| April Fools Day Contest 2025 |
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| Finished |
You are given a prime number $$$p$$$, $$$n$$$ integers $$$a_1, a_2, \ldots, a_n$$$, and an integer $$$k$$$.
Find the number of pairs of indexes $$$(i, j)$$$ ($$$1 \le i \lt j \le n$$$) for which $$$(a_i \oplus a_j)(a_i^2 \oplus a_j^2) \equiv k \bmod p$$$.
Here $$$\oplus$$$ denotes the bitwise XOR operation.
The first line contains integers $$$n, p, k$$$ ($$$2 \le n \le 3 \cdot 10^5$$$, $$$2 \le p \le 10^9$$$, $$$0 \le k \le p-1$$$). $$$p$$$ is guaranteed to be prime.
The second line contains $$$n$$$ integers $$$a_1, a_2, \ldots, a_n$$$ ($$$0 \le a_i \le p-1$$$). It is guaranteed that all elements are different.
Output a single integer — answer to the problem.
3 3 20 1 2
1
6 11 21 3 5 6 7 8
3
In the first example:
$$$(0\oplus1)(0^2 \oplus 1^2) = 1 \equiv 1 \bmod 3$$$.
$$$(0\oplus2)(0^2 \oplus 2^2) = 8 \equiv 2 \bmod 3$$$.
$$$(1\oplus2)(1^2 \oplus 2^2) = 15 \equiv 0 \bmod 3$$$.
So only $$$1$$$ pair satisfies the condition.
In the second example, there are $$$3$$$ such pairs: $$$(1, 5)$$$, $$$(1, 6)$$$, $$$(3, 6)$$$.
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