Le 01/02/2021 à 09:45, Marco Antoniotti a écrit :
Duh!
What?
(1- (integer-length x)) is not ffs.
76543 10 vvvvv vv #b11010100 ^ 2 (ffs #b11010100) = 2 (integer-length #b11010100) = 8 (1- (integer-length #b11010100)) = 7
(defun ffs/naive (n) (check-type n integer) (loop :for ffs :from 0 :while (evenp n) :do (setf n (ash n -1)) :finally (return ffs)))
(ffs/naive #b11010100) ; --> 2
The wikipedia page gives algorithms that are better than O(n), but with implementations in ℤ/2ⁿ instead of ℤ.
Instead, you can extract the least significant 1 bit using (compose 1+ lognot) and logand:
(progn (format t "~@{~32B~%~}" #b11010100 (logand #xffffffff (lognot #b11010100)) (logand #xffffffff (1+ (lognot #b11010100))) (logand #xffffffff (logand #b11010100 (1+ (lognot #b11010100))))) (format t "~D~%" (truncate (log (logand #b11010100 (1+ (lognot #b11010100))) 2))) (values)) 11010100 11111111111111111111111100101011 11111111111111111111111100101100 100 2
(defun ffs (n) (check-type n integer) (truncate (log (logand n (1+ (lognot n))) 2)))
(mapcar (function ffs) '(#b110101 #b1101010 #b11010100 #b110101000 #b1101010000)) --> (0 1 2 3 4)
On Mon, Feb 1, 2021 at 9:42 AM dbm@refined-audiometrics.com mailto:dbm@refined-audiometrics.com <dbm@refined-audiometrics.com mailto:dbm@refined-audiometrics.com> wrote:
(1- (integer-length x))On Feb 1, 2021, at 1:24 AM, Marco Antoniotti <marco.antoniotti@unimib.it <mailto:marco.antoniotti@unimib.it>> wrote: Hi I am wasti.... devoting some time to recreational hacking and I bumped into an interesting bit fiddling operation. I pored over the CLHS, but, while I may have missed something obvious, I am not sure what would be the best way to implement such a function using the standard operations. Any ideas?b Note that it appears that most HW does have an instruction to do that directly. Find first set - Wikipedia https://en.wikipedia.org/wiki/Find_first_set
Thanks Marco -- Marco Antoniotti, Associate Professortel.+39 - 02 64 48 79 01 DISCo, Università Milano Bicocca U14 2043http://bimib.disco.unimib.it <http://bimib.disco.unimib.it/> Viale Sarca 336 I-20126 Milan (MI) ITALY-- Marco Antoniotti, Associate Professortel.+39 - 02 64 48 79 01 DISCo, Università Milano Bicocca U14 2043http://bimib.disco.unimib.it http://bimib.disco.unimib.it/ Viale Sarca 336 I-20126 Milan (MI) ITALY