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We give an overview of theoretical and practical aspects of finding a simple polygon of minimum ( MinArea ) or maximum ( MaxArea ) possible area for a given set of n points in the plane. Both problems are known to be NP hard and were the subject of the 2019 Computational Geometry Challenge, which presented the quest of finding good solutions to more than 200 instances, ranging from n = 10 all the way to n = 1, 000, 000.more » « less

He, Meng ; Sheehy, Don (Ed.)We introduce basic, but heretofore generally unexplored, problems in computational origami that are similar in style to classic problems from discrete and computational geometry. We consider the problems of folding each corner of a polygon P to a point p and folding each edge of a polygon P onto a line segment L that connects two boundary points of P and compute the number of edges of the polygon containing p or L limited by crease lines and boundary edges.more » « less

null (Ed.)Imagine t ≤ mn unitsquare tiles in an m×n rectangular box that you can tilt to cause all tiles to slide maximally in one of the four orthogonal directions. Given two tiles of interest, is there a tilt sequence that brings them to adjacent squares? We give a lineartime algorithm for this problem, motivated by 2048 endgames. We also bound the number of reachable configurations, and design instances where all t tiles permute according to a cyclic permutation every four tilts.more » « less

null (Ed.)In this paper, we show that the rigidfoldability of a given crease pattern using all creases is weakly NPhard by a reduction from the partition problem, and that rigidfoldability with optional creases is NPhard by a reduction from the 1in3 SAT problem. Unlike flatfoldabilty of origami or flexibility of other kinematic linkages, whose complexity originates in the complexity of the layer ordering and possible selfintersection of the material, rigid foldabilltiy from a planar state is hard even though there is no potential selfintersection. In fact, the complexity comes from the combinatorial behavior of the different possible rigid folding configurations at each vertex. The results underpin the fact that it is harder to fold from an unfolded sheet of paper than to unfold a folded state back to a plane, frequently encountered problem when realizing foldingbased systems such as selffolding matters and reconfigurable robots.more » « less

We investigate algorithmic control of a large swarm of mobile particles (such as robots, sensors, or building material) that move in a 2D workspace using a global input signal (such as gravity or a magnetic field). Upon activation of the field, each particle moves maximally in the same direction until forward progress is blocked by a stationary obstacle or another stationary particle. In an open workspace, this system model is of limited use because it has only two controllable degrees of freedom—all particles receive the same inputs and move uniformly. We show that adding a maze of obstacles to the environment can make the system drastically more complex but also more useful. We provide a wide range of results for a wide range of questions. These can be subdivided into external algorithmic problems, in which particle configurations serve as input for computations that are performed elsewhere, and internal logic problems, in which the particle configurations themselves are used for carrying out computations. For external algorithms, we give both negative and positive results. If we are given a set of stationary obstacles, we prove that it is NPhard to decide whether a given initial configuration of unitsized particles can be transformed into a desired target configuration. Moreover, we show that finding a control sequence of minimum length is PSPACEcomplete. We also work on the inverse problem, providing constructive algorithms to design workspaces that efficiently implement arbitrary permutations between different configurations. For internal logic, we investigate how arbitrary computations can be implemented. We demonstrate how to encode dualrail logic to build a universal logic gate that concurrently evaluates AND, NAND, NOR, and OR operations. Using many of these gates and appropriate interconnects, we can evaluate any logical expression. However, we establish that simulating the full range of complex interactions present in arbitrary digital circuits encounters a fundamental difficulty: a FANOUT gate cannot be generated. We resolve this missing component with the help of 2 9 1 particles, which can create FANOUT gates that produce multiple copies of the inputs. Using these gates we provide rules for replicating arbitrary digital circuits.more » « less