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3:{a1-b1,a1-a2,a1-b2}
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Kings on edges have 5 potential target squares, thus there are 32 possible move-lists. All other kings have 8 all the 8 neighbors with up to 256 move-lists. Similar move-list enumeration is possible with knights and others. All possible move-target subsets of kings and knights for all 64 from-squares are [[Hash Table#MinimalPerfectHashing|perfectly minimal hashtablehashable]] with a magic factor of four one-bits set. 10016 possible king-move-lists and 5520 knight-move-lists. To reduce memory one may offset the sets to a "normalized" source square per king, knight and sliding piece line, implying some vector arithmetic in the board centric world considering the offset.
The less populated move-target subsets are, the less efficient this hashing technique. This might become a problem since bitboard move-generation is essentially about subsets of moves with certain properties, like most importantly fast winning captures at [[Node Types#CUT|Cut-nodes]].
