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diff --git a/src/object/sp-paint-server.h b/src/object/sp-paint-server.h
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+// SPDX-License-Identifier: GPL-2.0-or-later
+#ifndef SEEN_SP_PAINT_SERVER_H
+#define SEEN_SP_PAINT_SERVER_H
+
+/*
+ * Base class for gradients and patterns
+ *
+ * Author:
+ *   Lauris Kaplinski <lauris@kaplinski.com>
+ *   Jon A. Cruz <jon@joncruz.org>
+ *
+ * Copyright (C) 1999-2002 Lauris Kaplinski
+ * Copyright (C) 2000-2001 Ximian, Inc.
+ * Copyright (C) 2010 Authors
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
+ */
+
+#include <memory>
+#include <cairo.h>
+#include <2geom/rect.h>
+#include <sigc++/slot.h>
+#include "sp-object.h"
+
+namespace Inkscape {
+class Drawing;
+class DrawingPattern;
+class DrawingPaintServer;
+} // namespace Inkscape
+
+class SPPaintServer
+    : public SPObject
+{
+public:
+    SPPaintServer();
+    ~SPPaintServer() override;
+    int tag() const override { return tag_of<decltype(*this)>; }
+
+    bool isSwatch() const;
+    virtual bool isValid() const;
+
+    /*
+     * There are two ways to implement a paint server:
+     *
+     *  1. Simple paint servers (solid colors and gradients) implement the create_drawing_paintserver() method.
+     *     This returns a DrawingPaintServer instance holding a copy of the paint server's resources which is
+     *     used to produce a pattern on-demand using create_pattern().
+     *
+     *  2. The other paint servers (patterns and hatches) implement show(), hide() and setBBox().
+     *     The drawing item subtree returned by show() is attached as a fill/stroke child of the
+     *     drawing item the paint server is applied to, and used directly when rendering.
+     *
+     *  Paint servers only need to implement one method. If both are implemented, then option 2 is used.
+     */
+
+    virtual std::unique_ptr<Inkscape::DrawingPaintServer> create_drawing_paintserver();
+
+    virtual Inkscape::DrawingPattern *show(Inkscape::Drawing &drawing, unsigned key, Geom::OptRect const &bbox);
+    virtual void hide(unsigned key);
+    virtual void setBBox(unsigned key, Geom::OptRect const &bbox);
+
+protected:
+    bool swatch = false;
+};
+
+/**
+ * Returns the first of {src, src-\>ref-\>getObject(),
+ * src-\>ref-\>getObject()-\>ref-\>getObject(),...}
+ * for which \a match is true, or NULL if none found.
+ *
+ * The raison d'être of this routine is that it correctly handles cycles in the href chain (e.g., if
+ * a gradient gives itself as its href, or if each of two gradients gives the other as its href).
+ *
+ * \pre is<SPGradient>(src).
+ */
+template <class PaintServer>
+PaintServer *chase_hrefs(PaintServer *src, sigc::slot<bool (PaintServer const *)> match) {
+    /* Use a pair of pointers for detecting loops: p1 advances half as fast as p2.  If there is a
+       loop, then once p1 has entered the loop, we'll detect it the next time the distance between
+       p1 and p2 is a multiple of the loop size. */
+    PaintServer *p1 = src, *p2 = src;
+    bool do1 = false;
+    for (;;) {
+        if (match(p2)) {
+            return p2;
+        }
+
+        p2 = p2->ref->getObject();
+        if (!p2) {
+            return p2;
+        }
+        if (do1) {
+            p1 = p1->ref->getObject();
+        }
+        do1 = !do1;
+
+        if ( p2 == p1 ) {
+            /* We've been here before, so return NULL to indicate that no matching gradient found
+             * in the chain. */
+            return nullptr;
+        }
+    }
+}
+
+#endif // SEEN_SP_PAINT_SERVER_H
+/*
+  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 :