(import (only memory-mapped-files map-file-to-memory
                                  unmap-file-from-memory
                                  memory-mapped-file-pointer
                                  map/file map/shared prot/read))

(define MMAP-FLAGS (+ map/file map/shared))

;@siz will NOT be false
(define (memory-mapped-buffer loc fd siz chk)
  ;no size & no buff then byte, size & no buff then 1, else use buff
  (let ((amt (buffer-byte-size siz chk))
        (chunk (make-message-digest-raw-chunk #f 0 0))
        (rem siz)
        (mmap (the (or false (struct mmap)) #f)) )
    ;
    (define (finalize) (unmap-file-from-memory mmap rem))
    ;
    (if (= amt rem)
        ;then map whole file
        (begin
          (set! mmap (map-file-to-memory #f rem prot/read MMAP-FLAGS fd))
          (message-digest-raw-chunk-object-set! chunk (memory-mapped-file-pointer mmap))
          ;everything actually there - sizeof the allocated buffer!
          (message-digest-raw-chunk-size-set! chunk rem)
          (values chunk #f finalize) )
        ;else chunking mapping & unmapping unpdater
        (let ((off 0))
          ;FIXME pass fd in message-digest-raw-chunk?
          (define (updater)
            (define (reader amt)
              (when mmap (finalize))
              (set! mmap (map-file-to-memory #f amt prot/read MMAP-FLAGS fd off))
              (message-digest-raw-chunk-object-set! chunk (memory-mapped-file-pointer mmap))
              (message-digest-raw-chunk-size-set! chunk amt)
              (set! rem (- rem amt))
              (set! off (+ off amt))
              chunk )
            ;`finalize' invoked by caller when done (#f result)
            (and (not (zero? rem))
                 (reader (min rem (message-digest-raw-chunk-size chunk)))) )
          ;
          (values chunk updater finalize)) ) ) )

#|
;(assert (and (not (negative? siz)) (or (infinity? siz) (integer? siz))) 'XXX "invalid size" siz)
;-> read-until or (integer?/check siz) ->

;map-file-to-memory 1st arg ptr to alloc'ed buff of chunk-size, not #f
;(null-ptr) - so mapping chunk-size or remainder each on each update call

; (siz = true | buf < siz) ? read-until(fd-mmap) : read-all(fd-mmap)
(define (mapped-fileno-buffer loc fd siz buf)
  ;read fd until EOF using amt sized chunk
  ;EOF does not only mean read amount = 0 but also errno = 0
  ;what about interrupted reads
  (read-until loc fd (if (true? siz) buf (min siz buf))) )
|#