racket-bitsyntax

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1 Introduction

This library adds three features to Racket:

• library support for bit strings, a generalization of byte vectors;

• syntactic support for extracting integers, floats, sub-bit-strings and general values from bit strings; and

• syntactic support for constructing bit strings from integers, floats, other bit strings and general values.

It is heavily inspired by Erlang’s binaries, bitstrings, and binary pattern-matching. The Erlang documentation provides a good introduction to these features:

• Bit syntax expressions in the Erlang Reference Manual

• Bit syntax in the Programming Examples Manual

The binary matching (bit-string-case) and formatting (bit-string) languages can also be extended with custom parsers and formatters, giving lightweight syntactic support for domain-specific binary encodings of values.

2 Changes

Version 4.0 of this library changes the way custom parsers and formatters work, requiring them to be macros, where in previous releases they could have been implemented as functions. Version 4.0 of the library also supports the use of bit-string-case and bit-string in Typed Racket code.

Version 3.2 of this library adds support for custom parsers and formatters.

Version 3.0 of this library uses :: instead of : to separate expressions from encoding specifications in the bit-string-case and bit-string macros. The reason for this is to avoid a collision with Typed Racket, which uses : for its own purposes.

3 What is a bit string?

A bit string is either

• a byte vector, as returned by bytes and friends;

• a bit-resolution slice of a byte vector, as returned by sub-bit-string; or

• a splicing-together of two bit strings, as returned by bit-string-append.

The routines in this library are written, except where specified, to handle any of these three representations for bit strings.

If you need to flatten a bit string into a contiguous sequence of whole bytes, use bit-string->bytes or bit-string->bytes/align.

4 API

All the functionality below can be accessed with a single require:

4.1 Types

4.2 Pattern-matching bit strings

Each clause is then tried in turn. The first succeeding clause determines the result of the whole expression. A clause matches successfully if all its segment-patterns match some portion of the input, there is no unused input left over at the end, and the guard-expr (if there is one) evaluates to a true value. If a clause succeeds, then (begin body-expr ...) is evaluated, and its result becomes the result of the whole expression.

If none of the clauses succeed, and there is an else clause, its body-exprs are evaluated and returned. If there’s no else clause and none of the others succeed, an error is signalled.

Each segment-pattern matches zero or more bits of the input bit string. The given type, signedness, endianness and width are used to extract a value from the bit string, at which point it is either compared to some other value using equal? (if a comparison-pattern was used in the segment-pattern), bound to a pattern variable (if a binding-pattern was used), or discarded (if a discard-pattern was used) before matching continues with the next segment-pattern.

The supported segment types are

• integer – this is the default. A signed or unsigned, big- or little-endian integer of the given width in bits is read out of the bit string. Unless otherwise specified, integers default to big-endian, unsigned, and eight bits wide. Any width, not just multiples of eight, is supported.

• float – A 32- or 64-bit float in either big- or little-endian byte order is read out of the bit string using floating-point-bytes->real. Unless otherwise specified, floats default to big-endian and 64 bits wide. Widths other than 32 or 64 bits are unsupported.

• binary – A sub-bit-string is read out of the bit string. The bit string can be an arbitrary number of bits long, not just a multiple of eight. Unless otherwise specified, the entire rest of the input will be consumed and returned.

• custom-parser – An arbitrary amount of the input is consumed, analysed, and transformed. The built-in parsing options can’t be specified in conjunction with a custom parser: instead, each custom parser accepts its own option arguments. See Custom parsers and custom formatters.

Each type (except for custom-parser) has a default signedness, endianness, and width in bits, as described above. These can all (again, except for custom-parsers, which manage such issues on their own) be overridden individually:

• unsigned and signed specify that integers should be decoded in an unsigned or signed manner, respectively.

• big-endian, little-endian and native-endian specify the endianness to use in decoding integers or floats. Specifying native-endian causes Racket to use whatever is the native endianness of the platform the program is currently running on (discovered using system-big-endian?).

• default causes the decoder to use whatever the default width is for the type specified.

• bytes n causes the decoder to try to consume n bytes of input for this segment-pattern.

• bits n causes the decoder to try to consume n bits of input for this segment-pattern.

For example:

(bit-string-case some-input-value

([(= 0 :: bytes 2)] 'a)

([(f :: bits 10) (:: binary)]

(when (and (< f 123) (>= f 100)))

'between-100-and-123)

([(f :: bits 10) (:: bits 6)]

f)

([(f :: bits 10) (:: bits 6) (rest :: binary)]

(list f rest)))

This expression analyses some-input-value, which must be a (bit-string?). It may contain:

• 16 zero bits, in which case the result is 'a; or

• a ten-bit big-endian unsigned integer followed by 6 bits which are ignored, where the integer is between 100 (inclusive) and 123 (exclusive), in which case the result is 'between-100-and-123; or

• the same as the previous clause, but without the guard; if this succeeds, the result is the ten-bit integer itself; or

• the same as the previous clause, but with an arbitrary number of bits following the six discarded bits. The result here is a list containing the ten-bit integer and the trailing bit string.

The following code block parses a Pascal-style byte string (one length byte, followed by the right number of data bytes) and decodes it using a UTF-8 codec:

(bit-string-case input-bit-string

([len (body :: binary bytes len)]

(bytes->string/utf-8 (bit-string-pack body))))

Notice how the len value, which came from the input bit string itself, is used to decide how much of the remaining input to consume.

4.3 Assembling bit strings from pieces

Each spec can specify an integer or floating-point number to encode, a bit string to copy into the output, or a custom formatting routine to apply to the given value. If a type is not specified, integer is assumed. If an endianness is (relevant but) not specified, big-endian is assumed. If a width is not given, integers are encoded as 8-bit quantities, floats are encoded as 64-bit quantities, and binary objects are copied into the output in their entirety. Custom formatters do not accept these options, since they manage the encoding of the value they are given themselves, and take whatever options they need by other means.

If a width is specified, integers will be truncated or sign-extended to fit, and binaries will be truncated. If a binary is shorter than a specified width, an error is signalled. Floating-point encoding can only be done using 32- or 64-bit widths.

For example:

(define (string->pascal/utf-8 str)

(let ((bs (string->bytes/utf-8 str)))

(bit-string (bytes-length bs) [bs :: binary])))

This subroutine encodes its string argument using a UTF-8 codec, and then assembles it into a Pascal-style string with a prefix length byte. If the encoded string is longer than 255 bytes, note that the length byte will be truncated and so the encoding will be incorrect. A better encoder would ensure that bs was not longer than 255 bytes before encoding it as a Pascal string.

Note that if you wish to leave all the options at their defaults (that is, [... :: integer bits 8]), you can use the second form of spec given above.

4.4 Custom parsers and custom formatters

For simple uses of bit-string-case and bit-string, the built-in parsers and formatters will often be enough. Many binary data formats, however, make heavy use of domain-specific value encodings, and it quickly becomes either repetitive or awkward and error-prone to express these domain-specific formats. Custom parsers and custom formatters exist to allow you to extend both bit-string-case and bit-string to provide convenient shortcut syntax for domain-specific data formats.

For example, imagine a particular protocol makes heavy use of Pascal-style strings: sequences of UTF-8 encoded bytes prefixed by a single length byte, intrinsically limited to a maximum length of 255 bytes. Performing the necessary checks and transformations quickly gets repetitive, as you can see:

(bit-string-case packet

([(= PACKET-TYPE-ONE)

username-length (raw-username :: binary bytes username-length)

password-length (raw-password :: binary bytes password-length)]

(let ((username (bytes->string/utf-8 raw-username))

(password (bytes->string/utf-8 raw-password)))

...))

([(= PACKET-TYPE-TWO)

(error-code :: big-endian integer bytes 2)

error-text-length

(raw-error-text :: binary bytes error-text-length)]

(let ((error-text (bytes->string/utf-8 raw-error-text)))

...))

...)

On the formatting side, things are just as bad:

(define (encode-packet-type-one username password)

(let ((raw-username (string->bytes/utf-8 username))

(raw-password (string->bytes/utf-8 password)))

(when (> (bytes-length raw-username) 255)

(error 'encode-packet-type-one "Username too long"))

(when (> (bytes-length raw-password) 255)

(error 'encode-packet-type-one "Password too long"))

(bit-string PACKET-TYPE-ONE

(bytes-length raw-username)

(raw-username :: binary)

(bytes-length raw-password)

(raw-password :: binary))))

By introducing a custom extension, comprising both a parser and formatter together, we can improve the situation enormously:

(define-syntax pascal-string/utf-8

(syntax-rules ()

[(_ #t input ks kf)

; The first argument to the custom parser/formatter

; will be a literal #t to signal it is being used

; as a parser.

(bit-string-case input

([len (body :: binary bytes len) (rest :: binary)]

(ks (bytes->string/utf-8 (bit-string->bytes body)) rest))

(else

(kf)))]

[(_ #f str)

; The first argument to the custom parser/formatter

; will be a literal #f to signal it is being used

; as a formatter.

(let* ((bs (string->bytes/utf-8 str))

(len (bytes-length bs)))

(when (> len 255)

(error 'pascal-string/utf-8

"String of length ~v too long; max is 255 bytes"

len))

(bit-string len (bs :: binary)))]))

This single definition can now be used in any bit-string-case or bit-string expression where it is in scope. Here’s the earlier example, rewritten to use pascal-string/utf-8:

(bit-string-case packet

([(= PACKET-TYPE-ONE)

(username :: (pascal-string/utf-8))

(password :: (pascal-string/utf-8))]

...)

([(= PACKET-TYPE-TWO)

(error-code :: big-endian integer bytes 2)

(error-text :: (pascal-string/utf-8))]

...)

...)

Formatting is likewise much simplified:

(define (encode-packet-type-one username password)

(bit-string PACKET-TYPE-ONE

(username :: (pascal-string/utf-8))

(password :: (pascal-string/utf-8))))

4.4.1 Supplying arguments to custom parsers and formatters

Custom parser/formatters must be macros that accept one or more arguments. The first argument is a boolean flag, supplied as #t by bit-string-case or as #f by bit-string, indicating whether the custom extension is being used as a parser or a formatter, respectively. Following the flag,

• parsers (#t) are given the remaining input to be parsed, a success continuation function ks and a failure continuation function kf, and

• formatters (#f) are given the value to be formatted.

Subsequent arguments are supplied by the programmer at each use of the custom extension, and can be used to tweak the behaviour of the extension on a case-by-case basis.

For example, let’s suppose we didn’t want to restrict ourselves to the single length byte of Pascal-style strings, but wanted instead a more flexible way of indicating that a certain block of bytes should be interpreted and rendered as UTF-8 encoded text. We might define a custom parser/formatter like the following:

(define-syntax utf-8

(syntax-rules ()

; Consume entirety of input, decode as UTF-8

[(_ #t input ks kf)

(ks (bytes->string/utf-8 (bit-string->bytes input)) (bytes))]

; Consume a prefix of the input, decode as UTF-8

[(_ #t input ks kf length-in-bytes)

(bit-string-case input

([ (body :: binary bytes length-in-bytes)

(rest :: binary)]

(ks (bytes->string/utf-8 (bit-string->bytes body)) rest))

(else

(kf)))]

; Encode the entire string without length prefix

[(_ #f str)

(string->bytes/utf-8 str)]

; Encode the entire string with a length prefix

[(_ #f str (length-format-options ...))

(let* ((bs (string->bytes/utf-8 str))

(len (bytes-length bs)))

(bit-string (len :: length-format-options ...)

(bs :: binary)))]))

A more general utf-8 would be able to specify a length limit as well as a length format. Extending the example in this way is left as an exercise for the reader.

The utf-8 parser/formatter can then be used in any of four different ways:

• In bit-string-case, as (var :: (utf-8)) – will take the remainder of the input and UTF-8 decode it to a string.

• In bit-string-case, as (var :: (utf-8 123)) – will take the next 123 bytes of the input and UTF-8 decode it to a string. Note that the length, here 123, can come from some earlier field extracted from the input, leading to a form of dependent parsing.

• In bit-string, as (val :: (utf-8)) – will encode and output the entirety of val as UTF-8.

• In bit-string, as (val :: (utf-8 (option ...))) – will encode val as UTF-8, and will prepend the length of the encoded text in the output. The length will be formatted using the options, along the lines of ((bytes-length encoded-text) :: option ...), so the length can be encoded in any way at all. A recursive use of a custom formatter could even encode it in a variable-length fashion.

Giving arguments to custom parser/formatters opens the door to utilities such as variable-length integer codecs, generic zlib-based compressing codecs, generic encrypting codecs, generic transcoders and so on.

Applications of a custom extension macro are rewritten by bit-string-case from (extension arg ...) to (extension #t input ks kf arg ...), and by bit-string to (extension #f value arg ...).

4.4.2 The detailed anatomy of a custom extension

A custom extension should accept

• the boolean flag indicating whether it is being used as a parser or a formatter,

• additional arguments, depending on whether it is being used as a parser or a formatter, and

• any other arguments supplied at the time of use.

When used in "parser" mode (with #t as its first argument), expects a piece of syntax denoting an input bit-string, a "success continuation" and a "failure continuation" as its second through fourth arguments. The result of expansion should analyse the input bit-string as it sees fit. If it decides it has successfully matched a prefix of the input, it should call the success continuation with two arguments: the value extracted from the input prefix, and the remaining unconsumed input (as a bit-string). If, on the other hand, it decides it cannot match a prefix of the input, it should call the failure continuation with no arguments.

When called in "formatter" mode (with #f as its first argument), it should expect a piece of syntax denoting the value to be formatted as its second argument. The result of expansion should be an expression resulting in the encoded form of this value, as a bit-string.

The general form, then, of custom extensions, is:

(define-syntax my-custom-extension

(syntax-rules ()

[(_ #t input success-k failure-k other-arg ...)

(if (analyze input)

(success-k result-of-analysis remainder-of-input)

(failure-k))]

[(_ #f value other-arg ...)

(format-value-as-bit-string value)]))

4.5 Bit string utilities

The function bit-string->byte will raise an exception if given a bit string of any length other than exactly eight bits.

4.6 Debugging utilities

These procedures may be useful for debugging, but should not be relied upon otherwise.