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Could you please explain me what to do in this task from Croatian OI?
By Tigutor, history, 3 years ago, In English

Here is the statement https://wcipeg.com/problem/coi08p3

I found it very interesting, because restrictions are big. I was thinking about using array d[i] — number of cells with distance i to them, but I cannot recalculate it for one rectangle quickly.

So if you have any ideas, please share them

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3 years ago, # |
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Let $$$f_i(t)$$$ be the number of cells covered in the $$$i$$$-th sheet after $$$t$$$ minutes. It's easy to show that we can split the range $$$[0, \infty)$$$ into $$$O(1)$$$ ranges such that in each of these ranges, $$$f_i$$$ is a constant, linear or quadratic polynomial.

What the problem asks is the sum $$$f(t) = \sum_i f_i(t)$$$ for various values of $$$t$$$. Because each $$$f_i$$$ is a piecewise quadratic function with $$$O(1)$$$ pieces, $$$f$$$ is also a piecewise quadratic function with $$$O(n)$$$ pieces. You can calculate the coefficients of the polynomials in each of these pieces (by sweepline, for example) and evaluate a suitable function for each $$$t$$$ in the input.

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    3 years ago, # ^ |
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    What would be the formula to calculate covered area for one rectangle? Don’t fully get it

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      3 years ago, # ^ |
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      If you have a rectangle formed from $$$(x_1, y_1)$$$ and $$$(x_2, y_2)$$$ (here, I use them as points, not unit cells like in the problem), then the area covered at time $$$t$$$ is

      $$$\max(0, \min(t, x_2) - \max(-t, x_1)) \cdot \max(0, \min(t, y_2) - \max(-t, y_1)).$$$
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