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Implementing a new search algorithm

Starting with version v0.32, lc0 defines a search API that allows for multiple search algorithms to be implemented.

The API is deliberately high-level, so every search algorithm has to address the following aspects by itself:

• Search tree/nodes storage. The algorithm decides for themselves how and whether to store the search tree (should be be a tree, graph, stored in memory, on disk, etc.).
• Time management.

The plan is to have a set of reusable components that can be used by search for these aspects, but currently they are not implemented yet.

On the other hand, the following aspects are provided by the API:

• UCI interface. This includes support of configuration options.
• Move generation. Strictly speaking, this is not enforced by the API, but given that Move and Position are passed through the API, it’s natural to also use the GenerateLegalMoves function to generate moves.
• Backend interface, including caching.
• Syzygy tablebase API.