[gnumeric] LOG2: Improve accuracy.

[Morten Welinder <mortenw src gnome org>]

commit 7ba9593d9768e2a94e444ecdeea11931b7d7975b
Author: Morten Welinder <terra gnome org>
Date:   Wed Feb 19 14:04:05 2014 -0500

    LOG2: Improve accuracy.

 ChangeLog                   |    6 ++++++
 NEWS                        |    1 +
 plugins/fn-math/functions.c |    2 +-
 src/mathfunc.c              |   19 +++++++++++++++++++
 src/mathfunc.h              |    1 +
 5 files changed, 28 insertions(+), 1 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index cc41852..548461e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2014-02-19  Morten Welinder  <terra gnome org>
+
+       * plugins/fn-math/functions.c (gnumeric_log2): Use gnm_log2.
+
+       * src/mathfunc.c (gnm_log2): New function.
+
 2014-02-18  Morten Welinder  <terra gnome org>
 
        * src/ssdiff.c (xml_style_changed): Report differences for
diff --git a/NEWS b/NEWS
index c40cfda..691a40a 100644
--- a/NEWS
+++ b/NEWS
@@ -14,6 +14,7 @@ Morten:
        * Improve testing.
        * Fix XLSX number roundtrip issues.  [?] and [#724661]
        * Improve ssdiff's report of differences.
+       * Improve LOG2's accuracy.
 
 --------------------------------------------------------------------------
 Gnumeric 1.12.11
diff --git a/plugins/fn-math/functions.c b/plugins/fn-math/functions.c
index bc49553..b3c95af 100644
--- a/plugins/fn-math/functions.c
+++ b/plugins/fn-math/functions.c
@@ -1325,7 +1325,7 @@ gnumeric_log2 (GnmFuncEvalInfo *ei, GnmValue const * const *argv)
        if (t <= 0.0)
                return value_new_error_NUM (ei->pos);
 
-       return value_new_float (gnm_log (t) / M_LN2gnum);
+       return value_new_float (gnm_log2 (t));
 }
 
 /***************************************************************************/
diff --git a/src/mathfunc.c b/src/mathfunc.c
index 1b5ca50..3efa808 100644
--- a/src/mathfunc.c
+++ b/src/mathfunc.c
@@ -311,6 +311,25 @@ bd0(gnm_float x, gnm_float M)
        return yh + yl;
 }
 
+gnm_float
+gnm_log2 (gnm_float x)
+{
+       int e;
+
+       if (x == 0)
+               return gnm_ninf;
+
+       if (x < 0)
+               return gnm_nan;
+
+       if (!gnm_finite (x))
+               return x;
+
+       /* This split ensures accurate integer results for 2^x.  */
+       x = gnm_frexp (x, &e);
+
+       return e + gnm_log (x) / M_LN2gnum;
+}
 
 /* ------------------------------------------------------------------------- */
 /* --- BEGIN MAGIC R SOURCE MARKER --- */
diff --git a/src/mathfunc.h b/src/mathfunc.h
index e24fa05..d86d162 100644
--- a/src/mathfunc.h
+++ b/src/mathfunc.h
@@ -27,6 +27,7 @@ G_BEGIN_DECLS
 
 /* ------------------------------------------------------------------------- */
 
+gnm_float gnm_log2 (gnm_float x);
 gnm_float log1pmx (gnm_float x);
 gnm_float swap_log_tail (gnm_float lp);
 gnm_float pow1p (gnm_float x, gnm_float y);