I think I now understand why search (without QS) may see +mate score of mate-in-25(50 plies from the root) when it is still in the root iteration for a low depth of, say, 15. It is the working of chess position transposition in combination with TT hashing. If we have not encountered such situations before, the first impression is that since the search horizon is ply 14, search should not have knowledge about nodes beyond its horizon.
Transposition is the situation where two different nodes of the chess tree represent identical chess positions. What it means is that a position may be reached through different paths or sequences of moves. Say there is a node A at ply 3 which is found to have a +mate best-score of infi - 5 (mate-in 3) and which is hashed in the TT. If through a different path, the same position is reached at node B with ply 12, very near the horizon; there is no need to search this node B as a probe of the TT means we can return with an exact hit; the best score here retrieved from the TT would be: infi - 5 - 12 = infi - 17. And infi - 17 (17 plies from the root) refers to a mate node beyond the horizon of ply 14 - it seems a little illogical. It may be explained as a transplant of the sub-branch of the node A onto node B; and this sub-branch now will extend the chess tree beyond its original horizon of ply 14. So this is how transposition, together with TT hash tables, allows search to know of positions that seem to be beyond its tree horizon.
There is something which I still cannot understand with Natale's output. From what I now understand, at depth 20, the +mate score that search may have is limited to infi - 19 - 19, or infi - 38; ie MATE19. So I don't understand why the output has scores like MATE76.-------------------------------------------------------------------------------
00:15:06.642 20 Bd1-e2 8.9M +299.93 > 1
00:15:28.677 20 Bd1-e2 9.2M +MATE76 > 2
00:15:30.529 20 Bd1-e2 9.3M +MATE75 > 3
00:15:30.565 20 Bd1-e2 9.1M +MATE73 > 4
00:15:30.569 20 Bd1-e2 7.5M +MATE69 > 5
00:15:30.688 20 Bd1-e2 8.9M +MATE61 > 6
00:15:30.791 20 Bd1-e2 9.2M +MATE45 > 7
00:17:51.152 20 Bd1-e2 9.3M +6.00 1. Bd1-e2 Kh1-g2
Best Regards,
Rasjid.