For interview

NumberOfDiscIntersections.cs

namespace Codity;

public class NumberOfDiscIntersections
{
    const int MaxIntersectingPairs = 10000000;
    
    public int solution(int[] A) 
    {
        var result = 0;
        int[] left = new int[A.Length],
            right = new int[A.Length];

        for (int i = 0, j = A.Length - 1; i < A.Length; i++)
        {
            left[i > A[i]? i - A[i]: 0]++;
            right[j - i > A[i]? i + A[i]: j]++;
        }//TODO Parallel.For

        var temp = 0;
        for (var i = 0; i < A.Length; i++)
        {
            if (left[i] > 0)
            {    
                result += temp * left[i];
                result += left[i] * (left[i] - 1) / 2;
                if (result > MaxIntersectingPairs) 
                    return -1;
                temp += left[i];
            }
            temp -= right[i];
        }//TODO Paralell.For

        return result;
    }
}