#lang racket

;; Example from the Erlang Programming Examples section of the Erlang User's Guide:
;;
;; -define(IP_VERSION, 4).
;; -define(IP_MIN_HDR_LEN, 5).
;; DgramSize = byte_size(Dgram),
;; case Dgram of
;;     <<?IP_VERSION:4, HLen:4, SrvcType:8, TotLen:16,
;;       ID:16, Flgs:3, FragOff:13,
;;       TTL:8, Proto:8, HdrChkSum:16,
;;       SrcIP:32,
;;       DestIP:32, RestDgram/binary>> when HLen>=5, 4*HLen=<DgramSize ->
;;         OptsLen = 4*(HLen - ?IP_MIN_HDR_LEN),
;;         <<Opts:OptsLen/binary,Data/binary>> = RestDgram,
;;     ...
;; end.
;;
;; Translated into the syntax defined in this file:
;;
;; (define IP-VERSION 4)
;; (define IP-MINIMUM-HEADER-LENGTH 5)
;; (bit-string-case datagram
;;   ([ (= IP-VERSION :: bits 4)
;;      (header-length :: bits 4)
;;      service-type
;;      (total-length :: bits 16)
;;      (id :: bits 16)
;;      (flags :: bits 3)
;;      (fragment-offset :: bits 13)
;;      ttl
;;      protocol
;;      (header-checksum :: bits 16)
;;      (source-ip :: bits 32)
;;      (destination-ip :: bits 32)
;;      (rest :: binary) ]
;;    (when (and (>= header-length 5)
;;               (>= (bit-string-length datagram) (* header-length 4))))
;;    (let ((options-length (* 4 (- header-length IP-MINIMUM-HEADER-LENGTH))))
;;      (bit-string-case rest
;;        ([ (opts :: binary bytes options-length)
;;           (data :: binary) ]
;;         ...))))
;;   ...)

;; TODO: this should probably be a match extension

(require "bitstx.rkt")
(require "bitstring.rkt")

(provide :: bit-string-case)

(define-syntax bit-string-case
  (syntax-rules ()
    ((_ value clause ...)
     (let ((temp value))
       (when (not (bit-string? temp))
	 (error 'bit-string-case "Not a bit string: ~v" temp))
       (bit-string-case-helper temp clause ...)))))

(define-for-syntax (parse-options action options)
  (let loop ((options options)
	     (type (syntax integer))
	     (signedness (syntax unsigned))
	     (endianness (syntax big-endian))
	     (width (syntax default)))
    (syntax-case options (binary integer float
			  signed unsigned
			  little-endian big-endian native-endian
			  bytes bits default)
      (()
       #`(#,action #,type #,signedness #,endianness #,width))
      ((binary rest ...)
       (loop (syntax (rest ...)) (syntax binary) signedness endianness width))
      ((integer rest ...)
       (loop (syntax (rest ...)) (syntax integer) signedness endianness width))
      ((float rest ...)
       (loop (syntax (rest ...)) (syntax float) signedness endianness width))
      ((signed rest ...)
       (loop (syntax (rest ...)) type (syntax signed) endianness width))
      ((unsigned rest ...)
       (loop (syntax (rest ...)) type (syntax unsigned) endianness width))
      ((big-endian rest ...)
       (loop (syntax (rest ...)) type signedness (syntax big-endian) width))
      ((little-endian rest ...)
       (loop (syntax (rest ...)) type signedness (syntax little-endian) width))
      ((native-endian rest ...)
       (loop (syntax (rest ...)) type signedness (syntax native-endian) width))
      ((bytes n rest ...)
       (loop (syntax (rest ...)) type signedness endianness (syntax (* 8 n))))
      ((bits n rest ...)
       (loop (syntax (rest ...)) type signedness endianness (syntax n)))
      ((default rest ...)
       (loop (syntax (rest ...)) type signedness endianness (syntax default))))))

(define-for-syntax (parse-trailer action trailer)
  (syntax-case trailer (::)
    ((:: option ...)
     (parse-options action (syntax (option ...))))
    (()
     (parse-options action (syntax ())))))

(define-for-syntax (parse-bit-string-pattern pattern-clause)
  (syntax-case pattern-clause (= ::)
    ((= expr trailer ...)
     (parse-trailer (syntax (check expr)) (syntax (trailer ...))))
    ((:: trailer ...)
     (parse-trailer (syntax discard) (syntax (:: trailer ...))))
    ((id trailer ...)
     (parse-trailer (syntax (bind id)) (syntax (trailer ...))))
    (id
     (parse-trailer (syntax (bind id)) (syntax ())))))

(define-syntax bit-string-case-helper
  (lambda (stx)
    (syntax-case stx (when else)
      ((_ value (else body ...))
       (syntax (begin body ...)))
      ((_ value)
       (syntax (error 'bit-string-case "No matching clauses for ~v"
		      (bit-string-pack value))))
      ((_ value ((pattern-clause ...) body-and-guard ...) clause ...)
       (with-syntax ([tval (syntax-case (syntax (body-and-guard ...)) (when else)
			     (((when guard-exp) body ...)
			      (syntax (if guard-exp (begin body ...) (kf))))
			     ((body ...)
			      (syntax (begin body ...))))]
		     [canonical-pattern (map parse-bit-string-pattern
					     (syntax->list (syntax (pattern-clause ...))))])
	 (syntax
	  (let ((kf (lambda ()
		      (bit-string-case-helper value clause ...))))
	    (bit-string-case-arm value
				 tval
				 kf
				 canonical-pattern))))))))

(define-syntax bit-string-case-arm
  (syntax-rules (binary integer float default)
    ((_ value tval fthunk ())
     (if (zero? (bit-string-length value))
	 tval
	 (fthunk)))
    ((_ value tval fthunk (( action binary dontcare1 dontcare2 default ) ))
     (bit-string-perform-action action value fthunk tval))
    ((_ value tval fthunk (( action integer dontcare1 dontcare2 default )
			   remaining-clauses ...))
     (bit-string-case-arm value tval fthunk (( action integer dontcare1 dontcare2 8 )
					     remaining-clauses ...)))
    ((_ value tval fthunk (( action float dontcare1 dontcare2 default )
			   remaining-clauses ...))
     (bit-string-case-arm value tval fthunk (( action float dontcare1 dontcare2 64 )
					     remaining-clauses ...)))
    ((_ value tval fthunk (( action type signedness endianness width )
			   remaining-clauses ...))
     (let-values (((lhs rhs) (bit-string-split-at-or-false value width)))
       (if (not lhs)
	   (fthunk)
	   (let ((this-value (bit-string-case-extract-value
			      lhs type signedness endianness width)))
	     (bit-string-perform-action action this-value fthunk
					(bit-string-case-arm rhs tval fthunk
							     (remaining-clauses ...)))))))))

(define-syntax bit-string-perform-action
  (syntax-rules (check bind discard)
    ((_ (bind identifier) this-value fthunk tval)
     (let ((identifier this-value))
       tval))
    ((_ (check expr) this-value fthunk tval)
     (if (equal? this-value expr)
	 tval
	 (fthunk)))
    ((_ discard this-value fthunk tval)
     tval)))

(define-syntax bit-string-case-extract-value
  (syntax-rules (binary integer float)
    ((_ bin binary dontcare1 dontcare2 width-in-bits)
     ;; The width is already correct from the action of bit-string-split-at-or-false.
     bin)
    ((_ bin float dontcare1 endianness 32)
     (floating-point-bytes->real (bit-string->bytes bin)
				 (bit-string-case-endianness endianness)
				 0 4))
    ((_ bin float dontcare1 endianness 64)
     (floating-point-bytes->real (bit-string->bytes bin)
				 (bit-string-case-endianness endianness)
				 0 8))
    ((_ bin integer signedness endianness width-in-bits)
     ;; The width is already correct from the action of bit-string-split-at-or-false.
     (bit-string->integer bin
			  (bit-string-case-endianness endianness)
			  (bit-string-case-signedness signedness)))))

(define-syntax bit-string-case-endianness
  (syntax-rules (little-endian big-endian native-endian)
    ((_ little-endian)
     #f)
    ((_ big-endian)
     #t)
    ((_ native-endian)
     (system-big-endian?))))

(define-syntax bit-string-case-signedness
  (syntax-rules (signed unsigned)
    ((_ unsigned)
     #f)
    ((_ signed)
     #t)))