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Poker hand evaluation. Fast 3, 5 and 7-card hand evaluations.
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Poker hand evaluator

This go package provides a fast poker hand evaluator for 3-card, 5-card and 7-card hands.

When benchmarking on my machine, on a single core I get around 79 million 5-card evaluations per second, or roughly 47 CPU cycles per eval. And 50 million 7-card evaluations per second, or roughly 73 cycles per eval.

It works using the same principles as the 2+2 hand evaluator. It uses a huge state machine, with 52 transitions out for each state representing 52 possible next cards. The final nodes contain 52 ranks for each of the 52 last cards. By merging nodes for equivalent hands, the number of states is much smaller than (52*51*50*49*48*47*46) as it would be for a naive 7-card state machine. The number of states for the 5-card eval is only 3459, and 163060 for the 7-card eval.

The novelty (or at least, I think it's novel) is that each transition includes a remapping of suits to be applied to future cards, which greatly reduces the number of states. This remapping is done via relatively small lookup tables. (It could also be used to limit the number of transitions for later states, where there are only effectively 2 suits: the suit where a flush is possible, and the other suits. Clever use of this could further reduce the size of the state table. This isn't done yet).

TODO: further state merging as described above, and rewrite the eval code in assembler, to avoid bounds checking. I guess the suit transforms can be written faster.

Build modes

There are three modes of using this package, which can be chosen with built tags. These affect how the data tables are constructed.

First is the default (staticdata) in which case a large (7.7MB) source file is compiled into the package, which contains the data tables. This makes the binary roughly 7.7MB bigger, but also compiles a little slower relative to the other options. There is a little bit of startup time (0.2s), because the tables are stored compressed and are uncompressed at runtime.

Second is -tags gendata in which case a few seconds will be spent at binary startup time generating lookup tables.

Third is -tags filedata in which case the "poker.dat" file must be in the current directory, and it's loaded at startup time.

Timings on my workstation to build and run "cmd/holdemeval", running with arguments ./holdemeval -hands "AdAh QsQd 6c5c":

  • default (staticdata): 0.60sec to build, 0.34sec to run
  • -tags gendata : 0.17sec to build, 2.2sec to run
  • -tags filedata: 0.17sec to build, 0.32sec to run

The run times differ because -tags filedata and staticdata have to decompress the data before it can be used, and -tags gendata has to compute the tables from scratch.

My machine has 12 CPU cores, so with fewer cores the startup cost of generating the tables will be scaled up as you'd expect.

My recommendation is to use the default and for a release binary that is expected to run quickly and for development, and -tags gendata if you don't mind the slow startup time (for example, if you have a long-running server).


Here's the package documentation on godoc

The code is MIT licensed (see LICENSE.txt).

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