Kata

You are given array of integers, your task will be to count all pairs in that array and return their count.

Notes:

• Array can be empty or contain only one value; in this case return 0
• If there are more pairs of a certain number, count each pair only once. E.g.: for [0, 0, 0, 0] the return value is 2 (= 2 pairs of 0s)
• Random tests: maximum array length is 1000, range of values in array is between 0 and 1000

Examples

[1, 2, 5, 6, 5, 2]  -->  2

...because there are 2 pairs: 2 and 5

[1, 2, 2, 20, 6, 20, 2, 6, 2]  -->  4

...because there are 4 pairs: 2, 20, 6 and 2 (again)