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Subject: bug#55395: What does (1 2 3 . #2) mean?
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What, exactly, does the #N print notation mean (with print-circle=3Dnil)?

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) =3D> (1 . #0)
(rho 0 2) =3D> (1 2 1 2 . #2)
(rho 0 3) =3D> (1 2 3 1 2 . #2)
(rho 0 4) =3D> (1 2 3 4 1 2 3 4 1 2 . #5)
(rho 0 5) =3D> (1 2 3 4 5 1 2 3 4 5 1 . #5)
(rho 1 4) =3D> (1 2 3 4 5 2 3 4 5 2 . #5)
(rho 4 1) =3D> (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?

If #N means 'Nth object from the top along the path to the current =
object, starting at 0' then we should have

(rho 2 3) =3D> (1 2 3 4 5 . #2)
(list (rho 2 3)) =3D> ((1 2 3 4 5 . #3))

ie, adding the print depth to the index in the list. Do you agree?

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To: Mattias =?UTF-8?Q?Engdeg=C3=A5rd?= <mattiase@HIDDEN>
Subject: bug#55395: Acknowledgement (What does (1 2 3 . #2) mean?)
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Subject: bug#55395: What does (1 2 3 . #2) mean?
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From: Lars Ingebrigtsen <larsi@HIDDEN>
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Date: Fri, 13 May 2022 17:22:13 +0200
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 Content preview:  Mattias =?UTF-8?Q?Engdeg=C3=A5rd?= <mattiase@HIDDEN> writes: > 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 tortoi [...] 
 
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Mattias Engdeg=C3=A5rd <mattiase@HIDDEN> writes:

> It may have made more sense before the switch of cycle-detection algorith=
m 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 cor=
rect? What's the spec?

I don't think I've ever considered #x to be meaningful outside of
print-circle, but I guess if we wanted to have some semantics here, I
think I would have expected the index of the tortoise?  But...

> If #N means 'Nth object from the top along the path to the current object=
, starting at 0' then we should have
>
> (rho 2 3) =3D> (1 2 3 4 5 . #2)
> (list (rho 2 3)) =3D> ((1 2 3 4 5 . #3))
>
> ie, adding the print depth to the index in the list. Do you agree?

I've added Stefan to the CCs; I'm sure he has an opinion.

--=20
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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Subject: bug#55395: What does (1 2 3 . #2) mean?
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Date: Fri, 13 May 2022 18:08:33 +0200
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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@HIDDEN
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"And now for something completely different."

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>> It may have made more sense before the switch of cycle-detection algorit=
hm
>> 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 I've ever considered #x to be meaningful outside of
> print-circle, but I guess if we wanted to have some semantics here, I
> think I would have expected the index of the tortoise?  But...

Agreed (and agreed with Andreas as well).
The main goal is to avoid inf-looping and the #NNN chosen is
somewhat arbitrary.

I don't think it's worth it to try and make those #NNN more precise.
If the arbitrariness of the specific #NNN chosen is a problem, we could
replace it with a fixed `#=A1cycle!` or something like that (or if we
want it to be more user-readable we could print something like
#cycle:<OBJ> where OBJ is the first N layers of the rest of the cycle,
like (rho 0 2) =3D> (1 2 1 2 . #cycle:(1 2 1 ...))

I'm personally more bothered by the fact that those #NNN use exactly the
same syntax as used with `print-circle` yet they don't have the
same semantics.


        Stefan

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 Content preview:  Stefan Monnier <monnier@HIDDEN> writes: > Agreed
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Stefan Monnier <monnier@HIDDEN> writes:

> Agreed (and agreed with Andreas as well).
> The main goal is to avoid inf-looping and the #NNN chosen is
> somewhat arbitrary.
>
> I don't think it's worth it to try and make those #NNN more precise.

So it seems like everybody basically agrees on that...  so if Mattias
wants to implement something here to give them some semantics, he can
pretty much choose whatever he wants?

Or just document that there's really no semantics here without
print-circle, if he prefers.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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13 maj 2022 kl. 19.20 skrev Stefan Monnier <monnier@HIDDEN>:

> The main goal is to avoid inf-looping and the #NNN chosen is
> somewhat arbitrary.

It seems to have sort-of worked before and was broken, inadvertently and =
with the best intentions, by a later change.

Additionally the #N value appears to be correct for other object types. =
For example,

  [a [b #1=3D[[c #1# d]]]]

is printed as

  [a [b [[c #2 d]]]]

which is consistent with the manual.

> I don't think it's worth it to try and make those #NNN more precise.

I have a simple patch but it is not based on master so it will have to =
wait.

> I'm personally more bothered by the fact that those #NNN use exactly =
the
> same syntax as used with `print-circle` yet they don't have the
> same semantics.

The syntax isn't exactly the same (#N vs #N=3D and #N#) but annoyingly =
close.

For that matter I would have preferred numbering the other way, starting =
at the bottom going up, like de Bruijn indices. That way the numbering =
would be local and wouldn't depend on where the circular subtree occurs. =
Technically that would be an easy change, and if we agree that the =
numbers are unreliable right now, we might as well.

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>> The main goal is to avoid inf-looping and the #NNN chosen is
>> somewhat arbitrary.
> It seems to have sort-of worked before and was broken, inadvertently and
> with the best intentions, by a later change.
>
> Additionally the #N value appears to be correct for other object types. For example,
>
>   [a [b #1=[[c #1# d]]]]
>
> is printed as
>
>   [a [b [[c #2 d]]]]
>
> which is consistent with the manual.

I do have the impression that it used to be "correct", but I can't
remember of a single time where I actually made use of that NNN.

And even if you know what it means and it works correctly, it's pretty
hard for a normal human to correctly count the depth starting from
the root.

I suspect if we want to "do better", printing "the beginning" of the
object to which we're looping (and saying explicitly that there's
a cycle) will be a lot more useful to the average user.
[ Or we could even print "the period" rather than "the beginning".  ]

>> I'm personally more bothered by the fact that those #NNN use exactly the
>> same syntax as used with `print-circle` yet they don't have the
>> same semantics.
> The syntax isn't exactly the same (#N vs #N= and #N#) but annoyingly close.

Indeed, it's not as bad as I remembered.

> For that matter I would have preferred numbering the other way, starting at
> the bottom going up, like de Bruijn indices.

I love&hate de Bruijn indices as much as the next guy, but they're not
human-friendly (even more than the "like de Bruijn levels" we currently
use).


        Stefan

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 Content preview: 14 maj 2022 kl. 15.45 skrev Stefan Monnier
 <monnier@HIDDEN>:
 > I do have the impression that it used to be "correct", but I can't > remember
 of a single time where I actually made use of that NNN. > > And even if you
 know what it means and it works correctly, i [...] 
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14 maj 2022 kl. 15.45 skrev Stefan Monnier <monnier@HIDDEN>:

> I do have the impression that it used to be "correct", but I can't
> remember of a single time where I actually made use of that NNN.
>=20
> And even if you know what it means and it works correctly, it's pretty
> hard for a normal human to correctly count the depth starting from
> the root.

It is. Counting upwards from a leaf seems slightly easier.

Perhaps these attempts to generate a meaningful circularity reference is =
a fool's errand and we should just go with #!circle! or something =
similar.

Anyway, I'm attaching a patch that tries to put some meaning back in the =
` . #N` notation: it's an index into the same list, as it used to be, I =
think, prior to the switch from Floyd to Brent. This is done by keeping =
track of the index of the tortoise at all times, which isn't very =
expensive at all.

Without the patch: (rho 4 1) =3D> (1 2 3 4 5 5 5 . #3)
With the patch:    (rho 4 1) =3D> (1 2 3 4 5 5 5 . #6)

which should be a mild improvement.
It's still ambiguous:

 (1 1 . #1)

could mean either

 #1=3D(1 . #1#)

or

 #1=3D(1 1 . #1#)

and

 [(1 2 3 . #0)]

could mean either

 [#1=3D(1 2 3 . #1#)]

or
 #1=3D[(1 2 3 . #1#)]

but maybe it's less wrong, with room for future improvement?


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diff --git a/src/print.c b/src/print.c
index da4869e8fb..823bc07406 100644
--- a/src/print.c
+++ b/src/print.c
@@ -2028,12 +2028,12 @@ named_escape (int i)
   union {
     struct {
       Lisp_Object last;		/* cons whose car was just printed  */
-      ptrdiff_t idx;		/* index of next element */
-      intmax_t maxlen;		/* max length (from Vprint_length) */
+      intmax_t maxlen;		/* max number of elements left to print */
       /* state for Brent cycle detection */
       Lisp_Object tortoise;     /* slow pointer */
       ptrdiff_t n;		/* tortoise step countdown */
       ptrdiff_t m;		/* tortoise step period */
+      ptrdiff_t tortoise_idx;	/* index of tortoise */
     } list;
     struct {
       Lisp_Object obj;		/* object to print after " . " */
@@ -2408,10 +2408,10 @@ print_object (Lisp_Object obj, Lisp_Object printcharfun, bool escapeflag)
 		  .type = PE_list,
 		  .u.list.last = obj,
 		  .u.list.maxlen = print_length,
-		  .u.list.idx = 1,
 		  .u.list.tortoise = obj,
 		  .u.list.n = 2,
 		  .u.list.m = 2,
+		  .u.list.tortoise_idx = 0,
 		});
 	      /* print the car */
 	      obj = XCAR (obj);
@@ -2582,10 +2582,8 @@ print_object (Lisp_Object obj, Lisp_Object printcharfun, bool escapeflag)
 
 		printchar (' ', printcharfun);
 
-		/* FIXME: We wouldn't need to keep track of idx if we
-		   count down maxlen instead, and maintain a separate
-		   tortoise index if required.  */
-		if (e->u.list.idx >= e->u.list.maxlen)
+		--e->u.list.maxlen;
+		if (e->u.list.maxlen <= 0)
 		  {
 		    print_c_string ("...)", printcharfun);
 		    --prstack.sp;
@@ -2594,22 +2592,21 @@ print_object (Lisp_Object obj, Lisp_Object printcharfun, bool escapeflag)
 		  }
 
 		e->u.list.last = next;
-		e->u.list.idx++;
 		e->u.list.n--;
 		if (e->u.list.n == 0)
 		  {
 		    /* Double tortoise update period and teleport it.  */
+		    e->u.list.tortoise_idx += e->u.list.m;
 		    e->u.list.m <<= 1;
 		    e->u.list.n = e->u.list.m;
 		    e->u.list.tortoise = next;
 		  }
 		else if (BASE_EQ (next, e->u.list.tortoise))
 		  {
-		    /* FIXME: This #N tail index is bug-compatible with
-		       previous implementations but actually nonsense;
+		    /* FIXME: This #N tail index is somewhat ambiguous;
 		       see bug#55395.  */
 		    int len = sprintf (buf, ". #%" PRIdMAX ")",
-				       (e->u.list.idx >> 1) - 1);
+				       e->u.list.tortoise_idx);
 		    strout (buf, len, len, printcharfun);
 		    --prstack.sp;
 		    --print_depth;

--Apple-Mail=_9C60AD69-423C-4E73-823B-DAA20C4ABAB0--

Last modified: Wed, 18 May 2022 14:30:02 UTC

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997 nCipher Corporation Ltd, 1994-97 Ian Jackson.