[gnumeric] Fix linear averaging interpolation. [#680723]

[Jean BrÃfort <jbrefort src gnome org>]

commit 9c63b608867ecd2697fe671d1ef4bf1f7a9bc835
Author: Jean Brefort <jean brefort normalesup org>
Date:   Tue Jul 31 15:43:45 2012 +0200

    Fix linear averaging interpolation. [#680723]

 NEWS                       |    1 +
 plugins/fn-tsa/ChangeLog   |    4 ++++
 plugins/fn-tsa/functions.c |   10 ++++++----
 3 files changed, 11 insertions(+), 4 deletions(-)
---
diff --git a/NEWS b/NEWS
index 7eb164d..754b8fb 100644
--- a/NEWS
+++ b/NEWS
@@ -11,6 +11,7 @@ Andreas:
 
 Jean:
 	* Fix component references issues. [#680190]
+	* Fix linear averaging interpolation. [#680723]
 
 Morten:
 	* Fix xls read crash.  [#679992]
diff --git a/plugins/fn-tsa/ChangeLog b/plugins/fn-tsa/ChangeLog
index 932f6e7..3e2e63f 100644
--- a/plugins/fn-tsa/ChangeLog
+++ b/plugins/fn-tsa/ChangeLog
@@ -1,3 +1,7 @@
+2012-07-31  Jean Brefort  <jean brefort normalesup org>
+
+	* functions.c (linear_averaging): fixed it. [#680723]
+
 2012-07-27 Andreas J. Guelzow <aguelzow pyrshep ca>
 
 	* functions.c (gnumeric_interpolation): check for target size
diff --git a/plugins/fn-tsa/functions.c b/plugins/fn-tsa/functions.c
index 5ba4947..5dd3a4a 100644
--- a/plugins/fn-tsa/functions.c
+++ b/plugins/fn-tsa/functions.c
@@ -231,12 +231,13 @@ linear_averaging (const gnm_float *absc, const gnm_float *ord, int nb_knots,
 			slope = (ord[j] - ord[k]) / x0 / 2.;
 			res[i - 1] += x0 * (slope * x0 + ord[k]);
 		}
-		if (j > k - 1) {
+		if (j > k + 1) {
 			k = j - 1;
 			slope = (ord[j] - ord[k]) / (absc[j] - absc[k]) / 2.;
+			j = k;
 		}
-		x0 = targets[i] - absc[k];
-		res[i - 1] += x0 * (slope * x0 + ord[k]);
+		x0 = targets[i] - absc[j];
+		res[i - 1] += x0 * (slope * x0 + ord[j]);
 		res[i - 1] /= (targets[i] - targets[i - 1]);
 	}
 	return res;
@@ -391,7 +392,8 @@ static GnmFuncHelp const help_interpolation[] = {
 				 "('staircase with averaging'), and 5 ('natural cubic spline with "
 				 "averaging') is used, the number "
 				 "of returned values is one less than the number of targets and the targets "
-				 "values must be given in increasing order.") },
+				 "values must be given in increasing order. The interpolated values returned"
+				 "are the averages of the interpolation on each interval.") },
 	{ GNM_FUNC_HELP_NOTE, F_("Strings and empty cells in @{abscissae} and @{ordinates} are ignored.") },
 	{ GNM_FUNC_HELP_NOTE, F_("If several target data are provided they must be in the same column in consecutive cells.") },
 	{ GNM_FUNC_HELP_SEEALSO, "PERIODOGRAM" },