1520D · Same Differences

Problem: You are given an array $a$ of $n$ integers. Count the number of pairs of indices $(i, j)$ such that $i < j$ and $a_j - a_i = j - i$.

Input Format: The first line contains one integer $t$ ($1 \le t \le 10^4$). Then $t$ test cases follow.

The first line of each test case contains one integer $n$ ($1 \le n \le 2 \cdot 10^5$).

The second line of each test case contains $n$ integers $a_1, a_2, \ldots, a_n$ ($1 \le a_i \le n$) — array $a$.

It is guaranteed that the sum of $n$ over all test cases does not exceed $2 \cdot 10^5$.

Output Format: For each test case output the number of pairs of indices $(i, j)$ such that $i < j$ and $a_j - a_i = j - i$.