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authorBryan Newbold <bnewbold@robocracy.org>2017-02-20 00:05:25 -0800
committerBryan Newbold <bnewbold@robocracy.org>2017-02-20 00:05:25 -0800
commit8ffbc2df0fde83082610149d24e594c1cd879f4a (patch)
treea2be9aad5101c5e450ad141d15c514bc9c2a2963 /makcrc.scm
downloadslib-8ffbc2df0fde83082610149d24e594c1cd879f4a.tar.gz
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+;;;; "makcrc.scm" Compute Cyclic Checksums
+;;; Copyright (C) 1995, 1996 Aubrey Jaffer.
+;
+;Permission to copy this software, to redistribute it, and to use it
+;for any purpose is granted, subject to the following restrictions and
+;understandings.
+;
+;1. Any copy made of this software must include this copyright notice
+;in full.
+;
+;2. I have made no warrantee or representation that the operation of
+;this software will be error-free, and I am under no obligation to
+;provide any services, by way of maintenance, update, or otherwise.
+;
+;3. In conjunction with products arising from the use of this
+;material, there shall be no use of my name in any advertising,
+;promotional, or sales literature without prior written consent in
+;each case.
+
+;;;(define crc (eval (make-port-crc 16 #o010013)))
+;;;(define crc (eval (make-port-crc 08 #o053)))
+
+(define (file-check-sum file) (call-with-input-file file crc32))
+
+(define (make-port-crc . margs)
+ (define (make-mask hibit)
+ (+ (ash (+ -1 (ash 1 (+ 1 (- hibit 2)))) 1) 1))
+ (define accum-bits 32)
+ (define chunk-bits (integer-length (+ -1 char-code-limit)))
+ (define generator #f)
+ (cond ((pair? margs)
+ (set! accum-bits (car margs))
+ (cond ((pair? (cdr margs))
+ (set! generator (cadr margs))))))
+ (cond ((not generator)
+ (case accum-bits
+ ((32) (set! generator #b00000100110000010001110110110111))
+ (else (slib:error 'make-port-crc "no default polynomial for"
+ accum-bits "bits")))))
+ (let* ((chunk-mask (make-mask chunk-bits))
+ (crctab (make-vector (+ 1 chunk-mask))))
+ (define (accum src)
+ `(set!
+ crc
+ (logxor (ash (logand ,(make-mask (- accum-bits chunk-bits)) crc)
+ ,chunk-bits)
+ (vector-ref crctab
+ (logand ,chunk-mask
+ (logxor
+ (ash crc ,(- chunk-bits accum-bits))
+ ,src))))))
+ (define (make-crc-table)
+ (letrec ((r (make-vector chunk-bits))
+ (remd (lambda (m)
+ (define rem 0)
+ (do ((i 0 (+ 1 i)))
+ ((>= i chunk-bits) rem)
+ (if (logbit? i m)
+ (set! rem (logxor rem (vector-ref r i))))))))
+ (vector-set! r 0 generator)
+ (do ((i 1 (+ 1 i)))
+ ((>= i chunk-bits))
+ (let ((r-1 (vector-ref r (+ -1 i)))
+ (m-1 (make-mask (+ -1 accum-bits))))
+ (vector-set! r i (if (logbit? (+ -1 accum-bits) r-1)
+ (logxor (ash (logand m-1 r-1) 1) generator)
+ (ash (logand m-1 r-1) 1)))))
+ (do ((i 0 (+ 1 i)))
+ ((> i chunk-mask))
+ (vector-set! crctab i (remd i)))))
+ (cond ((>= (integer-length generator) accum-bits)
+ (slib:error 'make-port-crc
+ "generator longer than" accum-bits "bits")))
+ (make-crc-table)
+ `(lambda (port)
+ (define crc 0)
+ (define byte-count 0)
+ (define crctab ,crctab)
+ (do ((ci (read-char port) (read-char port)))
+ ((eof-object? ci))
+ ,(accum '(char->integer ci))
+ (set! byte-count (+ 1 byte-count)))
+ (do ((byte-count byte-count (ash byte-count ,(- chunk-bits))))
+ ((zero? byte-count))
+ ,(accum 'byte-count))
+ (logxor ,(make-mask accum-bits) crc))))