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; Binary parsing
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;----------------------------------------
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; Apologia
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;
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; Binary parsing and unparsing are transformations between primitive or
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; composite Scheme values and their external binary representations.
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;
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; Examples include reading and writing JPEG, TIFF, MP3, ELF file
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; formats, communicating with DNS, Kerberos, LDAP, SLP internet
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; services, participating in Sun RPC and CORBA/IIOP distributed systems,
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; storing and retrieving (arrays of) floating-point numbers in a
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; portable and efficient way. This project will propose a set of low- and
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; intermediate- level procedures that make binary parsing possible.
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; Scheme is a good language to do research in text compression. Text
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; compression involves a great deal of building and traversing
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; dictionaries, trees and similar data structures, where Scheme
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; excels. Performance doesn't matter in research, but the size of
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; compressed files does (to figure out the bpc for the common
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; benchmarks). Variable-bit i/o is a necessity. It is implemented
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; in the present file.
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; ASN.1 corresponds to a higher-level parsing (LR parser
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; vs. lexer). Information in LDAP responses and X.509 certificates is
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; structural and recursive, and so lends itself to be processed in
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; Scheme. Variable bit i/o is necessary, and so is a binary lexer for
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; a LR parser. Parsing of ASN.1 is a highly profitable enterprise
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;----------------------------------------
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; The outline of the project
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;
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; Primitives and streams
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;
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; - read-byte
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; - read-u8vector (cf. read-string)
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; - with-input-from-u8vector, with-input-from-encoded-u8vector 'base64,...
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; building binary i/o streams from a sequence of bytes. Streams over
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; u8vector, u16vector, etc. provide a serial access to memory. See SRFI-4
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;
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; - read-bit, read-bits via overlayed streams given read-byte
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; implemented in the present file.
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;
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; - mmap-u8vector, munmap-u8vector
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;
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; Conversions
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; - u8vector->integer u8vector endianness,
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; u8vector->sinteger u8vector endianness
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; These conversion procedures turn a sequence of bytes to an unsigned or
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; signed integer, minding the byte order. The u8vector in question can
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; have size 1,2,4,8, 3 etc. bytes. These two functions therefore can be
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; used to read shorts, longs, extra longs, etc. numbers.
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; - u8vector-reverse and other useful u8vector operations
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;
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; - modf, frexp, ldexp
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; The above primitives can be emulated in R5RS, yet they are quite handy
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; (for portable FP manipulation) and can be executed very efficiently by
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; an FPU.
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;
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; Higher-level parsing and combinators
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; These are combinators that can compose primitives above for more
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; complex (possibly iterative) actions.
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;
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; - skip-bits, next-u8token,...
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; - IIOP, RPC/XDR, RMI
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; - binary lexer for existing LR/LL-parsers
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;
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; The composition of primitives and combinators will represent binary
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; parsing language in a _full_ notation. This is similar to XPath
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; expressions in full notation. Later we need to find out the
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; most-frequently used patterns of the binary parsing language and
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; design an abbreviated notation. The latter will need a special
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; "interpreter". The abbreviated notation may turn out to look like
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; Olin's regular expressions.
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; $Id: binary-parse.scm,v 1.1 2003/04/14 06:38:47 mainzelM Exp $
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;----------------------------------------
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; Test harness
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;
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; The following macro runs built-in test cases -- or does not run,
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; depending on which of the two lines below you commented out
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;(define-syntax run-test
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; (syntax-rules ()
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; ((run-test body ...)
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; (begin (newline)
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; (display "-->Test")
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; (newline)
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; body ...))))
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(define-syntax run-test
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(syntax-rules ()
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((run-test body ...)
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(begin #f))))
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;(define-macro (run-test . body) '(begin #f))
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;(defmacro run-test body `(begin (display "\n-->Test\n") ,@body))
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;;========================================================================
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;; Configuration section
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;;
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; Performance is very important for binary parsing. We have to get all
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; help from a particular Scheme system we can get. If a Scheme function
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; can support the following primitives faster, we should take
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; advantage of that fact.
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;---
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; Configuration for Gambit. See below for other systems, as well as R5RS
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; implementations
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;(define-macro (logior x y) `(##fixnum.logior ,x ,y))
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;(define-macro (logand x y) `(##fixnum.logand ,x ,y))
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;(define-macro (lsh-left x n) `(##fixnum.shl ,x ,n))
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;(define-macro (lsh-right x n) `(##fixnum.lshr ,x ,n))
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;(define-macro (lsh-left-one x) `(##fixnum.shl ,x 1))
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;(define-macro (lsh-right-one x) `(##fixnum.lshr ,x 1))
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;(define-macro (-- x) `(##fixnum.- ,x 1))
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;(define-macro (++ x) `(##fixnum.+ ,x 1))
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;(define-macro (bit-set? x mask) ; return x & mask != 0
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; `(##not (##fixnum.zero? (logand ,x ,mask)))
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;)
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; End of the Gambit-specific configuration section
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;---
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; ---
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; Configuration for S48/scsh
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(define logior bitwise-ior)
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(define logand bitwise-and)
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(define (lsh-right x n) (arithmetic-shift x (- n)))
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(define lsh-left arithmetic-shift)
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(define (lsh-left-one x) (arithmetic-shift x 1))
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(define (lsh-right-one x) (arithmetic-shift x -1))
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(define (bit-set? x mask)
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(not (zero? (bitwise-and x mask))))
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; -- and ++ are not valid R5RS names:
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; ++ is not used in this file anyway
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; -- is replaced by decrement-one:
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(define (decrement-one n)
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(- n 1))
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(define-syntax time
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(syntax-rules ()
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((time expr)
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expr)))
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; End of S48/scsh configuration
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; ---
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; combine bytes in the MSB order. A byte may be #f
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(define (combine-two b1 b2) ; The result is for sure a fixnum
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(and b1 b2 (logior (lsh-left b1 8) b2)))
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(define (combine-three b1 b2 b3) ; The result is for sure a fixnum
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(and b1 b2 b3 (logior (lsh-left (logior (lsh-left b1 8) b2) 8) b3)))
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; Here the result may be a BIGNUM
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(define (combine-bytes . bytes)
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(cond
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((null? bytes) 0)
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((not (car bytes)) #f)
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(else
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(let loop ((bytes (cdr bytes)) (result (car bytes)))
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(cond
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((null? bytes) result)
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((not (car bytes)) #f)
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(else (loop (cdr bytes) (+ (car bytes) (* 256 result)))))))))
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;---
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; R5RS implementations of the primitives
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; This is the most portable -- and the slowest implementation
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; See also logical.scm from SLIB
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; (define (logior x y)
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; (cond ((= x y) x)
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; ((zero? x) y)
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; ((zero? y) x)
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; (else
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; (+ (* (logior (quotient x 2) (quotient y 2)) 2)
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; (if (and (even? x) (even? y)) 0 1)))))
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; (define (logand x y)
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; (cond ((= x y) x)
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; ((zero? x) 0)
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; ((zero? y) 0)
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; (else
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; (+ (* (logand (quotient x 2) (quotient y 2)) 2)
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; (if (or (even? x) (even? y)) 0 1)))))
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; (define (lsh-left x n) (* x (expt 2 n)))
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; (define (lsh-right x n) (quotient x (expt 2 n)))
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; (define (lsh-left-one x) (* x 2))
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; (define (lsh-right-one x) (quotient x 2))
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; (define (-- x) (- x 1))
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; (define (++ x) (+ x 1))
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;(define (bit-set? x mask) ; return x & mask != 0
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; (odd? (quotient x mask)) ; mask is an exact power of two
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;)
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;========================================================================
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; Reading a byte
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; Read-byte is a fundamental primitive; it needs to be
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; added to the standard. Most of the other functions are library
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; procedures. The following is an approximation, which clearly doesn't
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; hold if the port is a Unicode (especially UTF-8) character stream.
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; Return a byte as an exact integer [0,255], or the EOF object
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(define (read-byte port)
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(let ((c (read-char port)))
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(if (eof-object? c) c (char->ascii c))))
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; The same as above, but returns #f on EOF.
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(define (read-byte-f port)
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(let ((c (read-char port)))
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(and (not (eof-object? c)) (char->ascii c))))
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;========================================================================
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; Bit stream
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; -- Function: make-bit-reader BYTE-READER
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; Given a BYTE-READER (a thunk), construct and return a function
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; bit-reader N
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;
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; that reads N bits from a byte-stream represented by the BYTE-READER.
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; The BYTE-READER is a function that takes no arguments and returns
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; the current byte as an exact integer [0-255]. The byte reader
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; should return #f on EOF.
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; The bit reader returns N bits as an exact unsigned integer,
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; 0 -... (no limit). N must be a positive integer, otherwise the bit reader
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; returns #f. There is no upper limit on N -- other than the size of the
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; input stream itself and the amount of (virtual) memory an OS is willing
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; to give to your process. If you want to read 1M of _bits_, go ahead.
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;
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; It is assumed that the bit order is the most-significant bit first.
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;
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; Note the bit reader keeps the following condition true at all times:
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; (= current-inport-pos (ceiling (/ no-bits-read 8)))
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; That is, no byte is read until the very moment we really need (some of)
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; its bits. The bit reader does _not_ "byte read ahead".
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; Therefore, it can be used to handle a concatenation of different
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; bit/byte streams *STRICTLY* sequentially, _without_ 'backing up a char',
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; 'unreading-char' etc. tricks.
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; For example, make-bit-reader has been used to read GRIB files of
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; meteorological data, which made of several bitstreams with headers and
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; tags.
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; Thus careful attention to byte-buffering and optimization are the
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; features of this bit reader.
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;
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; Usage example:
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; (define bit-reader (make-bit-reader (lambda () #b11000101)))
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; (bit-reader 3) ==> 6
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; (bit-reader 4) ==> 2
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; The test driver below is another example.
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;
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; Notes on the algorithm.
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; The function recognizes and handles the following special cases:
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; - the buffer is empty and 8, 16, 24 bits are to be read
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; - reading all bits which are currently in the byte-buffer
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; (and then maybe more)
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; - reading only one bit
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; Since the bit reader is going to be called many times, optimization is
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; critical. We need all the help from the compiler/interpreter
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; we can get.
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(define (make-bit-reader byte-reader)
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(let ((buffer 0) (mask 0) ; mask = 128 means that the buffer is full and
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; the msb bit is the current (yet unread) bit
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(bits-in-buffer 0))
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; read the byte into the buffer and set up the counters.
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; return #f on eof
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(define (set-buffer)
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(set! buffer (byte-reader))
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(and buffer
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(begin
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(set! bits-in-buffer 8)
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(set! mask 128)
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#t)))
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; Read fewer bits than there are in the buffer
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(define (read-few-bits n)
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(let ((value (logand buffer ; all bits in buffer
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(decrement-one (lsh-left-one mask)))))
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(set! bits-in-buffer (- bits-in-buffer n))
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(set! mask (lsh-right mask n))
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(lsh-right value bits-in-buffer))) ; remove extra bits
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; read n bits given an empty buffer, and append them to value, n>=8
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(define (add-more-bits value n)
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(let loop ((value value) (n n))
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(cond
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((zero? n) value)
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((< n 8)
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(let ((rest (read-n-bits n)))
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(and rest (+ (* value (lsh-left 1 n)) rest))))
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(else
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(let ((b (byte-reader)))
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(and b (loop (+ (* value 256) b) (- n 8))))))))
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; The main module
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(define (read-n-bits n)
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; Check the most common cases first
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(cond
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((not (positive? n)) #f)
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((zero? bits-in-buffer) ; the bit-buffer is empty
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(case n
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((8) (byte-reader))
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((16)
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(let ((b (byte-reader)))
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(combine-two b (byte-reader))))
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((24)
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(let* ((b1 (byte-reader)) (b2 (byte-reader)))
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(combine-three b1 b2 (byte-reader))))
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(else
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(cond
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((< n 8)
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(and (set-buffer) (read-few-bits n)))
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((< n 16)
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(let ((b (byte-reader)))
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(and (set-buffer)
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(logior (lsh-left b (- n 8))
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(read-few-bits (- n 8))))))
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(else
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(let ((b (byte-reader)))
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(and b (add-more-bits b (- n 8)))))))))
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((= n 1) ; read one bit
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(let ((value (if (bit-set? buffer mask) 1 0)))
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(set! mask (lsh-right-one mask))
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(set! bits-in-buffer (decrement-one bits-in-buffer))
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value))
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((>= n bits-in-buffer) ; will empty the buffer
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(let ((n-rem (- n bits-in-buffer))
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(value (logand buffer ; for mask=64, it'll be &63
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(decrement-one (lsh-left-one mask)))))
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(set! bits-in-buffer 0)
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(cond
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((zero? n-rem) value)
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((<= n-rem 16)
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(let ((rest (read-n-bits n-rem)))
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(and rest (logior (lsh-left value n-rem) rest))))
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(else (add-more-bits value n-rem)))))
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(else (read-few-bits n))
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))
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read-n-bits)
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)
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; Validation tests
|
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(run-test
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(define (read-bits numbers nbits)
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(let* ((left-numbers numbers)
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(bit-reader
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(make-bit-reader
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(lambda ()
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(and (pair? left-numbers)
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(let ((byte (car left-numbers)))
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(set! left-numbers (cdr left-numbers))
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byte))))))
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(let loop ((result '()))
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|
|
(let ((num (bit-reader nbits)))
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(if num (loop (cons num result)) (reverse result))))))
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|
|
(define (do-test numbers nbits expected)
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|
|
(let ((result (read-bits numbers nbits)))
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|
|
|
(for-each display
|
|
|
|
|
(list "Reading " numbers " by " nbits " bits" ;newline
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|
"The result is: " result ))
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|
(or (equal? result expected)
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|
(error "the result differs from the expected: " expected))))
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|
(do-test '(1 2 3 4 5 6 7) 8 '(1 2 3 4 5 6 7))
|
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|
|
|
(do-test '(193 5 131 4) 1
|
|
|
|
|
'(1 1 0 0 0 0 0 1 0 0 0 0 0 1 0 1 1 0 0 0 0 0 1 1
|
|
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|
|
0 0 0 0 0 1 0 0))
|
|
|
|
|
(do-test '(193 5 131 4 5) 2
|
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|
|
|
'(3 0 0 1 0 0 1 1 2 0 0 3 0 0 1 0 0 0 1 1))
|
|
|
|
|
(do-test '(193 5 131 4) 3
|
|
|
|
|
'(6 0 2 0 2 6 0 3 0 1))
|
|
|
|
|
(do-test '(193 5 131 4 5 6 7) 4
|
|
|
|
|
'(12 1 0 5 8 3 0 4 0 5 0 6 0 7))
|
|
|
|
|
(do-test '(193 5 131 4 5 6 7) 5
|
|
|
|
|
'(24 4 2 24 6 1 0 5 0 24 3))
|
|
|
|
|
(do-test '(193 5 131 4 5 6 7 8 17 24) 8
|
|
|
|
|
'(193 5 131 4 5 6 7 8 17 24))
|
|
|
|
|
(do-test '(193 5 131 4 5 6 7 8 17 24) 9
|
|
|
|
|
'(386 22 24 64 160 385 388 17))
|
|
|
|
|
(do-test '(193 5 131 4 5 6 7 8 17) 16
|
|
|
|
|
'(49413 33540 1286 1800))
|
|
|
|
|
(do-test '(193 5 131 4 5 6 104) 17
|
|
|
|
|
'(98827 3088 10291))
|
|
|
|
|
(do-test '(193 5 131 4 5 6 104) 19
|
|
|
|
|
'(395308 49409))
|
|
|
|
|
(do-test '(193 5 131 4 5 6 104) 55
|
|
|
|
|
'(27165365385724724))
|
|
|
|
|
(do-test '(193 5 131 4 5 6 104) 56
|
|
|
|
|
'(54330730771449448))
|
|
|
|
|
)
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
; Timing test
|
|
|
|
|
; This test relies on a Gambit special form 'time' to clock
|
|
|
|
|
; evaluation of an expression.
|
|
|
|
|
; R5RS does not provide any timing facilities. So the test below
|
|
|
|
|
; might not run on your particular system, and probably needs
|
|
|
|
|
; adjustment anyway.
|
|
|
|
|
|
|
|
|
|
(run-test
|
|
|
|
|
(let ((fname "/tmp/a") (size 10240)
|
|
|
|
|
(pattern (ascii->char #x55)))
|
|
|
|
|
(define (read-by-bits n)
|
|
|
|
|
(for-each display
|
|
|
|
|
(list "Reading the file by " n " bits "))
|
|
|
|
|
(call-with-input-file fname
|
|
|
|
|
(lambda (port)
|
|
|
|
|
(let ((bit-reader (make-bit-reader
|
|
|
|
|
(lambda () (read-byte-f port)))))
|
|
|
|
|
(time
|
|
|
|
|
(do ((c (bit-reader n) (bit-reader n))) ((not c))))))))
|
|
|
|
|
|
|
|
|
|
(for-each display
|
|
|
|
|
(list "Creating a file " fname " of size " size " filled with "
|
|
|
|
|
pattern ;"\n"
|
|
|
|
|
))
|
|
|
|
|
(with-output-to-file fname
|
|
|
|
|
(lambda () (do ((i 0 (+ 1 i))) ((>= i size)) (write-char pattern))))
|
|
|
|
|
|
|
|
|
|
(newline)
|
|
|
|
|
(display "Reading the file by characters: the baseline ")
|
|
|
|
|
(call-with-input-file fname
|
|
|
|
|
(lambda (port)
|
|
|
|
|
(time
|
|
|
|
|
(do ((c (read-char port) (read-char port))) ((eof-object? c))))))
|
|
|
|
|
|
|
|
|
|
(newline)
|
|
|
|
|
(display "Reading the file by bytes: ")
|
|
|
|
|
(call-with-input-file fname
|
|
|
|
|
(lambda (port)
|
|
|
|
|
(time
|
|
|
|
|
(do ((c (read-byte-f port) (read-byte-f port))) ((not c))))))
|
|
|
|
|
|
|
|
|
|
(for-each read-by-bits
|
|
|
|
|
(list 1 2 3 4 5 6 7 8 9 10 11 15 16 17 23 24 25 30 32 65535
|
|
|
|
|
(* 8 size)))
|
|
|
|
|
))
|
Martin Gasbichler