TL;DR

On with Advent of Code puzzle 19 from 2016: using dynamic programming to attack the Josephus problem variant described in Aoc2016/19 - Halving Josephus.

I was very happy to get past puzzle 19 from the 2016 edition of Advent of Code, but letâs admit two facts:

• I didnât demonstrate that the heuristic is actually a rule;
• This wouldnât help in some other general case.

I meanâŚ what if I didnât spot the heuristic at all? Would I have been stuck with the brute force approach?

WellâŚ we have other cards to play.

# A recurrent situation

We are actually in aâŚ recurrent situation where knowing the solution to the problem for $N$ items might help us find out a solution for $N + 1$ items somehow easily (i.e. without having to actually place $N + 1$ elves and simulate the full game).

So letâs say we know the solution for $N$. Itâs $i$.

Now, letâs consider what we would do to solve the $N + 1$ case. Letâs first place all of them:

1 2 3 ... (k-3) (k-2) (k-1) k (k+1) (k+2) ... (N-3) (N-2) (N-1) N (N+1)

Here we are highlighting position $k$, which is the position that we will have to eliminate in this condition. We already know how to find $k$, because of the rules about the odd and even values of $N$.

Now, our first step is to eliminate element $k$, so weâre left with:

1 2 3 ... (k-3) (k-2) (k-1) (k+1) (k+2) ... (N-3) (N-2) (N-1) N (N+1)

At this point, we would have to move to the following item, that is the same as taking the initial item and move it to the end, like this:

2 3 ... (k-3) (k-2) (k-1) (k+1) (k+2) ... (N-3) (N-2) (N-1) N (N+1) 1

Now, theoretically, our brute force approach would ask us to just repeat the same steps with this new array of elements.

Array of $N$ elements.

Wait. A. Minute.

We already know what the solution is to this case: itâs $i$. Well no, itâs the element that is at position $i$. Letâs write the positions and the values then:

i> 1 2 3 ... (k-3) (k-2) (k-1) k     (k+1) ... (N-3) (N-2) (N-1) N
v> 2 3 4 ... (k-2) (k-1) (k+1) (k+2) (k+3) ... (N-2) (N-1) N     1

We can observe that:

• if $i$ was less than $k$, then it is only transformed into $i + 1$ after we âmove onâ to the next item, i.e. take the first item and put it in the final position;

• if $i$ was greater than, or equal to, $k$, then it gets shifted two positions ahead, one due to the elimination of $k$ itself, the other one for âmoving onâ like in the previous bullet;

• the only special case is when $i = N$, because in this case we reset back to $1$.

Hence, if we call $E(n)$ the winning elf in a game of $n$ elves, we have the following recursive relation:

$E(n) = \begin{cases} E(n-1) + 1, & \text{if E(n-1) < \lfloor\frac{n}{2}\rfloor }\\ E(n-1) + 2, & \text{if \lfloor\frac{n}{2}\rfloor \le E(n-1) < n - 1 } \\ 1, & \text{if E(n-1) = n - 1 } \end{cases}$

# A dynamic algorithm

To solve for $N$, then, we need the solution for $N - 1$, hence we need the solution for $N - 2$ and so on up toâŚ $N = 2$, where we already know that the solution is $1$.

Hence, an algorithm to solve for $N$ might be:

• start from $n = 2$ and $E(2) = 1$
• make a loop where $n$ is increased by one at each iteration and the corresponding value $E(n)$ is calculated from the value of the previous iteration;
• When $n$ lands on $N$âŚ weâre done!

This can be coded as follows:

sub josephus_part2_dynamic ($N) { my$i = 1;
my $n = 2; my$k = 1;
while ($n <$N) {
++$n;$k = int($n / 2);$i = $i <$k     ? $i + 1 :$i < $n - 1 ?$i + 2
:                 1;
}
return $i; } This approach as a complexity that is$O(n)$, as opposed to$O(1)\$ of the heuristicâŚ but itâs anyway a more general one, because it only requires us to figure out the mapping of the different indexes from an iteration to the following.

# Conclusion

This was an interesting ride! In particular, it gave me the opportunity to go a bit more in depth with the dynamic programming paradigmâŚ and learn something more on the way đ

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