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Commit ID 6e2ca97d93812b2f630f8b4ec1042105557abaec
Author Marco Costalba
Date 2016-06-07 13:06:26 UTC
Fix syzygy DTZ bug In this position: 3K4/8/3k4/8/4p3/4B3/5P2/8 w - - 0 5 Current DTZ probe returns 1 instead of 15 What happens is that the double push f4 is erroneously detected as a win move. After the push we have: [D]3K4/8/3k4/8/4pP2/4B3/8/8 b - f3 0 5 And here the code misses the possible ep capture exf3. The bug is in probe_dtz_no_ep() where is used probe_ab() that is blind to ep captures so it returns v == 2 (win) for position 3K4/8/3k4/8/4pP2/4B3/8/8 b - f3 0 5 Note that at the caller site the original position did not have any possible ep capture, so probe_dtz() returns immediately after calling probe_dtz_no_ep(). The fix is to call the ep-aware probe_wdl() instead of probe_ab() I have verified that DTZ is correct now and also there are no more mistmatches compared to the new 'syzygy' branch. Tested on a set of more than 600 endgame positions, included some tricky ones. For people interested to redo the test or doing additional tests please pull branch tb_dbg from repo. bench: 8450534 (bench unaffected because syzygy is not exercized during bench)
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