Re: simple-array vs displaced-to

Subject: Re: simple-array vs displaced-to
From: Erik Naggum <erik@naggum.no>
Date: 17 Jan 2004 21:38:04 +0000
Newsgroups: comp.lang.lisp
Message-ID: <3283364284060266KL2065E@naggum.no>

* Martin Raspaud
| One would say "why not give as parameter to the processing funtion
| the big array and an offset ?"  Well, I don't really like this, it
| doesn't feel the right way to do it to me.
| 
| So am I wrong with this or is there another solution, ie having a kind
| of displaced-to array that would be a simple-array ?

  One common optimization when working with non-simple arrays is to
  dig out the underlying simple-array and the start and end positions
  that a displaced array makes into one convenient object.

  The function ARRAY-DISPLACEMENT returns the values of the arguments
  :DISPLACED-TO and and :DISPLACED-INDEX-OFFSETgiven to MAKE-ARRAY or
  ADJUST-ARRAY, or NIL and 0 if it was not displaced.

(defun undisplace-array (array)
  "Return the fundamental array and the start and end positions into
it of a displaced array."
  (let ((length (length array))
        (start 0))
    (loop
      (multiple-value-bind (to offset) (array-displacement array)
        (if to
            (setq array to
                  start (+ start offset))
          (return (values array start (+ start length))))))))

-- 
Erik Naggum | Oslo, Norway

Act from reason, and failure makes you rethink and study harder.
Act from faith, and failure makes you blame someone and push harder.