permute-symmetrical

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;;;;;PERMUTE-SYMMETRCIAL -> seq of any lengths;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;;; SUB

(defun divide-seq-length (seq)
  (if (evenp (length seq))
    (/ (length seq) 2)
    (/ (1- (length seq)) 2)))
  

;;; MAIN

(defun permute-symmetrical (row &key (chance 0.5))
  (let ((1st (loop repeat (divide-seq-length row)
               for cnt = 0 then (incf cnt)
               collect (nth cnt row)))
        (2nd (loop repeat (divide-seq-length row)
                   for cnt = (- (length row) 1) then (decf cnt)
                   collect (nth cnt row))))

    (loop 
      for i in 1st
      for j in 2nd

      when (prob? chance)
      collect j into 1st-bag and collect i into 2nd-bag
      else collect i into 1st-bag and collect j into 2nd-bag

      when (= (length 1st-bag) (divide-seq-length row))
      do (return (if (evenp (length row))
                   (append 1st-bag (reverse 2nd-bag))
                   (append 1st-bag (list (nth (length 1st) row)) (reverse 2nd-bag)))))))
  

;;; EXAMPLE

(pitch-list-plot 
 (permute-symmetrical '(a4 bb4 ab4 b4 g4 c5 fs4 cs4 f4 d4 e4 eb4) ;zimmermann-row
                      :chance 0.3))