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diff --git a/src/helper/mathfns.h b/src/helper/mathfns.h
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+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * ... some mathematical functions
+ *
+ * Authors:
+ *   Johan Engelen <goejendaagh@zonnet.nl>
+ *
+ * Copyright (C) 2007 Johan Engelen
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
+ */
+
+#ifndef SEEN_INKSCAPE_UTIL_MATHFNS_H
+#define SEEN_INKSCAPE_UTIL_MATHFNS_H
+
+#include <2geom/point.h>
+
+namespace Inkscape {
+
+namespace Util {
+
+/**
+ * Returns area in triangle given by points; may be negative.
+ */
+inline double
+triangle_area (Geom::Point p1, Geom::Point p2, Geom::Point p3)
+{
+    using Geom::X;
+    using Geom::Y;
+    return (p1[X]*p2[Y] + p1[Y]*p3[X] + p2[X]*p3[Y] - p2[Y]*p3[X] - p1[Y]*p2[X] - p1[X]*p3[Y]);
+}
+
+/**
+ * \return x rounded to the nearest multiple of c1 plus c0.
+ *
+ * \note
+ * If c1==0 (and c0 is finite), then returns +/-inf.  This makes grid spacing of zero
+ * mean "ignore the grid in this dimension".
+ */
+inline double round_to_nearest_multiple_plus(double x, double const c1, double const c0)
+{
+    return floor((x - c0) / c1 + .5) * c1 + c0;
+}
+
+/**
+ * \return x rounded to the lower multiple of c1 plus c0.
+ *
+ * \note
+ * If c1==0 (and c0 is finite), then returns +/-inf.  This makes grid spacing of zero
+ * mean "ignore the grid in this dimension".
+ */
+inline double round_to_lower_multiple_plus(double x, double const c1, double const c0 = 0)
+{
+    return floor((x - c0) / c1) * c1 + c0;
+}
+
+/**
+ * \return x rounded to the upper multiple of c1 plus c0.
+ *
+ * \note
+ * If c1==0 (and c0 is finite), then returns +/-inf.  This makes grid spacing of zero
+ * mean "ignore the grid in this dimension".
+ */
+inline double round_to_upper_multiple_plus(double x, double const c1, double const c0 = 0)
+{
+    return ceil((x - c0) / c1) * c1 + c0;
+}
+
+}
+
+}
+
+#endif
+/*
+  Local Variables:
+  mode:c++
+  c-file-style:"stroustrup"
+  c-file-offsets:((innamespace . 0)(inline-open . 0)(case-label . +))
+  indent-tabs-mode:nil
+  fill-column:99
+  End:
+*/
+// vim: filetype=cpp:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:fileencoding=utf-8:textwidth=99 :