ENH: add arachne engine for narrow internal solid infill

ConcentricGapFill pattern was used for internal narrow
solid infill. Use arachne engine instead to remove
gap fill inside the pattern and improve the extrusion path

Signed-off-by: salt.wei <salt.wei@bambulab.com>
Change-Id: I758d7c72eb71cc37026b7cebf746cc345014c3f5
(cherry picked from commit 0b6bacd21a091afc13d7b36a69e5b10f155bc6f8)

src/libslic3r/Arachne/BeadingStrategy/DistributedBeadingStrategy.hpp

@@ -0,0 +1,40 @@
+// Copyright (c) 2022 Ultimaker B.V.
+// CuraEngine is released under the terms of the AGPLv3 or higher.
+
+#ifndef DISTRIBUTED_BEADING_STRATEGY_H
+#define DISTRIBUTED_BEADING_STRATEGY_H
+
+#include "BeadingStrategy.hpp"
+
+namespace Slic3r::Arachne
+{
+
+/*!
+ * This beading strategy chooses a wall count that would make the line width
+ * deviate the least from the optimal line width, and then distributes the lines
+ * evenly among the thickness available.
+ */
+class DistributedBeadingStrategy : public BeadingStrategy
+{
+protected:
+    float one_over_distribution_radius_squared; // (1 / distribution_radius)^2
+
+public:
+    /*!
+    * \param distribution_radius the radius (in number of beads) over which to distribute the discrepancy between the feature size and the optimal thickness
+    */
+    DistributedBeadingStrategy(coord_t optimal_width,
+                               coord_t default_transition_length,
+                               double  transitioning_angle,
+                               double  wall_split_middle_threshold,
+                               double  wall_add_middle_threshold,
+                               int     distribution_radius);
+
+    ~DistributedBeadingStrategy() override = default;
+
+    Beading compute(coord_t thickness, coord_t bead_count) const override;
+    coord_t getOptimalBeadCount(coord_t thickness) const override;
+};
+
+} // namespace Slic3r::Arachne
+#endif // DISTRIBUTED_BEADING_STRATEGY_H