We prove that two pushing-blocks puzzles are intractable in 2D. One of our
constructions improves an earlier result that established intractability in 3D
[OS99] for a puzzle inspired by the game PushPush. The second construction
answers a question we raised in [DDO00] for a variant we call Push-1. Both
puzzles consist of unit square blocks on an integer lattice; all blocks are
movable. An agent may push blocks (but never pull them) in attempting to move
between given start and goal positions. In the PushPush version, the agent can
only push one block at a time, and moreover when a block is pushed it slides
the maximal extent of its free range. In the Push-1 version, the agent can only
push one block one square at a time, the minimal extent---one square. Both
NP-hardness proofs are by reduction from SAT, and rely on a common
construction.Comment: 10 pages, 11 figures. Corrects an error in the conference version:
Proc. of the 12th Canadian Conference on Computational Geometry, August 2000,
pp. 211-21
