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[gnumeric] TDIST: fix problem with negative x.

commit d79d15242d61956209dd760195366719cfa46122
Author: Morten Welinder <terra gnome org>
Date:   Fri Apr 9 09:13:08 2010 -0400

    TDIST: fix problem with negative x.

 doc/C/func.defs             |   14 ++++++++------
 doc/C/functions.xml         |   20 ++++++++++++++------
 plugins/fn-stat/functions.c |   27 ++++++++++++++++++++++-----
 3 files changed, 44 insertions(+), 17 deletions(-)
---
diff --git a/doc/C/func.defs b/doc/C/func.defs
index ef29a89..beaaa0a 100644
--- a/doc/C/func.defs
+++ b/doc/C/func.defs
@@ -2889,7 +2889,7 @@ If any argument is zero, the result is zero.
 @SYNTAX=MATCH(seek,vector,type)
 @ARGUMENTDESCRIPTION= {seek}: value to find
 @{vector}: n by 1 or 1 by n range to be searched
- {type}: +1 to find the largest value â?¤ @{seek}, 0 to find the first value = @{seek}, or-1 to find the smallest value â?¥ @{seek}
+ {type}: +1 (the default) to find the largest value â?¤ @{seek}, 0 to find the first value = @{seek}, or-1 to find the smallest value â?¥ @{seek}
 @DESCRIPTION=MATCH searches @{vector} for @{seek} and returns the 1-based index.
 @NOTE= For @{type} = -1 the data must be sorted in descending order; for @{type} = +1 the data must be sorted in ascending order. If @{seek} could not be found, #N/A is returned. If @{vector} is neither n by 1 nor 1 by n, #N/A is returned.
 @SEEALSO=LOOKUP
@@ -4798,7 +4798,7 @@ If the data set does not contain any duplicates this function returns a #N/A err
 @ARGUMENTDESCRIPTION= {x}: total number of objects
 @{y}: number of selected objects
 @NOTE=If both @{x} and @{y} equal 0, PERMUTATIONA returns 1. If @{x} < 0 or @{y} < 0, PERMUTATIONA returns #NUM! If @{x} or @{y} are not integers, they are truncated
- ODF=This function is ODF compatible.
+ ODF=This function is OpenFormula compatible.
 @SEEALSO=POWER
 
 @CATEGORY=Statistics
@@ -5586,7 +5586,7 @@ Numbers, text and logical values are included in the calculation too. If the cel
 @SYNTAX=TDIST(x,dof,tails)
 @ARGUMENTDESCRIPTION= {x}: 
 @{dof}: number of degrees of freedom
- {tails}: 1 or 2. If this is 2, @{x} is replaced by the absolute value and the returned value is multiplied by 2
+ {tails}: 1 or 2.
 @DESCRIPTION=The survival function is 1 minus the cumulative distribution function.
 This function is Excel compatible for non-negative @{x}.
 @NOTE=If @{dof} < 1 this function returns a #NUM! error. If @{tails} is neither 1 or 2 this function returns a #NUM! error. The parameterization of this function is different from what is used for, e.g., NORMSDIST.  This is a common source of mistakes, but necessary for compatibility.
@@ -5695,13 +5695,15 @@ Numbers, text and logical values are included in the calculation too. If the cel
 
 @CATEGORY=Statistics
 @FUNCTION=ZTEST
- SHORTDESC=p-value of a 2-tailed z-test for a population mean
+ SHORTDESC=the probability of observing a sample mean as large as or larger than the mean of the given sample
 @SYNTAX=ZTEST(ref,x,stddev)
- ARGUMENTDESCRIPTION=@{ref}: data set
- {x}: mean as given in the null hypothesis
+ ARGUMENTDESCRIPTION=@{ref}: data set (sample)
+ {x}: population mean
 @{stddev}: population standard deviation, defaults to the sample standard deviation
+ DESCRIPTION=ZTEST calulates the probability of observing a sample mean as large as or larger than the mean of the given sample for samples drawn from a normal distribution with mean @{x} and standard deviation @{stddev}.
 @NOTE=If @{ref} contains less than two data items ZTEST returns #DIV/0! error.
 @EXCEL=This function is Excel compatible.
+ ODF=This function is OpenFormula compatible.
 @SEEALSO=CONFIDENCE,NORMDIST,NORMINV,NORMSDIST,NORMSINV,STANDARDIZE
 
 @CATEGORY=String
diff --git a/doc/C/functions.xml b/doc/C/functions.xml
index 00cdfc7..65a286b 100644
--- a/doc/C/functions.xml
+++ b/doc/C/functions.xml
@@ -9481,7 +9481,7 @@
         <title>Arguments</title>
         <para><parameter>seek</parameter>: value to find</para>
         <para><parameter>vector</parameter>: n by 1 or 1 by n range to be searched</para>
-        <para><parameter>type</parameter>: +1 to find the largest value â?¤ <parameter>seek</parameter>, 0 to find the first value = <parameter>seek</parameter>, or-1 to find the smallest value â?¥ <parameter>seek</parameter></para>
+        <para><parameter>type</parameter>: +1 (the default) to find the largest value â?¤ <parameter>seek</parameter>, 0 to find the first value = <parameter>seek</parameter>, or-1 to find the smallest value â?¥ <parameter>seek</parameter></para>
       </refsect1>
       <refsect1>
         <title>Description</title>
@@ -16631,7 +16631,7 @@
       </refsect1>
       <refsect1>
         <title>OpenDocument Format (ODF) Compatibility</title>
-        <para>This function is ODF compatible.</para>
+        <para>This function is OpenFormula compatible.</para>
       </refsect1>
       <refsect1>
         <title>See also</title>
@@ -19168,7 +19168,7 @@
         <title>Arguments</title>
         <para><parameter>x</parameter>: </para>
         <para><parameter>dof</parameter>: number of degrees of freedom</para>
-        <para><parameter>tails</parameter>: 1 or 2. If this is 2, <parameter>x</parameter> is replaced by the absolute value and the returned value is multiplied by 2</para>
+        <para><parameter>tails</parameter>: 1 or 2.</para>
       </refsect1>
       <refsect1>
         <title>Description</title>
@@ -19552,7 +19552,7 @@
           <function>ZTEST</function>
         </refname>
         <refpurpose>
-        p-value of a 2-tailed z-test for a population mean
+        the probability of observing a sample mean as large as or larger than the mean of the given sample
       </refpurpose>
       </refnamediv>
       <refsynopsisdiv>
@@ -19560,11 +19560,15 @@
       </refsynopsisdiv>
       <refsect1>
         <title>Arguments</title>
-        <para><parameter>ref</parameter>: data set</para>
-        <para><parameter>x</parameter>: mean as given in the null hypothesis</para>
+        <para><parameter>ref</parameter>: data set (sample)</para>
+        <para><parameter>x</parameter>: population mean</para>
         <para><parameter>stddev</parameter>: population standard deviation, defaults to the sample standard deviation</para>
       </refsect1>
       <refsect1>
+        <title>Description</title>
+        <para><function>ZTEST</function> calulates the probability of observing a sample mean as large as or larger than the mean of the given sample for samples drawn from a normal distribution with mean <parameter>x</parameter> and standard deviation <parameter>stddev</parameter>.</para>
+      </refsect1>
+      <refsect1>
         <title>Note</title>
         <para>If <parameter>ref</parameter> contains less than two data items <function>ZTEST</function> returns #DIV/0! error.</para>
       </refsect1>
@@ -19573,6 +19577,10 @@
         <para>This function is Excel compatible.</para>
       </refsect1>
       <refsect1>
+        <title>OpenDocument Format (ODF) Compatibility</title>
+        <para>This function is OpenFormula compatible.</para>
+      </refsect1>
+      <refsect1>
         <title>See also</title>
         <para><link linkend="gnumeric-CONFIDENCE"><function>CONFIDENCE</function></link>,
         <link linkend="gnumeric-NORMDIST"><function>NORMDIST</function></link>,
diff --git a/plugins/fn-stat/functions.c b/plugins/fn-stat/functions.c
index 8a19108..2e8930b 100644
--- a/plugins/fn-stat/functions.c
+++ b/plugins/fn-stat/functions.c
@@ -1188,8 +1188,7 @@ static GnmFuncHelp const help_tdist[] = {
 	{ GNM_FUNC_HELP_NAME, F_("TDIST:survival function of the Student t-distribution")},
 	{ GNM_FUNC_HELP_ARG, F_("x:")},
 	{ GNM_FUNC_HELP_ARG, F_("dof:number of degrees of freedom")},
-	{ GNM_FUNC_HELP_ARG, F_("tails:1 or 2. If this is 2, @{x} is replaced by the absolute value and "
-				"the returned value is multiplied by 2")},
+	{ GNM_FUNC_HELP_ARG, F_("tails:1 or 2.")},
 	{ GNM_FUNC_HELP_DESCRIPTION, F_("The survival function is 1 minus the cumulative distribution function.") },
 	{ GNM_FUNC_HELP_NOTE, F_("If @{dof} < 1 this function returns a #NUM! error.") },
 	{ GNM_FUNC_HELP_NOTE, F_("If @{tails} is neither 1 or 2 this function returns a #NUM! error.") },
@@ -1210,11 +1209,29 @@ gnumeric_tdist (GnmFuncEvalInfo *ei, GnmValue const * const *argv)
 	gnm_float x = value_get_as_float (argv[0]);
 	gnm_float dof = value_get_as_float (argv[1]);
 	int tails = value_get_as_int (argv[2]);
+	gnm_float p;
 
-	if (x < 0 || dof < 1 || (tails != 1 && tails != 2))
-		return value_new_error_NUM (ei->pos);
+	if (dof < 1)
+		goto bad;
+
+	if (tails == 1) {
+		gboolean lower_tail = FALSE;
+		if (x < 0) {
+			lower_tail = TRUE;
+			x = -x;
+		}
+		p = pt (x, dof, lower_tail, FALSE);
+	} else if (tails == 2) {
+		if (x < 0)
+			goto bad;
+		p = 2 * pt (x, dof, FALSE, FALSE);
+	} else
+		goto bad;
+
+	return value_new_float (p);
 
-	return value_new_float (tails * pt (x, dof, FALSE, FALSE));
+ bad:
+	return value_new_error_NUM (ei->pos);
 }
 
 /***************************************************************************/
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