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(slib.info)Cyclic Checksum

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Cyclic Checksum
===============

  `(require 'make-crc)'

 - Function: make-port-crc
 - Function: make-port-crc degree
     Returns an expression for a procedure of one argument, a port.
     This procedure reads characters from the port until the end of
     file and returns the integer checksum of the bytes read.

     The integer DEGREE, if given, specifies the degree of the
     polynomial being computed - which is also the number of bits
     computed in the checksums.  The default value is 32.

 - Function: make-port-crc generator
     The integer GENERATOR specifies the polynomial being computed.
     The power of 2 generating each 1 bit is the exponent of a term of
     the polynomial.  The value of GENERATOR must be larger than 127.

 - Function: make-port-crc degree generator
     The integer GENERATOR specifies the polynomial being computed.
     The power of 2 generating each 1 bit is the exponent of a term of
     the polynomial.  The bit at position DEGREE is implicit and should
     not be part of GENERATOR.  This allows systems with numbers
     limited to 32 bits to calculate 32 bit checksums.  The default
     value of GENERATOR when DEGREE is 32 (its default) is:

          (make-port-crc 32 #b00000100110000010001110110110111)

     Creates a procedure to calculate the P1003.2/D11.2 (POSIX.2) 32-bit
     checksum from the polynomial:

               32    26    23    22    16    12    11
            ( x   + x   + x   + x   + x   + x   + x   +
          
                10    8    7    5    4    2    1
               x   + x  + x  + x  + x  + x  + x  + 1 )  mod 2

     (require 'make-crc)
     (define crc32 (slib:eval (make-port-crc)))
     (define (file-check-sum file) (call-with-input-file file crc32))
     (file-check-sum (in-vicinity (library-vicinity) "ratize.scm"))
     
     => 157103930