Released about three years after the previous version, in November 2016, Houdini 5 is vastly improved being about 200 Elo stronger than Houdini 4. Houdini 4, released in November 2013, was about 50 Elo stronger than its predecessor, and features 6-men Syzygy bases. Since November 2012, Houdini 3 is available as ChessBase engine. If a different move becomes best at very high search depths at the root, re-searches to determine its exact score and the new principal variation are done faster due to a reduced search depth, presumably in conjunction with extensions along the PV. According to its author, Houdini 3 applies a accelerated principal variation search, also dubbed 'Smart' Fail-High. Houdini 3, released in October 2012, has further improved by more than 50 Elo points. Houdini 1.5a remains available as a free download. Houdini 2, released in September 2011, is commercial and bundled with a number of ChessOK products, such as Aquarium, Chess Assistant, and since December 2011, Chess King. In January 2011, Houdini 1.5 was leading Ingo Bauer's IPON rating list.
A chess engine by Robert Houdart, which appeared in 2010 as closed source engine, free for non-commercial use.
