GNU bug report logs - #75105
(cl-random -1.0e+INF)

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Package: emacs; Severity: wishlist; Reported by: Pip Cet <pipcet@HIDDEN>; dated Wed, 25 Dec 2024 23:28:02 UTC; Maintainer for emacs is bug-gnu-emacs@HIDDEN.

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Date: Mon, 17 Feb 2025 18:26:23 +0000
To: =?utf-8?Q?Mattias_Engdeg=C3=A5rd?= <mattias.engdegard@HIDDEN>
From: Pip Cet <pipcet@HIDDEN>
Subject: Re: bug#75105: (cl-random -1.0e+INF)
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Mattias Engdeg=C3=A5rd <mattias.engdegard@HIDDEN> writes:

> 16 feb. 2025 kl. 01.50 skrev Pip Cet <pipcet@HIDDEN>:
>
>> (cl-random 0.0) returns 0.0, but one could argue it should throw
>
> It definitely should throw, but perhaps it's not worth the
> incompatibility? Not sure, because existing code that passes 0.0 is
> likely buggy anyway.  Or we could say that it's just an ad-hoc
> extension, by vague analogy of (car nil) =3D nil.

On mathematical grounds, in my humble but correct opinion, the
pseudo-algorithm should be:

1. find a random real number in the closed interval [0, a]
2. round it towards zero.

IOW, that 'a' is never the result is a consequence of the rounding step,
not of using a half-open interval in step 1.

However, Common Lisp disagrees, and the cl- prefix kind of hints we
should heed its opinions.

And using (* x (cl-random 1.0)) if that's what you want seems easy
enough (and, either way, you get bitten by the low precision, with only
8 million possible outcomes).

Will modify the patch accordingly.

Pip
Information forwarded to bug-gnu-emacs@HIDDEN:
bug#75105; Package emacs. Full text available.

Message received at submit <at> debbugs.gnu.org:


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Date: Mon, 17 Feb 2025 18:26:23 +0000
To: =?utf-8?Q?Mattias_Engdeg=C3=A5rd?= <mattias.engdegard@HIDDEN>
From: Pip Cet <pipcet@HIDDEN>
Subject: Re: bug#75105: (cl-random -1.0e+INF)
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Mattias Engdeg=C3=A5rd <mattias.engdegard@HIDDEN> writes:

> 16 feb. 2025 kl. 01.50 skrev Pip Cet <pipcet@HIDDEN>:
>
>> (cl-random 0.0) returns 0.0, but one could argue it should throw
>
> It definitely should throw, but perhaps it's not worth the
> incompatibility? Not sure, because existing code that passes 0.0 is
> likely buggy anyway.  Or we could say that it's just an ad-hoc
> extension, by vague analogy of (car nil) =3D nil.

On mathematical grounds, in my humble but correct opinion, the
pseudo-algorithm should be:

1. find a random real number in the closed interval [0, a]
2. round it towards zero.

IOW, that 'a' is never the result is a consequence of the rounding step,
not of using a half-open interval in step 1.

However, Common Lisp disagrees, and the cl- prefix kind of hints we
should heed its opinions.

And using (* x (cl-random 1.0)) if that's what you want seems easy
enough (and, either way, you get bitten by the low precision, with only
8 million possible outcomes).

Will modify the patch accordingly.

Pip
Information forwarded to bug-gnu-emacs@HIDDEN:
bug#75105; Package emacs. Full text available.

Message received at submit <at> debbugs.gnu.org:


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Subject: Re: bug#75105: (cl-random -1.0e+INF)
From: =?utf-8?Q?Mattias_Engdeg=C3=A5rd?= <mattias.engdegard@HIDDEN>
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16 feb. 2025 kl. 01.50 skrev Pip Cet <pipcet@HIDDEN>:

> (cl-random 0.0) returns 0.0, but one could argue it should throw

It definitely should throw, but perhaps it's not worth the =
incompatibility? Not sure, because existing code that passes 0.0 is =
likely buggy anyway.
Or we could say that it's just an ad-hoc extension, by vague analogy of =
(car nil) =3D nil.
Information forwarded to bug-gnu-emacs@HIDDEN:
bug#75105; Package emacs. Full text available.

Message received at 75105 <at> debbugs.gnu.org:


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Subject: Re: bug#75105: (cl-random -1.0e+INF)
From: =?utf-8?Q?Mattias_Engdeg=C3=A5rd?= <mattias.engdegard@HIDDEN>
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16 feb. 2025 kl. 01.50 skrev Pip Cet <pipcet@HIDDEN>:

> (cl-random 0.0) returns 0.0, but one could argue it should throw

It definitely should throw, but perhaps it's not worth the =
incompatibility? Not sure, because existing code that passes 0.0 is =
likely buggy anyway.
Or we could say that it's just an ad-hoc extension, by vague analogy of =
(car nil) =3D nil.
Information forwarded to bug-gnu-emacs@HIDDEN:
bug#75105; Package emacs. Full text available.

Message received at 75105 <at> debbugs.gnu.org:


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Date: Sun, 16 Feb 2025 00:50:04 +0000
To: bug-gnu-emacs@HIDDEN, 75105 <at> debbugs.gnu.org, mattiasengdegard@HIDDEN,
 monnier@HIDDEN
From: Pip Cet <pipcet@HIDDEN>
Subject: Re: bug#75105: (cl-random -1.0e+INF)
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"Pip Cet via \"Bug reports for GNU Emacs, the Swiss army knife of text edit=
ors\"" <bug-gnu-emacs@HIDDEN> writes:

> The current behavior of (cl-random -1.0e+INF) is to return NaN with a
> probability of 1.1920928955078125e-07, and -1.0e+INF in the remaining
> cases.

Okay, so maybe I was wrong and that should be fixed.  This patch will
make cl-random behave consistently, though one could argue about some
cases:

(cl-random 0) involves a division by zero, as it should
(cl-random 1.0e+INF) returns 1.0e+INF, as it should
(cl-random 0.0) returns 0.0, but one could argue it should throw
(cl-random -0.0) should do the same as (cl-random -0.0) since negative
zero is a myth.

Updating the docstring would be nice, too.

From 22e59a481a7a1c2cbff349af0f6faa2f428981a8 Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@HIDDEN>
Subject: [PATCH] Make cl-random behave consistently for unusual arguments
 (bug#75105)

The old behavior was for (cl-random -1.0e+INF) to return NaN in about
one of eight million calls, and -1.0e+INF otherwise.  Other unusual
arguments were handled inconsistently as well.

* lisp/emacs-lisp/cl-extra.el (cl-random): Handle nonnegative
arguments consistently, error for negative arguments.
* test/lisp/emacs-lisp/cl-extra-tests.el (cl-extra-test-random): New
test.
---
 lisp/emacs-lisp/cl-extra.el            | 20 +++++++++++++-------
 test/lisp/emacs-lisp/cl-extra-tests.el | 10 ++++++++++
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/lisp/emacs-lisp/cl-extra.el b/lisp/emacs-lisp/cl-extra.el
index 09470457d93..8c218ff9d7e 100644
--- a/lisp/emacs-lisp/cl-extra.el
+++ b/lisp/emacs-lisp/cl-extra.el
@@ -494,13 +494,19 @@ cl-random
     (let* ((i (cl-callf (lambda (x) (% (1+ x) 55)) (cl--random-state-i sta=
te)))
 =09   (j (cl-callf (lambda (x) (% (1+ x) 55)) (cl--random-state-j state)))
 =09   (n (aset vec i (logand 8388607 (- (aref vec i) (aref vec j))))))
-      (if (integerp lim)
-=09  (if (<=3D lim 512) (% n lim)
-=09    (if (> lim 8388607) (setq n (+ (ash n 9) (cl-random 512 state))))
-=09    (let ((mask 1023))
-=09      (while (< mask (1- lim)) (setq mask (1+ (+ mask mask))))
-=09      (if (< (setq n (logand n mask)) lim) n (cl-random lim state))))
-=09(* (/ n '8388608e0) lim)))))
+      (cond
+       ((natnump lim)
+=09(if (<=3D lim 512) (% n lim)
+=09  (if (> lim 8388607) (setq n (+ (ash n 9) (cl-random 512 state))))
+=09  (let ((mask 1023))
+=09    (while (< mask (1- lim)) (setq mask (1+ (+ mask mask))))
+=09    (if (< (setq n (logand n mask)) lim) n (cl-random lim state)))))
+       ((< 0 lim 1.0e+INF)
+        (* (/ n '8388608e0) lim))
+       ((< lim -0.0)
+        (error "negative limit supplied to cl-random"))
+       (t
+        lim)))))
=20
 ;;;###autoload
 (defun cl-make-random-state (&optional state)
diff --git a/test/lisp/emacs-lisp/cl-extra-tests.el b/test/lisp/emacs-lisp/=
cl-extra-tests.el
index 75533b36f29..d87c0bfec5d 100644
--- a/test/lisp/emacs-lisp/cl-extra-tests.el
+++ b/test/lisp/emacs-lisp/cl-extra-tests.el
@@ -348,4 +348,14 @@ cl-extra-test-tailp
     (should (cl-tailp l l))
     (should (not (cl-tailp '(4 5) l)))))
=20
+(ert-deftest cl-extra-test-random ()
+  (should-error (cl-random -1))
+  (should-error (cl-random -0.5))
+  (should-error (cl-random -1.0e+INF))
+  (should-error (cl-random 0))
+  (should (eql (cl-random 1) 0)))
+  (should (eql (cl-random 0.0) 0.0))
+  (should (=3D (cl-random -0.0) 0.0))
+  (should (=3D (cl-random 1.0e+INF) 1.0e+INF))
+
 ;;; cl-extra-tests.el ends here
--=20
2.48.1
Information forwarded to bug-gnu-emacs@HIDDEN:
bug#75105; Package emacs. Full text available.

Message received at submit <at> debbugs.gnu.org:


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Date: Sun, 16 Feb 2025 00:50:04 +0000
To: bug-gnu-emacs@HIDDEN, 75105 <at> debbugs.gnu.org, mattiasengdegard@HIDDEN,
 monnier@HIDDEN
From: Pip Cet <pipcet@HIDDEN>
Subject: Re: bug#75105: (cl-random -1.0e+INF)
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"Pip Cet via \"Bug reports for GNU Emacs, the Swiss army knife of text edit=
ors\"" <bug-gnu-emacs@HIDDEN> writes:

> The current behavior of (cl-random -1.0e+INF) is to return NaN with a
> probability of 1.1920928955078125e-07, and -1.0e+INF in the remaining
> cases.

Okay, so maybe I was wrong and that should be fixed.  This patch will
make cl-random behave consistently, though one could argue about some
cases:

(cl-random 0) involves a division by zero, as it should
(cl-random 1.0e+INF) returns 1.0e+INF, as it should
(cl-random 0.0) returns 0.0, but one could argue it should throw
(cl-random -0.0) should do the same as (cl-random -0.0) since negative
zero is a myth.

Updating the docstring would be nice, too.

From 22e59a481a7a1c2cbff349af0f6faa2f428981a8 Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@HIDDEN>
Subject: [PATCH] Make cl-random behave consistently for unusual arguments
 (bug#75105)

The old behavior was for (cl-random -1.0e+INF) to return NaN in about
one of eight million calls, and -1.0e+INF otherwise.  Other unusual
arguments were handled inconsistently as well.

* lisp/emacs-lisp/cl-extra.el (cl-random): Handle nonnegative
arguments consistently, error for negative arguments.
* test/lisp/emacs-lisp/cl-extra-tests.el (cl-extra-test-random): New
test.
---
 lisp/emacs-lisp/cl-extra.el            | 20 +++++++++++++-------
 test/lisp/emacs-lisp/cl-extra-tests.el | 10 ++++++++++
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/lisp/emacs-lisp/cl-extra.el b/lisp/emacs-lisp/cl-extra.el
index 09470457d93..8c218ff9d7e 100644
--- a/lisp/emacs-lisp/cl-extra.el
+++ b/lisp/emacs-lisp/cl-extra.el
@@ -494,13 +494,19 @@ cl-random
     (let* ((i (cl-callf (lambda (x) (% (1+ x) 55)) (cl--random-state-i sta=
te)))
 =09   (j (cl-callf (lambda (x) (% (1+ x) 55)) (cl--random-state-j state)))
 =09   (n (aset vec i (logand 8388607 (- (aref vec i) (aref vec j))))))
-      (if (integerp lim)
-=09  (if (<=3D lim 512) (% n lim)
-=09    (if (> lim 8388607) (setq n (+ (ash n 9) (cl-random 512 state))))
-=09    (let ((mask 1023))
-=09      (while (< mask (1- lim)) (setq mask (1+ (+ mask mask))))
-=09      (if (< (setq n (logand n mask)) lim) n (cl-random lim state))))
-=09(* (/ n '8388608e0) lim)))))
+      (cond
+       ((natnump lim)
+=09(if (<=3D lim 512) (% n lim)
+=09  (if (> lim 8388607) (setq n (+ (ash n 9) (cl-random 512 state))))
+=09  (let ((mask 1023))
+=09    (while (< mask (1- lim)) (setq mask (1+ (+ mask mask))))
+=09    (if (< (setq n (logand n mask)) lim) n (cl-random lim state)))))
+       ((< 0 lim 1.0e+INF)
+        (* (/ n '8388608e0) lim))
+       ((< lim -0.0)
+        (error "negative limit supplied to cl-random"))
+       (t
+        lim)))))
=20
 ;;;###autoload
 (defun cl-make-random-state (&optional state)
diff --git a/test/lisp/emacs-lisp/cl-extra-tests.el b/test/lisp/emacs-lisp/=
cl-extra-tests.el
index 75533b36f29..d87c0bfec5d 100644
--- a/test/lisp/emacs-lisp/cl-extra-tests.el
+++ b/test/lisp/emacs-lisp/cl-extra-tests.el
@@ -348,4 +348,14 @@ cl-extra-test-tailp
     (should (cl-tailp l l))
     (should (not (cl-tailp '(4 5) l)))))
=20
+(ert-deftest cl-extra-test-random ()
+  (should-error (cl-random -1))
+  (should-error (cl-random -0.5))
+  (should-error (cl-random -1.0e+INF))
+  (should-error (cl-random 0))
+  (should (eql (cl-random 1) 0)))
+  (should (eql (cl-random 0.0) 0.0))
+  (should (=3D (cl-random -0.0) 0.0))
+  (should (=3D (cl-random 1.0e+INF) 1.0e+INF))
+
 ;;; cl-extra-tests.el ends here
--=20
2.48.1
Information forwarded to bug-gnu-emacs@HIDDEN:
bug#75105; Package emacs. Full text available.
Severity set to 'wishlist' from 'normal' Request was from Stefan Kangas <stefankangas@HIDDEN> to control <at> debbugs.gnu.org. Full text available.

Message received at submit <at> debbugs.gnu.org:


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To: bug-gnu-emacs@HIDDEN,
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 Stefan Monnier <monnier@HIDDEN>
From: Pip Cet <pipcet@HIDDEN>
Subject: (cl-random -1.0e+INF)
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The current behavior of (cl-random -1.0e+INF) is to return NaN with a
probability of 1.1920928955078125e-07, and -1.0e+INF in the remaining
cases.

There are at least two reasonable things cl-random can return for a
negative argument.  cl-random implements both, returning a float with
negative sign or a nonnegative integer.

There are at least two reasonable things cl-random can do in response to
a zero argument.  cl-random implements three of them:

(cl-random 0.0) is 0.0
(cl-random -0.0) is -0.0
(cl-random 0) throws an error

While "(while (not (isnan (cl-random -1.0e+INF))))" is an interesting
benchmark, its behavior may be somewhat surprising.

As this behavior has been in place since at least 1993 (when the
function was called random*), users may rely on it, and only the
docstring should be changed.
Acknowledgement sent to Pip Cet <pipcet@HIDDEN>:
New bug report received and forwarded. Copy sent to bug-gnu-emacs@HIDDEN. Full text available.
Report forwarded to bug-gnu-emacs@HIDDEN:
bug#75105; Package emacs. Full text available.
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