REVIEW 4 major objections 5 minor 1 cited by
Approximately Optimal Search on a Higher-dimensional Sliding Puzzle
T0 review · 4 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read On the paper's own account, exact A* search solves dimension-3 and most dimension-4 hypercube sliding puzzles, while evolutionary and reinforcement-learning searches still solve dimension-5 instances with median move counts usually below…
desk verdict A first benchmark of search methods on the hypercube k-rule sliding puzzle, but the preprint's own equations, pseudocode, and tables contradict the reported results. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the (d,k,l)-cubical sliding puzzle on the hypercube $Q_d$: $2^d-l$ colored rings occupy distinct vertices, and a k-move slides a ring to any vertex of a k-dimensional face that contains no other ring. The A* implementation depends on the admissible lower-bound heuristic $h_T(C)=\sum_i \lceil |C_i-T_i|/k\rceil$, whose monotonicity is used to guarantee optimality. The EA carries the argument through a fitness $f_a(C,T)=1/(1+L-h(C,T))$ and a selection force that linearly combines that fitness with the inverse move count. The RL method carries its share through a reward function seeded by a breadth-first expansion of the target configuration, so that early random walks are biased toward configurations with known short paths to the goal.
What would settle it
Run the released EA with the objective exactly as printed in Eqs. (3)-(4): if h(C,T) is the number of mismatched ring-target pairs, then $f_a(C,T)=1/(1+L-h)$ reaches its maximum 1 at h=L, i.e. at a configuration where every ring is in the wrong place, so the loop 'while $f_a \neq 1$' should never stop for any nontrivial puzzle. If the code still solves puzzles, the experiments are not testing the algorithm described; if it does not, the reported EA successes would not be reproducible.
Extended reading notes
Core claim
On the paper's own terms, the discovery is that the difficulty wall for exact search is not a wall for stochastic search. For d=3, A* yields provably optimal move counts between 4 and 10, and both RL and EA match or approach these optima. For d=4, A* still solves levels 0-3 optimally but fails on level 4 within the time limit; for d=5, A* finds no solution, while EA solves the k=1, k=2, and k=3 puzzles with median move counts of 12, 9, and 7, and RL solves them with medians of 8, 5, and 4. The paper's own tables record one sharp exception to the 'median below 30' slogan: for d=5 and k=4, RL has median 161 moves and EA never terminates successfully. The authors infer that EA is consistently the fastest method and RL the most accurate stochastic method, with A* remaining the method of choice whenever optimality is required and the dimension is small.
Load-bearing premise
The EA results presuppose that the implemented fitness function rewards rings that already match their target colours, but the printed equations define h as a mismatch count, so the printed fitness is maximized when zero rings match and the stopping condition $f_a=1$ can never be reached for $L>0$.
Editorial extensions
If this is right
- A* search certifies optimal solutions for d=3 and for most d=4 puzzles, with optimum move counts between 4 and 10, so for small dimensions exact search remains the reference method.
- For d=5, A* times out, but EA solves k=1,2,3 puzzles with median moves 12, 9, and 7, and RL solves the same cases with medians 8, 5, and 4; approximate solutions remain available where optimal ones are not.
- EA is consistently the fastest in CPU time but its success rate collapses for d=4, k=3 to under 5 percent, whereas RL keeps near-100 percent success across tested levels and face dimensions.
- The A* branching factor on these puzzles is at least $2^k-1$, so exact search cost grows at least like $(2^k-1)^n$ in solution length $n$; this is the concrete combinatorial wall that the stochastic methods are claimed to bypass.
Reading between the lines
- [Editorial inference] If the printed fitness in Eqs. (3)-(4) is taken literally—h counts mismatches—the EA as described cannot terminate, so the reported EA numbers must come from an unstated corrected objective; re-running with the stated equations would be a direct test of which algorithm the benchmark actually evaluates.
- [Editorial inference] The target-seeded breadth-first weighting used for RL could be fused with A* as a bidirectional or windowed search, potentially pushing exact solutions into some d=5 instances where the current A* stalls.
- [Editorial inference] The paper compares medians and CPU times, but the full distributions suggest a three-way trade-off among accuracy, variance, and speed; a Pareto-style summary would let practitioners choose a method by difficulty level rather than by dimension alone.
- [Editorial inference] Because the difficulty levels appear to have been generated only for k=d-1, the reported monotone increase of move count with level and k may be an artifact of level generation; generating levels independently per k could separate puzzle hardness from algorithm behavior.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies a higher-dimensional sliding puzzle on the d-dimensional hypercube with the k-rule move constraint. It proposes three search methods—A*, an evolutionary algorithm (EA), and a reinforcement learning (RL) agent—and benchmarks them on puzzles of dimensions d=3, 4, and 5 across several difficulty levels. The main reported conclusions are that A* solves the d=3 and most d=4 puzzles optimally, while for larger dimensions EA and RL still return approximate solutions, with the EA being fastest and RL being more accurate. The paper also provides a CPU-time comparison and claims that the methods go beyond the theoretical reachability results of prior work.
Significance. If the claims held, the paper would provide a useful practical benchmark showing that stochastic search can handle hypercube sliding puzzles beyond the reach of exact search. The authors ship a public code repository, report experiments over multiple dimensions, face dimensions, and difficulty levels, and provide CPU-time comparisons; these are genuine strengths. However, the central empirical and algorithmic claims are not currently supported: the EA objective function as written is inconsistent with the reported stopping behavior, the RL pseudocode is not executable as printed, and the paper's headline claim about median moves under 30 is contradicted by its own Table 5. Because these issues affect the core results, the contribution cannot be accepted in its present form.
major comments (4)
- [§3.3, Eq. (3)-(4), Algorithm 2] The fitness function in Eq. (3)-(4) is internally inconsistent with the stated stopping criterion. h(C,T) is defined as the number of mismatches (δ=1 when C_i≠T_i), so L−h is the number of matches. The fitness fa=1/(1+L−h) is therefore maximized when h=0, but its maximum value is 1/(1+L), not 1. Conversely, fa=1 only when h=L, i.e., when every ring is mismatched. This directly contradicts Algorithm 2 line 4, which stops when fa(C_b,T)=1, and Appendix A, which equates the maximum fitness with hamming distance 0. The EA results in Table 4 therefore cannot be produced by the algorithm as described unless the implemented objective differs from the printed one; the equations and the experimental code must be reconciled and the experiments rerun.
- [Appendix D, Algorithm 4] The RL pseudocode is not executable as written. Line 4 tests `n(T) < N`, but `n` is never defined and `N` is not an input parameter (the input is the branching parameter `P`). Line 17 contains the loop condition `c < 0`, which is never true because c starts at 0 and is only incremented; the surrounding text in §3.2 says episodes terminate when more steps have been taken than the best known path, which would require a condition such as `c < o` rather than `c < 0`. As printed, the algorithm either loops forever or terminates immediately, so the RL results in Table 5 are not reproducible from the stated method.
- [Abstract vs. Tables 4 and 5] The abstract's central claim that, when the dimension increases, "RL and EA methods can still provide a generally acceptable solution, i.e. a distribution of a number of moves with a median value of less than 30" is directly contradicted by the paper's own data. Appendix F, Table 5, for d=5, l=26, k=4, level 0, reports RL success 100% with min 62, max 316, and median 161 moves. Table 4 reports an EA success rate of 0% for the same puzzle. Thus the reported median is more than five times the advertised bound, and the plural "RL and EA methods" is not supported for this case. The headline claim must be corrected or substantially narrowed.
- [Tables 4 and 5] For d=4, k=1 and k=2, the EA performance table (Table 4) and the RL performance table (Table 5) report identical min, max, and median values for every difficulty level, including entries such as level 1: min 8, max 60, median 14, and level 4: min 12, max 28, median 16. Since these are independent stochastic algorithms run over 150 trials, identical summary statistics across all levels are not plausible unless the same data were used for both tables. This data-consistency problem must be resolved before the EA-vs-RL comparison in §4.2 can be trusted.
minor comments (5)
- [Algorithm 2 comments] The comments in Algorithm 2 refer to "the length of the target RNA structure," "base-pair distance," and a Lévy or Binomial distribution, which appear to be copied from an RNA-folding EA. These should be replaced with puzzle-specific terminology.
- [§4.3, Proposition 1] The proof states the complexity is "at least O((2^k−1)^n)", but the following sentence says "the complexity of A* search for a face dimension k=4 is at most O(7^n)"; the latter should be "at least" or "of order" to be consistent.
- [Figure 3 caption] The caption of Figure 3 says the puzzle parameters are k=2, d=3, l=4, but panel (b) shows d=4; the caption should describe both panels accurately.
- [§4.2] The sentence beginning "Among the three algorithms ... EA and RL techniques are ˚a both choose" contains a typographical error that obscures the intended meaning.
- [Appendix A] The stopping criteria in Appendix A state that the hamming distance of the best agent to the target is 1, whereas the main text (§3.3) states the stopping condition is hamming distance 0; these should be made consistent.
Circularity Check
No circularity: the paper is an empirical benchmark whose A*, RL, and EA results are independent of the algorithms' own outputs, and the only self-citation is not load-bearing.
full rationale
The paper is an empirical benchmark, not a derivation, and its central claims (A* optimal on d=3 and most d=4; EA/RL solving d=5 where A* times out) are supported by Tables 2, 4, and 5, which are independent of the algorithms' own outputs. No fitted parameter is relabeled as a prediction: EA hyperparameters (c*=1.8, alpha=0.2) are tuned on one d=3 instance in Appendix A and then fixed for the other puzzles, and RL hyperparameters are fixed as described in Section 3.2. The only self-citation, [4], is used in Proposition 1 to bound A*'s branching factor; that prior theorem is a mathematical fact about the puzzle model and is not used to define the experimental results, and the A* failure claim is established empirically in Table 2 rather than by the proposition. There is an internal inconsistency in the EA fitness function as printed: Eq. (3)-(4) define h as a mismatch count, so fa is maximized when rings do not match and the stopping condition fa=1 is unreachable; this is a correctness/bug concern, not a circular reduction, because no quantity is fit to its own output. The abstract's 'median less than 30' claim is also not supported by Table 5 for d=5, k=4 (median 161), but a data contradiction is distinct from circularity. No step in the paper equates a predicted quantity to its input by construction.
Assumptions & free parameters
free parameters (6)
- EA mutation Zipf exponent c =
c* = 1.8
- EA selection force alpha =
0.2
- EA population size N and max generations T =
N = 1000, T = 1000
- RL branching states P =
1000 (most puzzles), 100000 (d=5)
- RL learning rate alpha and discount gamma =
alpha = 0.05, gamma = 0.95
- RL episodes and trials =
1000 iterations, 150 runs per puzzle
assumptions (3)
- domain assumption Background solvability, parity, and branching-factor properties of cubical puzzles from Beyer et al. [4] are correct and apply to the random starting configurations used.
- standard math The A* evaluation heuristic defined in Definition 2 is admissible and monotonic.
- domain assumption Each ring's distance can be summed independently as a lower bound on total moves.
Cite this review
Pith. "Pith review of Approximately Optimal Search on a Higher-dimensional Sliding Puzzle." pith.science (2026). https://pith.science/paper/I57DUTOD
@misc{pith2026241201937,
author = {Pith},
title = {Pith review of: Approximately Optimal Search on a Higher-dimensional Sliding Puzzle},
year = {2026},
howpublished = {\url{https://pith.science/paper/I57DUTOD}},
note = {Machine review of arXiv:2412.01937}
}
abstract
Higher-dimensional sliding puzzles are constructed on the vertices of a $d$-dimensional hypercube, where $2^d-l$ vertices are distinctly coloured. Rings with the same colours are initially set randomly on the vertices of the hypercube. The goal of the puzzle is to move each of the $2^d-l$ rings to pre-defined target vertices on the cube. In this setting, the $k$-rule constraint represents a generalisation of edge collision for the movement of colours between vertices, allowing movement only when a hypercube face of dimension $k$ containing a ring is completely free of other rings. Starting from an initial configuration, what is the minimum number of moves needed to make ring colours match the vertex colours? An algorithm that provides us with such a number is called God's algorithm. When such an algorithm exists, it does not have a polynomial time complexity, at least in the case of the 15-puzzle corresponding to $k=1$ in the cubical puzzle. This paper presents a comprehensive computational study of different scenarios of the higher-dimensional puzzle. A benchmark of three computational techniques, an exact algorithm (the A* search) and two approximately optimal search techniques (an evolutionary algorithm (EA) and reinforcement learning (RL)) is presented in this work. The experiments show that all three methods can successfully solve the puzzle of dimension three for different face dimensions and across various difficulty levels. When the dimension increases, the A* search fails, and RL and EA methods can still provide a generally acceptable solution, i.e. a distribution of a number of moves with a median value of less than $30$. Overall, the EA method consistently requires less computational time, while failing in most cases to minimise the number of moves for the puzzle dimensions $d=4$ and $d=5$.
Figures
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Forward citations
Cited by 1 Pith paper
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Reference graph
Works this paper leans on
-
[4]
Higher-dimensional cubical sliding puzzles
Moritz Beyer, Stefano Mereta, ´Erika Rold´ an, and Peter Voran. Higher-dimensional cubical sliding puzzles. arXiv Preprint arXiv:2307.14143 , 2023
work page Pith review arXiv 2023
-
[1]
Graph puzzles, homotopy, and the alternating group
Richard M Wilson. Graph puzzles, homotopy, and the alternating group. Journal of Combinatorial Theory, Series B , 16(1):86–96, 1974
work page 1974
-
[2]
Discrete configuration spaces of squares and hexagons
Hannah Alpert. Discrete configuration spaces of squares and hexagons. Journal of Applied and Compu- tational Topology, 4(2):263–280, 2020
work page 2020
-
[3]
Parity Property of Hexagonal Sliding Puzzles
Ray Karpman and ´Erika Rold´ an. Parity property of hexagonal sliding puzzles. arXiv preprint arXiv:2201.00919, 2022
work page Pith review arXiv 2022
-
[5]
Finding the shortest move-sequence in the graph-generalized 15-puzzle is NP-hard
Oded Goldreich. Finding the shortest move-sequence in the graph-generalized 15-puzzle is NP-hard. In Studies in complexity and cryptography. Miscellanea on the interplay between randomness and computa- tion, pages 1–5. Springer, 2011
work page 2011
-
[6]
A formal basis for the heuristic determination of minimum cost paths
Peter E Hart, Nils J Nilsson, and Bertram Raphael. A formal basis for the heuristic determination of minimum cost paths. IEEE Transactions on Systems Science and Cybernetics , 4(2):100–107, 1968
work page 1968
-
[7]
Reinforcement Learning: An Introduction
Alex M Andrew. Reinforcement Learning: An Introduction. Robotica, 17(2):229–235, 1999. 17
work page 1999
-
[8]
Markov games as a framework for multi-agent reinforcement learning
Michael L Littman. Markov games as a framework for multi-agent reinforcement learning. In Machine Learning Proceedings, pages 157–163. Elsevier, 1994
work page 1994
Show all 38 references
-
[9]
Reinforcement learning and approximate dynamic programming for feedback control
Frank L Lewis and Derong Liu. Reinforcement learning and approximate dynamic programming for feedback control. John Wiley & Sons, 2013
2013
-
[10]
Approximate Dynamic Programming: Solving the curses of dimensionality , volume
Warren B Powell. Approximate Dynamic Programming: Solving the curses of dimensionality , volume
-
[11]
Reinforcement learning: An introduction
Richard S Sutton. Reinforcement learning: An introduction. A Bradford Book, 2018
2018
-
[12]
Simulation-based optimization, volume 62
Abhijit Gosavi. Simulation-based optimization, volume 62. Springer, 2015
2015
-
[13]
A comprehensive survey of multiagent rein- forcement learning
Lucian Busoniu, Robert Babuska, and Bart De Schutter. A comprehensive survey of multiagent rein- forcement learning. IEEE Transactions on Systems, Man, and Cybernetics, Part C (Applications and Reviews), 38(2):156–172, 2008
2008
-
[14]
Nature-inspired optimization algorithms, volume 2
Xin-She Yang. Nature-inspired optimization algorithms, volume 2. Elsevier, 2020
2020
-
[15]
Optimal dynamic treatment regimes
Susan A Murphy. Optimal dynamic treatment regimes. Journal of the Royal Statistical Society Series B: Statistical Methodology, 65(2):331–355, 2003
2003
-
[16]
A Markovian Decision Process
Richard Bellman. A Markovian Decision Process. Journal of Mathematics and Mechanics , pages 679– 684, 1957
1957
-
[17]
Reinforcement learning for combinatorial optimization: A survey
Nina Mazyavkina, Sergey Sviridov, Sergei Ivanov, and Evgeny Burnaev. Reinforcement learning for combinatorial optimization: A survey. Computers & Operations Research, 134:105400, 2021
2021
-
[18]
Neural combinatorial optimization with reinforcement learning
Irwan Bello, Hieu Pham, Quoc V Le, Mohammad Norouzi, and Samy Bengio. Neural combinatorial optimization with reinforcement learning. arXiv Preprint arXiv:1611.09940 , 2016
2016 arXiv
-
[19]
Reinforcement learning for solving the vehicle routing problem
Mohammadreza Nazari, Afshin Oroojlooy, Lawrence Snyder, and Martin Tak´ ac. Reinforcement learning for solving the vehicle routing problem. Advances in Neural Information Processing Systems , 31, 2018
2018
-
[20]
Exploratory combinatorial opti- mization with reinforcement learning
Thomas Barrett, William Clements, Jakob Foerster, and Alex Lvovsky. Exploratory combinatorial opti- mization with reinforcement learning. In Proceedings of the AAAI Conference on Artificial Intelligence , volume 34, pages 3243–3250, 2020
2020
-
[21]
Combinatorial optimization with graph convolutional networks and guided tree search
Zhuwen Li, Qifeng Chen, and Vladlen Koltun. Combinatorial optimization with graph convolutional networks and guided tree search. Advances in Neural Information Processing Systems , 31, 2018
2018
-
[22]
Learning combinatorial optimization algorithms over graphs
Elias Khalil, Hanjun Dai, Yuyu Zhang, Bistra Dilkina, and Le Song. Learning combinatorial optimization algorithms over graphs. Advances in Neural Information Processing Systems , 30, 2017
2017
-
[23]
Learning local search heuristics for boolean satisfiability
Emre Yolcu and Barnab´ as P´ oczos. Learning local search heuristics for boolean satisfiability. Advances in Neural Information Processing Systems , 32, 2019
2019
-
[24]
Deep-pack: A vision-based 2d online bin packing algorithm with deep reinforcement learning
Olyvia Kundu, Samrat Dutta, and Swagat Kumar. Deep-pack: A vision-based 2d online bin packing algorithm with deep reinforcement learning. In 2019 28th IEEE International Conference on Robot and Human Interactive Communication (RO-MAN) , pages 1–7. IEEE, 2019
2019
-
[25]
Attend2pack: Bin packing through deep reinforcement learning with attention
Jingwei Zhang, Bin Zi, and Xiaoyu Ge. Attend2pack: Bin packing through deep reinforcement learning with attention. arXiv preprint arXiv:2107.04333 , 2021
2021 arXiv
-
[26]
John H. Holland. Adaptation in Natural and Artificial Systems: an introductory analysis with applica- tions to biology, control, and artificial intelligence . The MIT Press London, 1992
1992
-
[27]
aRNAque: an evolutionary algorithm for inverse pseudoknotted RNA folding inspired by L´ evy flights.BMC Bioinformatics , 23(1):335, 2022
Nono SC Merleau and Matteo Smerlak. aRNAque: an evolutionary algorithm for inverse pseudoknotted RNA folding inspired by L´ evy flights.BMC Bioinformatics , 23(1):335, 2022
2022
-
[28]
A simple evolutionary algorithm guided by local mutations for an efficient rna design
Nono SC Merleau and Matteo Smerlak. A simple evolutionary algorithm guided by local mutations for an efficient rna design. In Proceedings of the Genetic and Evolutionary Computation conference , pages 1027–1034, 2021. 18
2021
-
[29]
Evolutionary solution for the RNA design problem
Ali Esmaili-Taheri, Mohammad Ganjtabesh, and Morteza Mohammad-Noori. Evolutionary solution for the RNA design problem. Bioinformatics, 30(9):1250–1258, 2014
2014
-
[30]
ERD: a fast and reliable tool for RNA design including constraints
Ali Esmaili-Taheri and Mohammad Ganjtabesh. ERD: a fast and reliable tool for RNA design including constraints. BMC Bioinformatics , 16(1):20, 2015
2015
-
[31]
RnaPredict—an evolutionary algorithm for RNA secondary structure prediction
Kay Wiese, Alain Deschenes, and Andrew Hendriks. RnaPredict—an evolutionary algorithm for RNA secondary structure prediction. IEEE/ACM Transactions on Computational Biology and Bioinformatics, 5(1):25–41, 2008
2008
-
[32]
An introduction to genetic algorithms
Melanie Mitchell. An introduction to genetic algorithms . MIT Press, 1998
1998
-
[33]
An evolutionary algorithm for the vehicle routing problem with route balancing
Nicolas Jozefowiez, Fr´ ed´ eric Semet, and El-Ghazali Talbi. An evolutionary algorithm for the vehicle routing problem with route balancing. European Journal of Operational Research, 195(3):761–769, 2009
2009
-
[34]
A hybrid genetic algorithm for the vehicle routing problem with simultaneous pickup and delivery
Fanggeng Zhao, Dong Mei, Jiangsheng Sun, and Weimin Liu. A hybrid genetic algorithm for the vehicle routing problem with simultaneous pickup and delivery. In 2009 Chinese Control and Decision Conference, pages 3928–3933. IEEE, 2009
2009
-
[35]
Graph puzzles, homotopy, and the alternating group
Richard M Wilson. Graph puzzles, homotopy, and the alternating group. Journal of Combinatorial Theory, Series B , 16:88–96, 1974
1974
-
[36]
Artificial Intelligence: A Modern Approach (2nd ed.)
Stuart Russell and Peter Norvig. Artificial Intelligence: A Modern Approach (2nd ed.) . Prentice Hall, 2003
2003
-
[37]
Reinforcement learning-assisted evolutionary algorithm: A survey and research opportunities
Yanjie Song, Yutong Wu, Yangyang Guo, Ran Yan, Ponnuthurai Nagaratnam Suganthan, Yue Zhang, Witold Pedrycz, Swagatam Das, Rammohan Mallipeddi, Oladayo Solomon Ajani, et al. Reinforcement learning-assisted evolutionary algorithm: A survey and research opportunities. Swarm and E...
2024
-
[703]
John Wiley & Sons, 2007
2007
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