Re: What to do for faster `remove-duplicates'?

[Artur Malabarba]

2015-05-06 19:41 GMT+01:00 Oleh Krehel <address@hidden>:
> Artur Malabarba <address@hidden> writes:
>
>> But that nreverse could be optimized out if the first loop followed a
>> `while'+`setcdr' strategy like the second.
>
> Please check the optimized version:
>
> (defun delete-dups (list)
>   "Destructively remove `equal' duplicates from LIST.
> Store the result in LIST and return it.  LIST must be a proper list.
> Of several `equal' occurrences of an element in LIST, the first
> one is kept."
>   (if (> (length list) 100)
>       (let ((hash (make-hash-table :test #'equal)))
>         (let ((tail list)
>               elt retail)
>           (while (setq retail (cdr tail))
>             (setq elt (car retail))
>             (if (gethash elt hash)
>                 (setcdr tail (cdr retail))
>               (puthash elt t hash))
>             (setq tail retail))))
>     (let ((tail list))
>       (while tail
>         (setcdr tail (delete (car tail) (cdr tail)))
>         (setq tail (cdr tail)))))
>   list)

I may be wrong, but it looks like it's completely skipping the car of the list.