(define-module (web socket frame)
  #:use-module (ice-9 format)
  #:use-module (ice-9 match)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-9 gnu)
  #:use-module (srfi srfi-26)
  #:use-module (rnrs bytevectors)
  #:use-module (rnrs io ports)
  #:export (make-frame
            frame?
            frame-final?
            frame-type
            frame-masking-key
            frame-data

            continuation-frame?
            text-frame?
            binary-frame?
            close-frame?
            ping-frame?
            pong-frame?
            fragment-frame?
            final-fragment-frame?
            control-frame?
            data-frame?

            frame-length
            frame-concatenate
            text-frame->string
            text-frames->string

            read-frame
            write-frame))

;;;
;;; WebSocket frames
;;;

(define-record-type <frame>
  (make-frame final? type masking-key data)
  frame?
  (final? frame-final?)
  (type frame-type)
  (masking-key frame-masking-key)
  (data frame-data))

(define (display-frame frame port)
  (format port "#<frame final?: ~a type: ~a masking-key: ~a length: ~d>"
          (frame-final? frame)
          (frame-type frame)
          (frame-masking-key frame)
          (frame-length frame)))

(set-record-type-printer! <frame> display-frame)

(define (continuation-frame? frame)
  "Return #t if FRAME is a continuation frame."
  (eq? (frame-type frame) 'continuation))

(define (text-frame? frame)
  "Return #t if FRAME is a text frame."
  (eq? (frame-type frame) 'text))

(define (binary-frame? frame)
  "Return #t if FRAME is a binary frame."
  (eq? (frame-type frame) 'binary))

(define (close-frame? frame)
  "Return #t if FRAME is a close frame."
  (eq? (frame-type frame) 'close))

(define (ping-frame? frame)
  "Return #t if FRAME is a ping frame."
  (eq? (frame-type frame) 'ping))

(define (pong-frame? frame)
  "Return #t if FRAME is a pong frame."
  (eq? (frame-type frame) 'pong))

;; See section 5.4 - Fragmentation
(define (fragment-frame? frame)
  "Return #t if FRAME is an incomplete message."
  (or (continuation-frame? frame)
      (not (frame-final? frame))))

(define (final-fragment-frame? frame)
  "Return #t if FRAME is the final piece of a fragmented message."
  (and (frame-final? frame)
       (continuation-frame? frame)))

;; See section 5.5 - Control Frames
(define (control-frame? frame)
  "Return #t if FRAME is a control frame."
  (or (close-frame? frame)
      (ping-frame? frame)
      (pong-frame? frame)))

;; See section 5.6 - Data Frames
(define (data-frame? frame)
  "Return #t if FRAME is a data frame."
  (or (text-frame? frame)
      (binary-frame? frame)))

(define (frame-length frame)
  "Return the length of the data bytevector in FRAME."
  (bytevector-length (frame-data frame)))

(define (text-frame->string frame)
  "Convert FRAME, an unfragmented text frame, into a string."
  (utf8->string (frame-data frame)))

(define (frame-concatenate frames)
  "Concatenate the data in FRAMES, a list of fragmented frames, into a
single bytevector."
  (let ((bv (make-bytevector (reduce + 0 (map frame-length frames)))))
    (let loop ((frames frames)
               (offset 0))
      (match frames
        (() bv)
        ((frame . rest)
         (let ((length (frame-length frame)))
           (bytevector-copy! (frame-data frame) 0 bv offset length)
           (loop rest (+ offset length))))))))

(define (text-frames->string frames)
  "Convert FRAMES, a list of fragmented text frames, into a single
concatenated string."
  (utf8->string (frame-concatenate frames)))


;;;
;;; Frame reader
;;;

(define (websocket-error message . args)
  (apply error message args))

;; See section 5.2 - Base Framing Protocol
(define (read-frame port)
  (define (opcode->frame-type opcode)
    (match opcode
      (#x0 'continuation)
      (#x1 'text)
      (#x2 'binary)
      (#x8 'close)
      (#x9 'ping)
      (#xA 'pong)
      (else (websocket-error "invalid opcode: " opcode))))

  (define (control-frame? type)
    (memq type '(close ping pong)))

  (define (parse-fin-bit octet)
    ;; Test the first bit of the octet.
    (not (zero? (logand #x80 octet))))

  (define (parse-opcode octet final?)
    ;; The opcode is stored in the least-significant nibble of the
    ;; octet.
    (let ((type (opcode->frame-type (logand #x0f octet))))
      ;; Section 5.5 specifies that control frames must not be
      ;; fragmented.
      (when (and (not final?) (control-frame? type))
        (websocket-error "fragmented control frame: " type))
      type))

  (define (parse-mask-bit octet)
    (not (zero? (logand #x80 octet))))

  (define (parse-length octet)
    ;; For lengths <= 125, the frame length is encoded in the last 7
    ;; bits of the octet.  If this number is 126, then the true length
    ;; is encoded in the following 16 bits.  If the number is 127,
    ;; then the true length is encoded in the following 64 bits.
    (match (logand #x7f octet)
      (126
       (bytevector-u16-ref (get-bytevector-n port 2) 0 (endianness big)))
      (127
       (bytevector-u64-ref (get-bytevector-n port 8) 0 (endianness big)))
      (length length)))

  (define (parse-masking-key)
    ;; Masking keys are always 32 bits.
    (get-bytevector-n port 4))

  ;; See section 5.3 - Client-to-Server Masking
  (define (unmask-bytevector! bv masking-key)
    (let loop ((i 0))
      (when (< i (bytevector-length bv))
        (let ((unmasked (logxor (u8vector-ref bv i)
                                (u8vector-ref masking-key (modulo i 4)))))
          (u8vector-set! bv i unmasked)
          (loop (1+ i))))))

  (define (read-data type masking-key length)
    ;; Section 5.5 specifies that control frame bodies may not exceed
    ;; 125 bytes.
    (when (and (> length 125)
               (control-frame? type))
      (websocket-error "control frame to large: " type length))

    (let ((bv (get-bytevector-n port length)))
      (when masking-key
        (unmask-bytevector! bv masking-key))
      bv))

  (let* ((type-byte (get-u8 port))
         (length-byte (get-u8 port))
         (final? (parse-fin-bit type-byte))
         (type (parse-opcode type-byte final?))
         (mask? (parse-mask-bit length-byte))
         (length (parse-length length-byte))
         (masking-key (and mask? (parse-masking-key)))
         (data (read-data type masking-key length)))
    (make-frame final? type masking-key data)))


;;;
;;; Frame writer
;;;

(define* (write-frame frame #:optional (port (current-output-port)))
  ;; Packs an unsigned integer into a bytevector in network byte
  ;; order.
  (define (uint->bytevector n size)
    (uint-list->bytevector (list n) (endianness big) size))

  (let ((length (frame-length frame))
        (mask   (frame-masking-key frame)))
    ;; Write FIN bit and opcode.
    (put-u8 port
            (logior (if (frame-final? frame) #x80 #x00)
                    (match (frame-type frame)
                      ('continuation #x00)
                      ('text         #x01)
                      ('binary       #x02)
                      ('close        #x08)
                      ('ping         #x09)
                      ('pong         #x0A))))

    ;; Write mask bit and length.
    (put-u8 port
            (logior (if mask #x80 #x00)
                    (cond
                     ((< length 126) length)
                     ((< length (expt 2 16)) 126)
                     (else 127))))

    ;; Write true size when size is greater than 125.
    (cond
     ((< length 126) #f)
     ((< length (expt 2 16))
      (put-bytevector port (uint->bytevector length 2)))
     (else
      (put-bytevector port (uint->bytevector length 8))))

    ;; Write masking key, if present.
    (let ((mask (frame-masking-key frame)))
      (when mask (put-bytevector port mask)))

    ;; Write data.
    (put-bytevector port (frame-data frame))))