data-tests-popcnt n))))
+  ;; https://oeis.org/A000120
+  (should (= 11 (logcount 9727)))
+  (should (= 8 (logcount 9999))))
+
 ;; Bool vector tests.  Compactly represent bool vectors as hex
 ;; strings.