#| -*-Scheme-*-

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    2017, 2018, 2019 Massachusetts Institute of Technology

This file is part of MIT/GNU Scheme.

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it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at
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WITHOUT ANY WARRANTY; without even the implied warranty of
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General Public License for more details.

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|#

;;;; Bit String Primitives
;;; package: (runtime bit-string)

(declare (usual-integrations))

(define-primitives
  bit-string-allocate make-bit-string bit-string?
  bit-string-length bit-string-ref bit-string-clear! bit-string-set!
  bit-string-zero? bit-string=?
  bit-string-fill! bit-string-move! bit-string-movec!
  bit-string-or! bit-string-and! bit-string-andc!
  bit-string-xor! bit-substring-move-right!
  bit-string->unsigned-integer unsigned-integer->bit-string
  read-bits! write-bits!
  bit-substring-find-next-set-bit)

(define (bit-string-copy bit-string)
  (let ((result (bit-string-allocate (bit-string-length bit-string))))
    (bit-string-move! result bit-string)
    result))

(define (bit-string-not bit-string)
  (let ((result (bit-string-allocate (bit-string-length bit-string))))
    (bit-string-movec! result bit-string)
    result))

(define (bit-string-or x y)
  (let ((result (bit-string-allocate (bit-string-length x))))
    (bit-string-move! result x)
    (bit-string-or! result y)
    result))

(define (bit-string-and x y)
  (let ((result (bit-string-allocate (bit-string-length x))))
    (bit-string-move! result x)
    (bit-string-and! result y)
    result))

(define (bit-string-andc x y)
  (let ((result (bit-string-allocate (bit-string-length x))))
    (bit-string-move! result x)
    (bit-string-andc! result y)
    result))

(define (bit-string-xor x y)
  (let ((result (bit-string-allocate (bit-string-length x))))
    (bit-string-move! result x)
    (bit-string-xor! result y)
    result))

(define (bit-substring bit-string start end)
  (let ((result (bit-string-allocate (- end start))))
    (bit-substring-move-right! bit-string start end result 0)
    result))

(define (bit-substring-extend string start end length)
  ;; Assumption: (<= (- end start) length)
  (let ((result (make-bit-string length false)))
    (bit-substring-move-right! string start end result 0)
    result))

(define (bit-string-append x y)
  (declare (integrate x y))
  (let ((x-length (bit-string-length x))
	(y-length (bit-string-length y)))
    (let ((result (bit-string-allocate (+ x-length y-length))))
      (bit-substring-move-right! x 0 x-length result 0)
      (bit-substring-move-right! y 0 y-length result x-length)
      result)))

(define (bit-string-append-reversed x y)
  (declare (integrate bit-string-append))
  (bit-string-append y x))

(define (signed-integer->bit-string nbits number)
  (unsigned-integer->bit-string
   nbits
   (cond ((negative? number)
	  (if (>= number (- (expt 2 (-1+ nbits))))
	      (+ number (expt 2 nbits))
	      (error "Integer too small to be encoded" number)))
	 ((< number (expt 2 (-1+ nbits))) number)
	 (else (error "Integer too large to be encoded" number)))))

(define (bit-string->signed-integer bit-string)
  (let ((unsigned-result (bit-string->unsigned-integer bit-string))
	(nbits (bit-string-length bit-string)))
    (if (bit-string-ref bit-string (-1+ nbits))	;Sign bit.
	(- unsigned-result (expt 2 nbits))
	unsigned-result)))