(module stx mzscheme
  (require "util.rkt"
           syntax/id-table)
  
  (provide parse)

  (define (bad-args stx num)
    (raise-syntax-error
     #f
     (format "incorrect number of arguments (should have ~a)" num)
     stx))
  
  ;; char-range-arg: syntax-object syntax-object -> nat
  ;; If c contains is a character or length 1 string, returns the integer
  ;; for the character.  Otherwise raises a syntax error.
  (define (char-range-arg stx containing-stx)
    (let ((c (syntax-e stx)))
      (cond
        ((char? c) (char->integer c))
        ((and (string? c) (= (string-length c) 1))
         (char->integer (string-ref c 0)))
        (else
         (raise-syntax-error
          #f
          "not a char or single-char string"
          containing-stx stx)))))
  (test-block ()
              ((char-range-arg #'#\1 #'here) (char->integer #\1))
              ((char-range-arg #'"1" #'here) (char->integer #\1)))
  
  (define orig-insp (variable-reference->module-declaration-inspector
                     (#%variable-reference)))
  (define (disarm stx)
    (syntax-disarm stx orig-insp))

  ;; parse : syntax-object (box (list-of syntax-object)) -> s-re (see re.rkt)
  ;; checks for errors and generates the plain s-exp form for s
  ;; Expands lex-abbrevs and applies lex-trans.
  (define (parse stx disappeared-uses)
    (let loop ([stx stx]
               [disappeared-uses disappeared-uses]
               ;; seen-lex-abbrevs: id-table
               [seen-lex-abbrevs (make-immutable-free-id-table)])
      (let ([recur (lambda (s)
                     (loop (syntax-rearm s stx)
                           disappeared-uses
                           seen-lex-abbrevs))]
            [recur/abbrev (lambda (s id)
                            (loop (syntax-rearm s stx)
                                  disappeared-uses
                                  (free-id-table-set seen-lex-abbrevs id id)))])
        (syntax-case (disarm stx) (repetition union intersection complement concatenation
                                              char-range char-complement)
          (_
           (identifier? stx)
           (let ((expansion (syntax-local-value stx (lambda () #f))))
             (unless (lex-abbrev? expansion)
               (raise-syntax-error 'regular-expression
                                   "undefined abbreviation"
                                   stx))
             ;; Check for cycles.
             (when (free-id-table-ref seen-lex-abbrevs stx (lambda () #f))
               (raise-syntax-error 'regular-expression
                                   "illegal lex-abbrev cycle detected"
                                   stx
                                   #f
                                   (list (free-id-table-ref seen-lex-abbrevs stx))))
             (set-box! disappeared-uses (cons stx (unbox disappeared-uses)))
             (recur/abbrev ((lex-abbrev-get-abbrev expansion)) stx)))
          (_
           (or (char? (syntax-e stx)) (string? (syntax-e stx)))
           (syntax-e stx))
          ((repetition arg ...)
           (let ((arg-list (syntax->list (syntax (arg ...)))))
             (unless (= 3 (length arg-list))
               (bad-args stx 2))
             (let ((low (syntax-e (car arg-list)))
                   (high (syntax-e (cadr arg-list)))
                   (re (caddr arg-list)))
               (unless (and (number? low) (exact? low) (integer? low) (>= low 0))
                 (raise-syntax-error #f
                                     "not a non-negative exact integer"
                                     stx
                                     (car arg-list)))
               (unless (or (and (number? high) (exact? high) (integer? high) (>= high 0))
                           (eq? high +inf.0))
                 (raise-syntax-error #f
                                     "not a non-negative exact integer or +inf.0"
                                     stx
                                     (cadr arg-list)))
               (unless (<= low high)
                 (raise-syntax-error
                  #f
                  "the first argument is not less than or equal to the second argument"
                  stx))
               `(repetition ,low ,high ,(recur re)))))
          ((union re ...)
           `(union ,@(map recur (syntax->list (syntax (re ...))))))
          ((intersection re ...)
           `(intersection ,@(map recur (syntax->list (syntax (re ...))))))
          ((complement re ...)
           (let ((re-list (syntax->list (syntax (re ...)))))
             (unless (= 1 (length re-list))
               (bad-args stx 1))
             `(complement ,(recur (car re-list)))))
          ((concatenation re ...)
           `(concatenation ,@(map recur (syntax->list (syntax (re ...))))))
          ((char-range arg ...)
           (let ((arg-list (syntax->list (syntax (arg ...)))))
             (unless (= 2 (length arg-list))
               (bad-args stx 2))
             (let ((i1 (char-range-arg (car arg-list) stx))
                   (i2 (char-range-arg (cadr arg-list) stx)))
               (if (<= i1 i2)
                   `(char-range ,(integer->char i1) ,(integer->char i2))
                   (raise-syntax-error
                    #f
                    "the first argument does not precede or equal second argument" 
                    stx)))))
          ((char-complement arg ...)
           (let ((arg-list (syntax->list (syntax (arg ...)))))
             (unless (= 1 (length arg-list))
               (bad-args stx 1))
             (let ((parsed (recur (car arg-list))))
               (unless (char-set? parsed)
                 (raise-syntax-error #f
                                     "not a character set"
                                     stx
                                     (car arg-list)))
               `(char-complement ,parsed))))
          ((op form ...)
           (identifier? (syntax op))
           (let* ((o (syntax op))
                  (expansion (syntax-local-value o (lambda () #f))))
             (set-box! disappeared-uses (cons o (unbox disappeared-uses)))
             (cond
               ((lex-trans? expansion)
                (recur ((lex-trans-f expansion) (disarm stx))))
               (expansion
                (raise-syntax-error 'regular-expression
                                    "not a lex-trans"
                                    stx))
               (else
                (raise-syntax-error 'regular-expression
                                    "undefined operator"
                                    stx)))))
          (_ 
           (raise-syntax-error
            'regular-expression
            "not a char, string, identifier, or (op args ...)"
            stx))))))
       

  
  ;; char-set? : s-re -> bool
  ;; A char-set is an re that matches only strings of length 1.
  ;; char-set? is conservative.
  (define (char-set? s-re)
    (cond
      ((char? s-re) #t)
      ((string? s-re) (= (string-length s-re) 1))
      ((list? s-re) 
       (let ((op (car s-re)))
         (case op
           ((union intersection) (andmap char-set? (cdr s-re)))
           ((char-range char-complement) #t)
           ((repetition)
            (and (= (cadr s-re) (caddr s-re)) (char-set? (cadddr s-re))))
           ((concatenation)
            (and (= 2 (length s-re)) (char-set? (cadr s-re))))
           (else #f))))
      (else #f)))
  (test-block ()
              ((char-set? #\a) #t)
              ((char-set? "12") #f)
              ((char-set? "1") #t)
              ((char-set? '(repetition 1 2 #\1)) #f)
              ((char-set? '(repetition 1 1 "12")) #f)
              ((char-set? '(repetition 1 1 "1")) #t)
              ((char-set? '(union "1" "2" "3")) #t)
              ((char-set? '(union "1" "" "3")) #f)
              ((char-set? '(intersection "1" "2" (union "3" "4"))) #t)
              ((char-set? '(intersection "1" "")) #f)
              ((char-set? '(complement "1")) #f)
              ((char-set? '(concatenation "1" "2")) #f)
              ((char-set? '(concatenation "" "2")) #f)
              ((char-set? '(concatenation "1")) #t)
              ((char-set? '(concatenation "12")) #f)
              ((char-set? '(char-range #\1 #\2)) #t)
              ((char-set? '(char-complement #\1)) #t))

  (test-block ()
              ((parse #'#\a) #\a)
              ((parse #'"1") "1")
              ((parse #'(repetition 1 1 #\1)) '(repetition 1 1 #\1))
              ((parse #'(repetition 0 +inf.0 #\1)) '(repetition 0 +inf.0 #\1))
              ((parse #'(union #\1 (union "2") (union)))
               '(union #\1 (union "2") (union)))
              ((parse #'(intersection #\1 (intersection "2") (intersection)))
               '(intersection #\1 (intersection "2") (intersection)))
              ((parse #'(complement (union #\1 #\2)))
               '(complement (union #\1 #\2)))
              ((parse #'(concatenation "1" "2" (concatenation)))
               '(concatenation "1" "2" (concatenation)))
              ((parse #'(char-range "1" #\1)) '(char-range #\1 #\1))
              ((parse #'(char-range #\1 "1")) '(char-range #\1 #\1))
              ((parse #'(char-range "1" "3")) '(char-range #\1 #\3))
              ((parse #'(char-complement (union "1" "2")))
               '(char-complement (union "1" "2"))))
  )