[PATCH] Add support for log2.

[PATCH] Add support for log2.

[Rüdiger Sonderfeld]

log2(3) is a new function in C99.  I think it makes sense adding
support for it in Emacs because of the improved accuracy and
logarithmus dualis is common in computer science and information
theory.  The following code snipped (from jlf) shows the improved
accuracy for larger numbers:

  (mapcar (lambda (n)
            (let ((float-log (log (expt 2 n) 2)))
              (list (if (> float-log n) '> '≯)
                    (if (= float-log n) '= '≠)
                    (if (< float-log n) '< '≮))))
          (number-sequence 0 31))

A feature test is added to configure.ac and a fallback for legacy
systems included.

* src/floatfns.c (Flog): Add special case for `log2'.
  (Flog2): New function.

* configure.ac: Test for `log2'.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
---
 configure.ac   |  2 +-
 src/floatfns.c | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index a16a52d..6e5c888 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3235,7 +3235,7 @@ gai_strerror mkstemp getline getdelim sync \
 difftime posix_memalign \
 getpwent endpwent getgrent endgrent \
 touchlock \
-cfmakeraw cfsetspeed copysign __executable_start)
+cfmakeraw cfsetspeed copysign __executable_start log2)
 
 ## Eric Backus <ericb@lsid.hp.com> says, HP-UX 9.x on HP 700 machines
 ## has a broken `rint' in some library versions including math library
diff --git a/src/floatfns.c b/src/floatfns.c
index d7514ec..a7e03e7 100644
--- a/src/floatfns.c
+++ b/src/floatfns.c
@@ -241,12 +241,29 @@ The function returns the cons cell (SGNFCAND . EXP).
 
       if (b == 10.0)
 	d = log10 (d);
+#ifdef HAVE_LOG2
+      else if (b == 2.0)
+        d = log2 (d);
+#endif
       else
 	d = log (d) / log (b);
     }
   return make_float (d);
 }
 
+DEFUN ("log2", Flog2, Slog2, 1, 1, 0,
+       doc: /* Return the logarithm base 2 of ARG.  */)
+  (Lisp_Object arg)
+{
+  double d = extract_float (arg);
+#ifdef HAVE_LOG2
+  d = log2 (d);
+#else
+  d = log (d) / log (2.0);
+#endif
+  return make_float(d);
+}
+
 DEFUN ("log10", Flog10, Slog10, 1, 1, 0,
        doc: /* Return the logarithm base 10 of ARG.  */)
   (Lisp_Object arg)
@@ -553,6 +570,7 @@ The function returns the cons cell (SGNFCAND . EXP).
   defsubr (&Sexp);
   defsubr (&Sexpt);
   defsubr (&Slog);
+  defsubr (&Slog2);
   defsubr (&Slog10);
   defsubr (&Ssqrt);
 
-- 
1.8.3.1

Re: [PATCH] Add support for log2.

[Stefan Monnier]

> A feature test is added to configure.ac and a fallback for legacy
> systems included.
> * src/floatfns.c (Flog): Add special case for `log2'.
>   (Flog2): New function.
> * configure.ac: Test for `log2'.

Looks OK, feel free to install (with an entry to etc/NEWS as well).


        Stefan

Re: [PATCH] Add support for log2.

[Paul Eggert]

Elisp already has a syntax for log2 (X), namely (log X 2),
so how about the following more-conservative change?
This avoids adding the "log2" function to the Emacs namespace,
thus no need for a NEWS entry, doc change, etc.

(I know the same argument applies to log10, but
that horse left the barn long ago.)

=== modified file 'ChangeLog'
--- ChangeLog	2013-06-19 20:10:57 +0000
+++ ChangeLog	2013-06-20 02:32:16 +0000
@@ -1,3 +1,7 @@
+2013-06-20  Rüdiger Sonderfeld <ruediger@c-plusplus.de>
+
+	* configure.ac (log2): Check for this function.
+
 2013-06-19  Juanma Barranquero  <lekktu@gmail.com>
 
 	* .bzrignore: Add GNU GLOBAL files.

=== modified file 'configure.ac'
--- configure.ac	2013-06-17 06:03:19 +0000
+++ configure.ac	2013-06-20 02:31:58 +0000
@@ -3235,7 +3235,7 @@
 difftime posix_memalign \
 getpwent endpwent getgrent endgrent \
 touchlock \
-cfmakeraw cfsetspeed copysign __executable_start)
+cfmakeraw cfsetspeed copysign __executable_start log2)
 
 ## Eric Backus <ericb@lsid.hp.com> says, HP-UX 9.x on HP 700 machines
 ## has a broken `rint' in some library versions including math library

=== modified file 'src/ChangeLog'
--- src/ChangeLog	2013-06-19 20:10:57 +0000
+++ src/ChangeLog	2013-06-20 02:31:58 +0000
@@ -1,3 +1,8 @@
+2013-06-20  Rüdiger Sonderfeld <ruediger@c-plusplus.de>
+
+	* floatfns.c (Flog) [HAVE_LOG2]: Use log2 if available and if the
+	base is 2; in practice it's more accurate.
+
 2013-06-19  Juanma Barranquero  <lekktu@gmail.com>
 
 	* sound.c (string_default): Move to !WINDOWSNT section.

=== modified file 'src/floatfns.c'
--- src/floatfns.c	2013-06-20 01:19:43 +0000
+++ src/floatfns.c	2013-06-20 02:31:58 +0000
@@ -33,10 +33,10 @@
    acosh, atanh, cbrt, *copysign, erf, erfc, exp2, expm1, fdim, fma,
    fmax, fmin, fpclassify, hypot, ilogb, isfinite, isgreater,
    isgreaterequal, isinf, isless, islessequal, islessgreater, *isnan,
-   isnormal, isunordered, lgamma, log1p, log2, *logb (approximately),
-   lrint/llrint, lround/llround, nan, nearbyint, nextafter,
-   nexttoward, remainder, remquo, *rint, round, scalbln, scalbn,
-   signbit, tgamma, trunc.
+   isnormal, isunordered, lgamma, log1p, *log2 [via (log X 2)], *logb
+   (approximately), lrint/llrint, lround/llround, nan, nearbyint,
+   nextafter, nexttoward, remainder, remquo, *rint, round, scalbln,
+   scalbn, signbit, tgamma, trunc.
  */
 
 #include <config.h>
@@ -252,6 +252,10 @@
 
       if (b == 10.0)
 	d = log10 (d);
+#if HAVE_LOG2
+      else if (b == 2.0)
+	d = log2 (d);
+#endif
       else
 	d = log (d) / log (b);
     }

Re: [PATCH] Add support for log2.

[Rüdiger Sonderfeld]

On Wednesday 19 June 2013 19:36:14 Paul Eggert wrote:
> Elisp already has a syntax for log2 (X), namely (log X 2),
> so how about the following more-conservative change?
> This avoids adding the "log2" function to the Emacs namespace,
> thus no need for a NEWS entry, doc change, etc.
> 
> (I know the same argument applies to log10, but
> that horse left the barn long ago.)

I would be fine with that.  Common Lisp has no log2 or log10 either, so it 
probably makes sense.  However I think in that case we should consider 
deprecating log10 for the sake of consistency.  The existence of a log10 seems 
to imply a lack of a log2.

Regards,
Rüdiger

Re: [PATCH] Add support for log2.

[Paul Eggert]

On 06/20/2013 05:56 AM, Rüdiger Sonderfeld wrote:
> we should consider 
> deprecating log10 for the sake of consistency.

Sounds good.  I committed all that, as trunk bzr 113098.

Re: [PATCH] Add support for log2.

[Rüdiger Sonderfeld]

On Thursday 20 June 2013 07:27:45 Paul Eggert wrote:
> On 06/20/2013 05:56 AM, Rüdiger Sonderfeld wrote:
> > we should consider
> > deprecating log10 for the sake of consistency.
> 
> Sounds good.  I committed all that, as trunk bzr 113098.

Thanks!