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;;; Harmonic Flow web site
(define-module (apps aux lists)
#:use-module (apps aux numbers)
#:use-module (srfi srfi-1)
#:use-module (srfi srfi-27)
#:export (list-group
list-slice
rest
separate
take-random))
(define (list-group los limit)
(map (lambda (index)
(list-slice los index (+ index limit)))
;; TODO: Use a skip-count procedure instead of iota.
(iota (ceiling (/ (length los) limit)) 0 limit)))
(define* (list-slice los index-a #:optional (index-b #false))
(let ((start index-a)
(end (if (or (not index-b) (> index-b (length los)))
(- (length los) 1)
(- index-b 1))))
(map (lambda (index)
(list-ref los index))
(range index-a end))))
(define (rest los)
(cond ((<= (length los) 1) (list))
(else (list-tail los 1))))
(define (separate los separator)
"Return a list with the elements of LOS separated by SEPARATOR.
LOS (list)
A list of s-expressions.
SEPARATOR (s-expression)
Any s-expression that will be added between the elements of the
given list.
RETURN VALUE (list)
A list of s-expressions."
(cond ((or (null? los) (= (length los) 1)) los)
(else
(cons (first los)
(cons separator (separate (rest los) separator))))))
(define (take-random list n)
"Return a list containing N different elements from LIST, if
possible, chosen randomly and evenly distributed. If LIST has less
than N elements, the result is a permutation of LIST."
(let loop ((list list)
(n n)
(len (length list)))
(if (<= (min n len) 0)
'()
(let ((r (random-integer len)))
(cons (list-ref list r)
(loop (append (take list r)
(drop list (1+ r)))
(- len 1)
(- n 1)))))))
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