31902: rationalise use of gamma function.

Make zsh/mathfunc consistent across systems and use tgamma() where
available

ChangeLog

@@ -1,3 +1,8 @@
+2013-10-26  Peter Stephenson  <p.w.stephenson@ntlworld.com>
+
+	* Jun T: 31902: rationalise gamma function in mathfunc for
+	consistency with various libraries and user expectations.
+
 2013-10-25  Peter Stephenson  <p.w.stephenson@ntlworld.com>
 
 	* 31890 (with Mikael's fixes in 31891): NEWS: add descriptions

Doc/Zsh/mod_mathfunc.yo

@@ -33,7 +33,7 @@ returns an integer.
 The function tt(signgam) takes no arguments, and returns an integer, which
 is the C variable of the same name, as described in manref(gamma)(3).  Note
 that it is therefore only useful immediately after a call to tt(gamma) or
-tt(lgamma).  Note also that `tt(signgam+LPAR()RPAR)' and `tt(signgam)' are
+tt(lgamma).  Note also that `tt(signgam+LPAR()RPAR())' and `tt(signgam)' are
 distinct expressions.
 
 The following functions take two floating point arguments: tt(copysign),

Src/Modules/mathfunc.c

@@ -340,7 +340,16 @@ math_func(char *name, int argc, mnumber *argv, int id)
       break;
 
   case MF_GAMMA:
+#ifdef HAVE_TGAMMA
+      retd = tgamma(argd);
+#else
+#ifdef HAVE_SIGNGAM
+      retd = lgamma(argd);
+      retd = signgam*exp(retd);
+#else /*XXX assume gamma(x) returns Gamma(x), not log(|Gamma(x)|) */
       retd = gamma(argd);
+#endif
+#endif
       break;
 
   case MF_HYPOT:

Test/V03mathfunc.ztst

@@ -136,3 +136,9 @@ F:This test fails if your math library doesn't have erand48().
   done
   (( ok ))
 0:Test random number generator distributions are not grossly broken
+
+  float -F 5 g l
+  (( g = gamma(2), l = lgamma(2) ))
+  print $g, $l
+0:Test Gamma function gamma and lgamma
+>1.00000, 0.00000

configure.ac

@@ -1222,7 +1222,7 @@ AC_CHECK_FUNCS(strftime strptime mktime timelocal \
 	       getrlimit getrusage \
 	       setlocale \
 	       uname \
-	       signgam \
+	       signgam tgamma \
 	       putenv getenv setenv unsetenv xw\
 	       brk sbrk \
 	       pathconf sysconf \