From: Andreas Schwab <schwab@linux-m68k.org>
To: "Mattias Engdegård" <mattiase@acm.org>
Cc: 55395@debbugs.gnu.org
Subject: bug#55395: What does (1 2 3 . #2) mean?
Date: Fri, 13 May 2022 18:08:33 +0200 [thread overview]
Message-ID: <87czghl8ji.fsf@igel.home> (raw)
In-Reply-To: <4B70C9BE-B784-4B74-A081-7A8B3F8D5136@acm.org> ("Mattias Engdegård"'s message of "Fri, 13 May 2022 13:32:34 +0200")
On Mai 13 2022, Mattias Engdegård wrote:
> Let's define (rho LEAD LOOP) as the iota list that has a loop LOOP long after LEAD initial elements:
>
> (defun rho (lead loop)
> (let ((l (number-sequence 1 (+ lead loop))))
> (setcdr (nthcdr (+ lead loop -1) l) (nthcdr lead l))
> l))
>
> Then we have:
>
> (rho 0 1) => (1 . #0)
> (rho 0 2) => (1 2 1 2 . #2)
> (rho 0 3) => (1 2 3 1 2 . #2)
> (rho 0 4) => (1 2 3 4 1 2 3 4 1 2 . #5)
> (rho 0 5) => (1 2 3 4 5 1 2 3 4 5 1 . #5)
> (rho 1 4) => (1 2 3 4 5 2 3 4 5 2 . #5)
> (rho 4 1) => (1 2 3 4 5 5 5 . #3)
>
> and so on. The pattern is not obvious to me.
>
> It may have made more sense before the switch of cycle-detection algorithm from Floyd to Brent. This can be fixed by hand-coding the list iteration and explicitly remembering the index of the tortoise, but would that be correct? What's the spec?
I don't think there is a defined meaning behind the number, it's more an
implementation detail. If you want to have precise cycle detection you
need to enable print-circle.
--
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510 2552 DF73 E780 A9DA AEC1
"And now for something completely different."
prev parent reply other threads:[~2022-05-13 16:08 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-13 11:32 bug#55395: What does (1 2 3 . #2) mean? Mattias Engdegård
2022-05-13 15:22 ` Lars Ingebrigtsen
2022-05-13 17:20 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2022-05-13 19:54 ` Lars Ingebrigtsen
2022-05-13 20:01 ` Mattias Engdegård
2022-05-14 13:45 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2022-05-18 14:29 ` Mattias Engdegård
2022-05-18 21:16 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2022-05-23 14:59 ` Mattias Engdegård
2022-05-13 16:08 ` Andreas Schwab [this message]
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