import java.util.Arrays;
import java.util.OptionalInt;
 
class EqualPairCounter {
    int numberOfEqualPairsLinearWithExtraArrayAndStreams(int[] input) {
        int max = input[0];
        int min = input[0];
        for (int v : input) {
            max = Math.max(max, v);
            min = Math.min(min, v);
        }
        int shift = min;
        int[] countArr = new int[max - min + 1];
        for (int v : input) {
            int countIdx = v - shift;
            countArr[countIdx]++;
        }
 
        OptionalInt res = Arrays.stream(countArr)
            .parallel()
            .filter(count -> count > 1)
            .map(n -> (n - 1) * n / 2)
            .reduce(Integer::sum);
 
        return res.orElse(0);
    }
 
    public static void main(String[] args) {
        int[] input = {1, 2, 4, 1, 2, 1, 2, -3, 4, 5, 1, 2, 4, 5, 1, -5, 2 ,5, 6, 7, -3, 7, 8, 2, 1, 2, -3, 4, 5};
        int pairCount = new EqualPairCounter().numberOfEqualPairsLinearWithExtraArrayAndStreams(input);
        System.out.println("Res: " + pairCount); // => 52
    }
}