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Added &auxome function NUNZIP-ALIST

It even has comments. What more could be desired?
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commit f577a1ce2e6eb1a1f11038923e97c75bb48a64bb 1 parent bf6eec2
@adlai adlai authored
Showing with 25 additions and 0 deletions.
  1. +25 −0 src/utils.lisp
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25 src/utils.lisp
@@ -19,6 +19,31 @@
collect (cdr pair) into vals
finally (return (values keys vals))))
+(defun nunzip-alist (alist &aux (keys alist) (vals (car alist)))
+ "Destructive, non-consing version of `unzip-alist'."
+ (do (
+ ;; Pointers to the list tails, so we can push to their ends
+ (keys-tail keys (cdr keys-tail))
+ (vals-tail vals (cdr vals-tail))
+
+ ;; And pointers to the elements that we pick out at each stage
+ (next-key (caar alist) (caar alist))
+ (next-val (cdar alist) (cdar alist)))
+
+ ;; We're done when the tails are empty
+ ((null keys-tail)
+ (values keys vals))
+
+ ;; Since we already have pointers to the elements of this stage, and
+ ;; to the relevant list tails, we just need to hang on to the remainder
+ ;; of the original alist -- a pointer which is lost at the next stage.
+ (setf alist (cdr alist))
+
+ ;; Scalpel please.
+ (setf (car keys-tail) next-key ; Must... not... use... RPLACA!
+ (car vals-tail) next-val ; Must... resist... the urge...
+ (cdr vals-tail) (car alist)))) ; Gaaaah!
+
(defmacro fun (&body body)
"This macro puts the FUN back in FUNCTION."
`(lambda (&optional _) (declare (ignorable _)) ,@body))
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