bitwise-rotate-bit-field, fxrotate-bit-field - bitwise rotate left

LIBRARY

(import (rnrs))                     ;R6RS
(import (rnrs arithmetic bitwise))  ;R6RS
(import (rnrs arithmetic fixnums))  ;R6RS

SYNOPSIS

(bitwise-rotate-bit-field n start end count)
(fxrotate-bit-field n start end count)

DESCRIPTION

Returns the result of cyclically permuting in n the bits at positions from start (inclusive) to end (exclusive) by bits towards the more significant bits.

IMPLEMENTATION NOTES

The following are reference implementations from R6RS. 

;; bitwise-rotate-bit-field
(let ((width (- end start)))
  (if (positive? width)
      (let* ((count (mod count width))
             (field0
               (bitwise-bit-field n start end))
             (field1 (bitwise-arithmetic-shift-left
                       field0 count))
             (field2 (bitwise-arithmetic-shift-right
                       field0
                       (- width count)))
             (field (bitwise-ior field1 field2)))
        (bitwise-copy-bit-field n start end field))
      n))

;; fxrotate-bit-field
(let ((width (fx- end start)))
  (fxcopy-bit-field n start end
    (fxior
      (fxarithmetic-shift-left
        (fxbit-field n start (fx- end count))
        count)
      (fxarithmetic-shift-right
        (fxbit-field n start end)
        (fx- width count)))))

RETURN VALUES

Returns a single value; an exact integer.

EXAMPLES

(fxrotate-bit-field #x1234 4 12 4)  =>  #x1324

APPLICATION USAGE

Bitwise rotation is a common operation in cryptography.

COMPATIBILITY

These procedures are unique to R6RS. Similar alternatives can be found in SRFI-60 and SRFI-151.

ERRORS

These procedures can raise exceptions with the following condition types:
&assertion (R6RS)
The wrong number of arguments was passed or an argument was outside its domain. In particular, all arguments must be exact integers, start, end and count must be non-negative, and start must be less than or equal to end.

For the fixnum variant there are further restrictions. All arguments must be fixnums, and start, end and count must be less than the implementation's fixnum width, and count must be less than or equal to end-start.

SEE ALSO

bitwise-rotate-bit-field(3scm), bitwise-arithmetic-shift(3scm), bitwise-copy-bit-field(3scm)

STANDARDS

R6RS.

HISTORY

Many processor instruction set architectures have an integer rotate instruction. R6RS was the first Scheme report to standardize this operation. Previously SRFI-60 had slightly different bit-field-rotate procedure. Early Scheme running on MacLISP had access to the rot procedure that rotated a whole 36-bit fixnum by some amount.

AUTHORS

This page is part of the scheme-manpages project. It includes materials from the RnRS documents. More information can be found at https://github.com/schemedoc/manpages/.

Markup created by unroff 1.0sc,    March 04, 2023.