Home * Chess * Position * Copy-Make

While traversing a Search Tree, the Copy-Make approach keeps and updates local copies of certain aspects of a chess position inside an array indexed by ply, which could also be interpreted as explicit, random accessible search stack. It usually refers the irreversible aspects of the position, like ep state, castling rights and the halfmove clock, which can not incrementally updated during unmake move. Some programs even keep reversible stuff inside an array, to avoid incremental update during unmake. Copy-Make is required, if aspects need to be accessed randomly in the current branch from the root (or even starting game position) to the current one.

Copy-Make

// make
memcpy (&position[ply+1].irrvrsAspects, 
        &position[ply  ].irrvrsAspects, 
        sizeof(irrvrsAspects));
ply++;
update (position[ply], move)
...
// unmake
ply--;
// position[ply] is still valid

Stack

The alternative, to maintain those irreversible aspects inside a global structure, would require a stack (LIFO), with push and global update during make, and pop from stack to global structure during unmake, and therefor higher memory bandwidth for copying back and forth.

// make
push (position.irreversibleAspects);
ply++;
update (position, move)
...
// unmake
ply--;
pop (position.irreversibleAspects);
// position is restored from stack