Duh again! MA
On Mon, Feb 1, 2021 at 7:47 PM Pascal Bourguignon <p...@informatimago.com> wrote: > 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 d...@refined-audiometrics.com > > <mailto:d...@refined-audiometrics.com> <d...@refined-audiometrics.com > > <mailto:d...@refined-audiometrics.com>> wrote: > > > > (1- (integer-length x)) > > > > > >> On Feb 1, 2021, at 1:24 AM, Marco Antoniotti > >> <marco.antonio...@unimib.it <mailto:marco.antonio...@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 > > > -- > __Pascal Bourguignon__ > > -- Marco Antoniotti, Associate Professor tel. +39 - 02 64 48 79 01 DISCo, Università Milano Bicocca U14 2043 http://bimib.disco.unimib.it Viale Sarca 336 I-20126 Milan (MI) ITALY