chromate00's blog

By chromate00, 7 weeks ago, In English

Two days ago, the Div.3 (Codeforces Round 938 (Div. 3)) suffered from severe issues of hacks, because the problem G (1955G - GCD on a grid) did not have proper validation for the sum of $$$nm$$$ in hacks, which was specified as at most $$$2 \cdot 10^5$$$ in the statements. Sure, I won't ask about why that happened, that is not constructive discussion. Instead, I will discuss about something very interesting about the task.

During that incident, I was wondering. There has to be a way to solve this without the constraint on $$$n \cdot m$$$, right? Of course, $$$7\times 10^8$$$ bytes of input is impossible anyways, but if we ignore that, $$$10^8$$$ is not a very "dreaded" number under these ages of optimizations. There has to be a way to do this.

Then the idea came to me.


Before we cover the solution, we cover a few basic facts on number theory — it might not be necessary to know this to understand the solution, but it will be helpful. Basically, every integer is a point on the grid of infinite dimensions. Each dimension represents a prime factor, so if we restrict the domain to divisors of some integer, it becomes $$$O(\log x)$$$ dimensions because there are only $$$O(\log x)$$$ prime factors of an integer. $$$\gcd$$$ and $$$\text{lcm}$$$ becomes much more tractable to deal with on this grid, because they become simply $$$\min$$$ and $$$\max$$$ on each dimension. Same with divisibility, if each exponent on $$$a$$$ is no less than the corresponding exponent on $$$b$$$, then $$$a$$$ is divisible by $$$b$$$.

Now to the solution. The grid of divisors of $$$a_{1,1}$$$ has $$$O(\log a)$$$ dimensions and $$$d(a)$$$ points, so if we use the same idea on how one flattens a multidimensional array to one dimension, we can map each divisor (point) to their corresponding indices in one array. So, let us consider using a bitset of divisors, so each cell in the DP table can comfortably store the status of each divisor comfortably.

Let us make a bitmask for each divisor $$$mask_d$$$, defined as the union of all divisors of $$$d$$$. Let the multiplier on prime $$$p$$$ while flattening the multidimensional grid be $$$mult_p$$$ (From the facts above, one can see this is essentially the product of $$$\text{exponent}+1$$$ for all smaller primes). Then, $$$mask_1=\texttt{0000...0001}$$$, and $$$mask_d=mask_{(d/p)}|(mask_{(d/p)} \ll mult_p)$$$ if $$$d$$$ is divisible by some prime $$$p$$$. From preprocessing a sieve we have information on all such values of $$$p$$$, so this can be computed nicely as well.

Now we assume WLOG all values in $$$a$$$ are divisors of $$$a_{1,1}$$$ (if it isn't then we can take GCD to make it so). Let $$$b_{i,j}$$$ be the $$$mask$$$ corresponding to the value of $$$a_{i,j}$$$. Then the DP transition becomes as follows —

$$$dp_{i,j}=(dp_{i-1,j}\mathbin{|}dp_{i,j-1})\mathbin{\&}b_{i,j}$$$

And of course, the base condition is $$$dp_{1,1}=b_{1,1}$$$.

After finding $$$dp_{n,m}$$$, we can see that if $$$mask_d$$$ for some $$$d$$$ is completely contained in $$$dp_{n,m}$$$, then there exists a path whose GCD is divisible by $$$d$$$. So we try that for each $$$d$$$, and take the maximum $$$d$$$ where the divisibility condition holds.

The time complexity analysis is simple. Because it takes $$$\mathcal{O}(\sqrt{a})$$$ time to enumerate divisors of $$$a_{1,1}$$$, and processing $$$mask$$$-s takes $$$\mathcal{O}(\frac{d(a)^2}{w})$$$ time, we must use $$$\mathcal{O}(\sqrt{a}+\frac{d(a)^2}{w})$$$ time per test case. Then, there are $$$\mathcal{O}(nm)$$$ transitions in the DP, each taking $$$\mathcal{O}(\frac{d(a)}{w})$$$ time. So the DP takes $$$\mathcal{O}(nm\frac{d(a)}{w})$$$ time. Also as we did $$$\gcd$$$ for each cell, the $$$\gcd$$$ must take $$$\mathcal{O}(nm\log(a))$$$ Finally, trying the divisibility for each $$$d$$$ takes $$$\mathcal{O}(\frac{d(a)^2}{w})$$$ again, but that is already counted in the time complexity per test case so we are fine. The final time complexity required is $$$\mathcal{O}(t(\sqrt{a}+\frac{d(a)^2}{w})+\sum{nm}({\log(a)+\frac{d(a)}{w}}))$$$. Because $$$\frac{d(a)}{w}$$$ is such a small constant (precisely $$$4$$$), it should scale well for much larger values of $$$nm$$$, and even possibly run even when there were no constraints on the sum of $$$nm$$$, that is $$$\sum{nm}=10^8$$$ in the worst situation.


255932465 is the accepted submission, and the benchmark is as follows. For all cases $$$a_{i,j}=720\,720$$$ was used for all cells because that is the worst case for $$$d(a)$$$ (though $$$\gcd$$$ might be worse for other values). Only informations of $$$n$$$ and $$$m$$$ were input for each test case, to minimize the effect from IO bound. All benchmark results are from custom invocations. The result was as follows.

Case Runtime
$$$t=100,n=100,m=100$$$ $$$46\text{ms}$$$
$$$t=1000,n=100,m=100$$$ $$$217\text{ms}$$$
$$$t=10000,n=100,m=100$$$ $$$1358\text{ms}$$$
$$$t=100,n=300,m=300$$$ $$$171\text{ms}$$$
$$$t=1,n=1000,m=1000$$$ $$$92\text{ms}$$$
$$$t=1,n=2000,m=2000$$$ $$$187\text{ms}$$$
$$$t=1,n=3000,m=3000$$$ $$$359\text{ms}$$$ $$$^\dagger$$$
$$$t=1,n=4000,m=4000$$$ MLE $$$^\dagger$$$

$$$^\dagger$$$ The stack overflowed, so I had to move the array dp to static (global range).

May you have any question, please ask in comments! Thank you for reading this far.

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7 weeks ago, # |
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why dont use bfs with target set as every divisors of a(0, 0) and a(n-1, m-1) by checking if there's a path? That's much easier tho a bit slow, but if optimize this you can avoid TLE. I just realised this right after the contest, when my computer went back after 2 hours crashing

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    7 weeks ago, # ^ |
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    The BFS takes $$$\mathcal{O}(nmd(a))$$$, but there isn't a very nice way to optimize it further with bitsets, at least there are none that I know of. If you mean using $$$\gcd(a_{1,1},a_{n,m})$$$ instead of $$$a_{1,1}$$$, I tried it (255928562), the difference was marginal.

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7 weeks ago, # |
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I just replaced vector<vector<bool>> vis(n, vector<bool>(m)) inside of BFS function to bitset<10005> vis and got AC, thank you for inspiration. if you wondering 255935755 TL using 2d-vector, 255935445 AC using bitset.

to be honest, it's so stupid. I mean isn't this problem setters responsibility which check this kind of silly "optimization" on pretest? idk who would think of this issue after pass pretest. the only reason 2/3 of ac disappeared after systest is not due to complexity of problem itself but weak pretest.

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7 weeks ago, # |
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nice

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7 weeks ago, # |
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Segment trees, sparse table, square root should all just submit to bitset supremacy at this point. It is the master data-structure to rule over all the subservient data-structures.

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    7 weeks ago, # ^ |
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    Agreed. As the representative of the bitset community, I suggest that we ban all non-bitset data structures. That way, we will be able to focus on solving the problems with a bitset rather than developing a huge data structure which will ofc fail to work correctly.

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7 weeks ago, # |
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You can get rid of MLE by processing row by row (example)

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7 weeks ago, # |
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7 weeks ago, # |
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Nice find, although I’d say that referring to it as ‘bitset where you least expect’ is a bit of an overstatement. Basically, the intended solution is to compute for each $$$d$$$ a 0/1 dp indicating whether or not you reached cell $$$(i, j)$$$ going through only multiples of $$$d$$$. You just parallelized the computations over all $$$d$$$ (as the required dp tables are binary matrices).