(module servlet-tables mzscheme
  (require (lib "contract.ss")
           (lib "url.ss" "net")
           (lib "list.ss"))
  (provide (struct exn:servlet-instance ())
           (struct exn:servlet-continuation ())
           (struct execution-context (connection request suspend))
           (struct servlet-instance (id k-table custodian context mutex))
           current-servlet-instance)

  ;; current-servlet-instance. The server will parameterize
  ;; over the current-servlet-instance before invoking a servlet
  ;; or invoking a continuation. The current-servlet-instance
  ;; will be in affect for the entire dynamic extent of every
  ;; continuation associated with that instance.
  (define current-servlet-instance (make-parameter #f))
  (define-struct servlet-instance (id k-table custodian context mutex))
  (define-struct execution-context (connection request suspend))

  ;; Notes:
  ;; * The servlet-instance-id is the key used for finding the servlet-instance in
  ;;   instance table.
  ;; * The servlet-instance-k-table stores continuations that were created
  ;;   during this instance.
  ;; * The servlet-instance-execution-context stores the context in which the
  ;;   instance is executing. The servlet-instance can have only one
  ;;   execution-context at any particular time. The execution-context will be
  ;;   updated whenever a continuation associated with this instance is
  ;;   invoked.
  ;; * The servlet-instance-mutex is used to guarentee mutual-exclusion in the
  ;;   case when it is attempted to invoke multiple continuations
  ;;   simultaneously.

  (provide/contract
   [continuation-url? (url? . -> . (union boolean? (list/c symbol? number? number?)))]
   [store-continuation! (procedure? url? servlet-instance? . -> . string?)]
   [create-new-instance! (hash-table? custodian? execution-context? semaphore?
                                      . -> . servlet-instance?)]
   [remove-instance! (hash-table? servlet-instance? . -> . any)]
   [clear-continuations! (servlet-instance? . -> . any)]
   )

  ;; not found in the instance table
  (define-struct (exn:servlet-instance exn) ())
  ;; not found in the continuatin table
  (define-struct (exn:servlet-continuation exn) ())

  (define-values (make-k-table get-k-id!)
    (let ([id-slot 'next-k-id])
      (values

       ;; make-k-table: -> (hash-table-of continuation)
       ;; Create a continuation table with an initial value for the next
       ;; continuation id.
       (lambda ()
         (let ([k-table (make-hash-table)])
           (hash-table-put! k-table id-slot 0)
           k-table))

       ;; get-k-id!: hash-table -> number
       ;; get the current-continuation id and increment the internal value
       (lambda (k-table)
         (let ([id (hash-table-get k-table id-slot)])
           (hash-table-put! k-table id-slot (add1 id))
           id)))))

  ;; store-continuation!: continuation execution-context servlet-instance -> url-string
  ;; store a continuation in a k-table for the provided servlet-instance
  (define (store-continuation! k uri inst)
    (let ([k-table (servlet-instance-k-table inst)])
      (let ([next-k-id (get-k-id! k-table)]
            [salt      (random 100000000)])
        (hash-table-put! k-table next-k-id (list k salt))
        (embed-ids (servlet-instance-id inst) next-k-id salt uri))))

  ;; clear-continuations!: servlet-instance -> void
  ;; replace the k-table for the given servlet-instance
  (define (clear-continuations! inst)
    (set-servlet-instance-k-table!
     inst
     (make-k-table)))

  ;; create-new-instance! hash-table custodian execution-context semaphore-> servlet-instance
  (define (create-new-instance! instance-table cust ctxt sema)
    (let* ([inst-id (string->symbol (symbol->string (gensym 'id)))]
           [inst
            (make-servlet-instance
             inst-id (make-k-table) cust ctxt sema)])
      (hash-table-put! instance-table inst-id inst)
      inst))

  ;; remove-instance!: hash-table servlet-instance -> void
  (define (remove-instance! instance-table inst)
    (hash-table-remove! instance-table (servlet-instance-id inst)))

  ;; ********************************************************************************
  ;; Parameter Embedding

  (define URL-PARAMS:REGEXP (regexp "([^\\*]*)\\*([^\\*]*)\\*([^\\*]*)"))

  (define (match-url-params x) (regexp-match URL-PARAMS:REGEXP x))

  ;; embed-ids: number number number url -> string
  ;; embedd the two numbers in a url
  (define (embed-ids inst-id k-id salt in-url)
    (insert-param
     in-url
     (format "~a*~a*~a" inst-id k-id salt)))

  ;; continuation-url?: url -> (union (list number number number) #f)
  ;; determine if this url encodes a continuation and extract the instance id and
  ;; continuation id.
  (define (continuation-url? a-url)
    (let ([str (url->param a-url)])
      (and str
           (let ([param-match (cdr (match-url-params str))])
             (list (string->symbol (car param-match))
                   (string->number (cadr param-match))
                   (string->number (caddr param-match)))))))

  ;; url->param: url -> (union string #f)
  (define (url->param a-url)
    (let ([l (filter path/param? (url-path a-url))])
      (and (not (null? l))
           (path/param-param (car l)))))

  ;; insert-param: url string -> string
  ;; add a path/param to the path in a url
  ;; (assumes that there is only one path/param)
  (define (insert-param in-url new-param-str)
    (url->string
     (replace-path
      (lambda (old-path)
        (if (null? old-path)
            (list (make-path/param "" new-param-str))
            (let* ([car-old-path (car old-path)])
              (cons (make-path/param (if (path/param? car-old-path)
                                         (path/param-path car-old-path)
                                         car-old-path)
                                     new-param-str)
                    (cdr old-path)))))
      in-url)))

  ;; replace-path: (url-path -> url-path) url -> url
  ;; make a new url by replacing the path part of a url with a function
  ;; of the url's old path
  ;; also remove the query
  (define (replace-path proc in-url)
    (let ([new-path (proc (url-path in-url))])
      (make-url
       (url-scheme in-url)
       (url-user in-url)
       (url-host in-url)
       (url-port in-url)
       new-path
       '()
       (url-fragment in-url))))

  ;; **************************************************

  )