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__
