Paste number 7527: interruptable map

Paste number 7527: interruptable map
Pasted by: offby1
When:9 years, 12 hours ago
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#! /bin/sh
exec mzscheme -qr "$0" ${1+"$@"}

;; A quick (but not very general-purpose, alas) way to get partial
;; results from a function that would ordinarily take a long time to
;; run.

(require (lib "" "srfi" "1")
         (lib ""))

(define (permute l)
  ;; (distribute 3 (list         )) => '()
  ;; (distribute 3 (list 'a      )) => ((3 a    ) (a 3    )                    )
  ;; (distribute 3 (list 'a 'b   )) => ((3 a b  ) (a 3 b  ) (a b 3  )          )
  ;; (distribute 3 (list 'a 'b 'c)) => ((3 a b c) (a 3 b c) (a b 3 c) (a b c 3))
  (define (distribute item l)
     ((null? l)
     ((null? (cdr l))
      (list (list item (car l))
            (list (car l) item)))
      (cons (cons item l)
            (map (lambda (seq)
                   (cons (car l) seq)) (distribute item (cdr l)))))

   ((null? l)
   ((null? (cdr l))
    (list l))
    (apply append
           (partial-map 2 (lambda (seq)
                            (distribute (car l) seq))
                        (permute (cdr l)))))))

;; like `map', but might stop before hitting the end of LIST, and
;; hence return only partial results.
(define (partial-map max-seconds-to-run func list)
  (define result '())
  (define (my-map func list)
    (let loop ((l list))
      (if (not (null? l))
            (set! result (cons (func (car l))
            (loop (cdr l))))))
   (let ((t (thread (lambda () (my-map func list)))))
     (sync/timeout max-seconds-to-run t)

     ;; if we don't do this, the thread will continue to run and eat
     ;; CPU and memory.
     (kill-thread t)
   (reverse result)

(let ((n 10))
  (printf "There are at least ~a permutations of the first ~a nonnegative integers~%"
          (length (permute (iota n)))

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