import java.util.HashMap;
import java.util.Map;
 
class EqualPairCounter {
    int numberOfEqualPairsLinearWithExtraMap(int[] input) {
        int res = 0;
        Map<Integer, Integer> vMap = new HashMap<>();
 
        for (final int v : input) {
            int count = vMap.getOrDefault(v, 0);
            vMap.put(v, ++count);
        }
 
        for(final int v : vMap.keySet()) {
            if (vMap.get(v) > 1) {
                int pairs = vMap.get(v);
                res += (pairs - 1) * pairs / 2;
            }
        }
        return res;
    }
 
    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().numberOfEqualPairsLinearWithExtraMap(input);
        System.out.println("Res: " + pairCount); // => 52
    }
}