CF1520D Same Differences

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 $ .

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 $ .

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