Thanks to Kapt for proofreading this blog.

Recently, I came across a couple of AtCoder problems on modular arithmetic. In short, they both involved sequences $$$\{ A, A+C, A+2C, \dots \}$$$ modulo $$$M$$$. The usual way to deal with these sequences is floor sums:

Problem (Floor Sum). Calculate $$$\displaystyle \sum_{k=0}^n \left\lfloor \frac{Ck+A}{M} \right\rfloor$$$.

This is solvable in $$$O(\log M)$$$ with a recursive Euclid-like algorithm.

If you want to learn more about floor sums, I recommend this blog by _timelord.

However, what I want to show you is how you can solve problems like that with a completely different idea. The basis of our reasoning is the following statement:

(For the rest of this blog, let $$$0 \le C, A \lt M$$$, $$$\gcd(C, M) = 1$$$, $$$n \lt M$$$, just to avoid hassle.)

Statement. The sequence $$$\{A, C + A, 2C + A, \dots, Cn + A\} \bmod M$$$ can be split into $$$O(\sqrt{M})$$$ disjoint arithmetic progressions (not necessarily contiguous).

Proof. Suppose either $$$C$$$ or $$$M - C$$$ is small. Then you can simply divide the sequence every time the progression overflows, with each segment being about $$$\frac{M}{C}$$$ elements long. Thus we split our sequence into $$$O(1 + \frac{nC}{M})$$$ arithmetic progressions.

Now let's consider the first $$$\sqrt{M} + 1$$$ members of our sequence. By the pigeonhole principle, some two of them, e.g. $$$(Ci + A) \bmod M$$$ and $$$(Cj + A) \bmod M$$$ ($$$i \lt j$$$), will differ by less than $$$\sqrt{M}$$$. Thus, for $$$d := j - i$$$, either $$$(Cd \bmod M)$$$ or $$$(M - Cd \bmod M)$$$ will be less than $$$\sqrt{M}$$$ (let's call this value $$$H$$$).

Consider the subsequence $$$\{A, Cd + A, 2Cd + A, \dots, Cd \cdot \left\lfloor \frac{n}{d} \right\rfloor + A\} \bmod M$$$. As shown above, we can split it into $$$O(1 + \frac{nH}{dM})$$$ arithmetic progressions.

Summing this result over all remainders modulo $$$d$$$, we get $$$O(d + \frac{nH}{M})$$$ arithmetic progressions. Since $$$d \le \sqrt{M}$$$ and $$$H \lt \sqrt{M}$$$, this boils down to $$$O(\sqrt{M})$$$.

The first question is surely 'what are we going to do with this thing'.

Problems

Problem 1. Calculate $$$\displaystyle \sum_{k=0}^n ((Ck + A) \bmod M)$$$.

This sum is the total sum of all arithmetic progressions. Sum of arithmetic progression $$$\{ x, H + x, 2H + x, \dots, nH + x \}$$$ is

This also allows us to solve Floor Sum in $$$O(\sqrt{M})$$$. Hooray!

Problem 2. Calculate $$$\displaystyle \sum_{k=0}^n ((Ck + A) \bmod M)^2$$$.

Nothing changes except the formula for summing squares of elements of the arithmetic progression $$$\{ x, H + x, 2H + x, \dots, nH + x \}$$$:

This is a real problem that I prepared for a local event, and I sincerely apologize to anyone who was forced to try to solve it.

Problem 3 (relevant part of Fizz Buzz Difference). Given $$$0 \lt R \lt M$$$, find minimum $$$k \gt 0$$$, s.t. $$$Ck \bmod M \lt R$$$.

Since the indices of elements of arithmetic progressions in original sequence are increasing (that is, progression $$$\{ x, H + x, 2H + x, \dots, nH + x \}$$$ corresponds to indices $$$\{i, i + d, i + 2d, \dots, i + nd\}$$$), all we have to do is find the first element in each progression that satisfies the condition.

Problem 4 (Sum of Min of Mod of Linear). Given $$$0 \le A_1 \le \dots \le A_m \lt M$$$, calculate $$$\displaystyle \sum_{k=0}^n \min_{1 \le i \le m} ((Ck + A_i) \bmod M)$$$.

If $$$(Ck \bmod M) \in [M - A_i; M - A_{i-1})$$$, then the value of minimum is $$$((Ck + A_i) \bmod M)$$$; note that this is just $$$(Ck \bmod M) + A_i$$$. For $$$i = 1$$$ it's a bit trickier: the value of minimum is

• $$$(Ck \bmod M) + A_1$$$ if $$$(Ck \bmod M) \in [0; M - A_m)$$$
• $$$(Ck \bmod M) + A_1 - M$$$ if $$$(Ck \bmod M) \in [M - A_1; M)$$$

Either way, calculating the sum of $$$(Ck \bmod M)$$$ is Problem 1, so what we need is to find the number of elements of the sequence that lie in each segment.

Suppose we consider each progression separately. If $$$H \gt 0$$$, then the progression $$$\{ x, H + x, 2H + x, \dots, nH + x \}$$$ has

elements less than $$$R$$$. Using this formula, we can find the number of elements in our progression that lie on each of the $$$m + 1$$$ subsegments. This gives us the solution in $$$O(m \sqrt{M})$$$. Unfortunately, this is not fast enough.

Note that all of our arithmetic progressions have the same step $$$H$$$, so we can solve the problem independently for each residue modulo $$$H$$$.

For example, consider elements of the form $$$Hk + r$$$. Each progression either doesn't have any of them or contains a contiguous segment of them. Since subsegments of $$$[0; M)$$$ also correspond to some segments, all we need is to add $$$1$$$ on some segments and then find sums on $$$m + 1$$$ segments. This is a simple scanline.

We need to do scanline $$$H$$$ times, but we only have $$$O(d + H)$$$ segments in total. If we make $$$d$$$ larger and $$$H$$$ smaller, we can sort all segments in $$$O(d \log d)$$$ and do scanlines in $$$O(mH)$$$.

To do this, recall the way we obtained $$$d$$$ in the first place. Suppose we instead look through the first $$$M^{2/3}$$$ members of our sequence. Then $$$d \le M^{2/3}$$$ and $$$H \le M^{1/3}$$$. Thus we can solve the problem in $$$O(M^{2/3} \log M + mM^{1/3})$$$.

Takeaway

I guess there isn't much moral to this story. If you're a problemsetter, this blog should probably serve as a warning and a prompt to try to counter heuristics like this. If you're a contestant, this blog should, more than anything, be a reason to get acquainted with floor sums, but also to try to find some clever ways around the intended solution.

If you know any problems that can be solved with this technique, or if you want to tell me off for rambling about a well-known idea, feel free to comment below.

Anyway, stay safe until I'm back with yet another way of torturing myself!