{"type":"ScholarlyArticle","title":"A*+IDA*: A Simple Hybrid Search Algorithm","authors":[{"name":"Zhaoxing Bu","slug":"zhaoxing-bu","given":"Zhaoxing","family":"Bu","affiliation":"Computer Science Department University of California , Los Angeles Los Angeles , CA 90095"},{"name":"Richard E. Korf","slug":"richard-e-korf","given":"Richard E.","family":"Korf","affiliation":"Computer Science Department University of California , Los Angeles Los Angeles , CA 90095"}],"abstract":"We present a simple combination of A* and IDA*, which we call A*+IDA*. It runs A* until memory is almost exhausted, then runs IDA* below each frontier node without duplicate checking. It is widely believed that this algorithm is called MREC, but MREC is just IDA* with a transposition table. A*+IDA* is the first algorithm to run significantly faster than IDA* on the 24-Puzzle, by a factor of almost 5. A complex algorithm called dual search was reported to significantly outperform IDA* on the 24-Puzzle, but the original version does not. We made improvements to dual search and our version combined with A*+IDA* outperforms IDA* by a factor of 6.7 on the 24-Puzzle. Our disk-based A*+IDA* shows further improvement on several hard 24-Puzzle instances. We also found optimal solutions to a subset of random 27 and 29-Puzzle problems. A*+IDA* does not outperform IDA* on Rubik’s Cube, for reasons we explain.","doi":"10.24963/ijcai.2019/168","url":"https://ijcai.org/proceedings/2019/168","pdf":"https://ijcai.org/proceedings/2019/0168.pdf","markdown":"https://ijcai.org/proceedings/2019/168.md","bibtex":"https://ijcai.org/proceedings/2019/bibtex/168","volume":{"slug":"2019","title":"Proceedings of the Twenty-Eighth International Joint Conference on Artificial Intelligence","short_title":"IJCAI 2019","url":"https://ijcai.org/proceedings/2019/","year":2019,"location":"Macao","dates":"10-16 August 2019","editors":"Sarit Kraus","isbn":"978-0-9992411-4-1","publisher":"International Joint Conferences on Artificial Intelligence Organization","doi_prefix":"10.24963/ijcai.2019"},"section":"Heuristic Search and Game Playing","track":"Main track","number":168,"pages":{"first":1206,"last":1212},"keywords":["Heuristic Search and Game Playing: Heuristic Search","Planning and Scheduling: Search in Planning and Scheduling","Heuristic Search and Game Playing: Combinatorial Search and Optimisation"],"videos":[],"references":[{"position":1,"raw":"[Akagi et al., 2010] Yuima Akagi, Akihiro Kishimoto, and Alex Fukunaga. 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