1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
|
(in-package :cl-utilities)
(defun rotate-byte (count bytespec integer)
"Rotates a field of bits within INTEGER; specifically, returns an
integer that contains the bits of INTEGER rotated COUNT times
leftwards within the byte specified by BYTESPEC, and elsewhere
contains the bits of INTEGER. See http://www.cliki.net/ROTATE-BYTE"
(declare (optimize (speed 3) (safety 0) (space 0) (debug 1)))
#-sbcl
(let ((size (byte-size bytespec)))
(when (= size 0)
(return-from rotate-byte integer))
(let ((count (mod count size)))
(labels ((rotate-byte-from-0 (count size integer)
(let ((bytespec (byte size 0)))
(if (> count 0)
(logior (ldb bytespec (ash integer count))
(ldb bytespec (ash integer (- count size))))
(logior (ldb bytespec (ash integer count))
(ldb bytespec (ash integer (+ count size))))))))
(dpb (rotate-byte-from-0 count size (ldb bytespec integer))
bytespec
integer))))
;; On SBCL, we use the SB-ROTATE-BYTE extension.
#+sbcl-uses-sb-rotate-byte (sb-rotate-byte:rotate-byte count bytespec integer))
;; If we're using the SB-ROTATE-BYTE extension, we should inline our
;; call and let SBCL handle optimization from there.
#+sbcl-uses-sb-rotate-byte (declaim (inline rotate-byte))
|
