Fix logb on zero, infinite, NaN args

Change logb to return -infinity, +infinity, and NaN respectively.
Formerly logb returned an extreme fixnum to represent
infinity, but this is no longer the right thing to do now that
we have bignums and there is no extreme integer.
* doc/lispref/numbers.texi (Float Basics), etc/NEWS: Document.
* src/floatfns.c (Flogb): Implement this.

diff --git a/doc/lispref/numbers.texi b/doc/lispref/numbers.texi
index cffc634169f8b0c3709001ccaf4b3de71961ee83..fbdd83fa86ed033d4fcbd632cb3edc977d90b8a5 100644 (file)
--- a/doc/lispref/numbers.texi
+++ b/doc/lispref/numbers.texi
@@ -313,14 +313,18 @@ and returns the result.  @var{x1} and @var{x2} must be floating point.
 
 @defun logb x
 This function returns the binary exponent of @var{x}.  More
-precisely, the value is the logarithm base 2 of @math{|x|}, rounded
-down to an integer.
+precisely, if @var{x} is finite and nonzero, the value is the
+logarithm base 2 of @math{|x|}, rounded down to an integer.
+If @var{x} is zero, infinite, or a NaN, the value is minus infinity,
+plus infinity, or a NaN respectively.
 
 @example
 (logb 10)
      @result{} 3
 (logb 10.0e20)
      @result{} 69
+(logb 0)
+     @result{} -1.0e+INF
 @end example
 @end defun
 

diff --git a/etc/NEWS b/etc/NEWS
index b316aecbfa444402f046c555a6339e29ac4f9c48..3670ab5bf44943fc5821a45450d702aa373ec940 100644 (file)
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1112,6 +1112,10 @@ old-style backquotes as new-style, bind the new variable
 integer, Emacs now signals an error if the number is too large for the
 implementation to format.
 
+** logb now returns infinity when given an infinite or zero argument,
+and returns a NaN when given a NaN.  Formerly, it returned an extreme
+fixnum for such arguments.
+
 ---
 ** Some functions and variables obsolete since Emacs 22 have been removed:
 archive-mouse-extract, assoc-ignore-case, assoc-ignore-representation,

diff --git a/src/floatfns.c b/src/floatfns.c
index 2d76b97eec7c2b74ec92e5dedde339dea4f3fc95..a913aad5aacf2ebe695c4fca79ae9a07e3034ee1 100644 (file)
--- a/src/floatfns.c
+++ b/src/floatfns.c
@@ -306,27 +306,22 @@ This is the same as the exponent of a float.  */)
   if (FLOATP (arg))
     {
       double f = XFLOAT_DATA (arg);
-
       if (f == 0)
-       value = MOST_NEGATIVE_FIXNUM;
-      else if (isfinite (f))
-       {
-         int ivalue;
-         frexp (f, &ivalue);
-         value = ivalue - 1;
-       }
-      else
-       value = MOST_POSITIVE_FIXNUM;
+       return make_float (-HUGE_VAL);
+      if (!isfinite (f))
+       return f < 0 ? make_float (-f) : arg;
+      int ivalue;
+      frexp (f, &ivalue);
+      value = ivalue - 1;
     }
-  else if (BIGNUMP (arg))
+  else if (!FIXNUMP (arg))
     value = mpz_sizeinbase (XBIGNUM (arg)->value, 2) - 1;
   else
     {
-      eassert (FIXNUMP (arg));
-      EMACS_INT i = eabs (XFIXNUM (arg));
-      value = (i == 0
-              ? MOST_NEGATIVE_FIXNUM
-              : EMACS_UINT_WIDTH - 1 - ecount_leading_zeros (i));
+      EMACS_INT i = XFIXNUM (arg);
+      if (i == 0)
+       return make_float (-HUGE_VAL);
+      value = EMACS_UINT_WIDTH - 1 - ecount_leading_zeros (eabs (i));
     }
 
   return make_fixnum (value);