# It seems like a nim variant

Now this is a more of a computer science problem than a math problem but it is concerning game theory and it does seem a lot like nim (it's from an online judge), so i'm kinda stuck on this one i'd really appreciate any help you could give me.

You have $n$ piles of stones, the $i$-th pile has $a_i$ stones on it; you also have a list of available moves, where moves are described by "remove $x_1$ stones from pile $1$, $x_2$ stones from pile $2$, $x_3$ stones from pile $3$, etc., etc." ($x_i$ can be $0$). This is a two-player game where each player in his/her turn can choose one move and play it, and the player that can't make any move loses. For a given configuration you should determine if the winner is the first or the second player.

• Assuming both players play optimally of course. Feb 4 '13 at 19:05

You're correct that this is a nim variant. More generally, all impartial games (those where both players have the same moves available to them) of this sort fall under the broad category of the Sprage-Grundy theorem which says that their values are equivalent to the value of some nim-pile.

But for determining who's to win from a given 'single-position' configuration, you don't need any of the broader information; instead, you just need the concepts of 'next player wins' ($\mathcal{N}$-positions) and 'previous player has won' ($\mathcal{P}$-positions), along with the rules for determining the value of a position from all of its options: if any of the moves available from a given position is to a $\mathcal{P}$-position, then that position is an $\mathcal{N}$-position (since the next player can win by moving to the $\mathcal{P}$-position). Otherwise (i.e., if all of the positions that can be reached from the given position are $\mathcal{N}$-positions), the position is a $\mathcal{P}$-position; there are no good moves available.

This evaluation can then be used in one of two ways; if you're interested in the value of a single position then you can simply build the tree (more accurately, DAG — Directed Acyclic Graph) of moves from that position (being careful to cache 'transpositions' - e.g., since making move $m_1$ followed by $m_2$ leads to the same position $P_{12}$ as making move $m_2$ followed by $m_1$, you don't want to compute the value of $P_{12}$ twice) and evaluate it.

If you're going to be computing the value of many positions then it will generally be better to build a table of values, but in games which have many piles this is likely to be a very large table. Since all of the moves have to decrease one or more coordinates, the table can be built outwards from the origin in 'shells': first fill in the value for the origin (which by definition will be a $\mathcal{P}$-position, since there's no legal move to make). Next, fill in the values for all cells with the sum of their pile-sizes equal to 1 (i.e., $(1,0,0,\ldots),\ (0, 1, 0, \ldots),\ (0, 0, 1,\ldots),\ \ldots$); since any move decreases one or more coordinates it has to decrease the sum of the pile-sizes and so you'll already have processed all of the positions a given position's value can depend on before you process that position itself. Note that for a fixed number of dimensions (i.e., piles), it's easy to write the iteration over each shell as a series of nested loops with appropriate limits. For instance, the four-pile case would look like:

for ( totalSum = 0; totalSum <= 16; totalSum++ ) {
for ( pile0 = 0; pile0 <= totalSum; pile0++ ) {
for ( pile1 = 0; pile1 <= totalSum-pile0; pile1++ ) {
for ( pile2 = 0; pile2 <= totalSum-(pile0+pile1); pile2++ ) {
pile3 = totalSum-(pile0+pile1+pile2);
...
}
}
}
}


If the number of piles is given as part of the input you'll have a bit more work to do, but there are still relatively straightforward ways of iterating over all combinations summing to a given number.

• The size of the table will be about $\prod_{i=1}^n (1+a_i)$ unless there are modular constraints to exploit (say the only move for a pile with $73$ is to remove $10$ stones, then that's equivalent to a pile of size $7$ with a $1$-stone move). The DAG will also be about that size if the move sets are arithmetically dense. Feb 4 '13 at 19:20
• Thanks for the fast answer, anyway i forgot to mention that there is a constraint that says that the sum of sizes of all piles is less than 16. So i guess the table is the way to go.I know about the DAG method, but how do you build said table?I would upvote you but it requires 15 rep :( Feb 4 '13 at 19:26
• @DraganMarkovic I've updated my answer with a description of how to build such a table. Feb 4 '13 at 19:58