Number Of Disc Intersections

We draw N discs on a plane. The discs are numbered from 0 to N − 1. An array A of N non-negative integers, specifying the radiuses of the discs, is given. The J-th disc is drawn with its center at (J, 0) and radius A[J].

We say that the J-th disc and K-th disc intersect if J ≠ K and the J-th and K-th discs have at least one common point (assuming that the discs contain their borders).

The figure below shows discs drawn for N = 6 and A as follows:
A[0] = 1
A[1] = 5
A[2] = 2
A[3] = 1
A[4] = 4
A[5] = 0

There are eleven (unordered) pairs of discs that intersect, namely:
• discs 1 and 4 intersect, and both intersect with all the other discs;
• disc 2 also intersects with discs 0 and 3.

Write a function:
function solution(A);
that, given an array A describing N discs as explained above, returns the number of (unordered) pairs of intersecting discs. The function should return −1 if the number of intersecting pairs exceeds 10,000,000.

Given array A shown above, the function should return 11, as explained above.

Write an efficient algorithm for the following assumptions:
• N is an integer within the range [0..100,000];
• each element of array A is an integer within the range [0..2,147,483,647].

Hints:

• convert 2D discs into 1D line segments on X axis into touples with Left-x and Right-x ([x0, x1])
• sort the touples by Left-x (x0)
• add counters if next segment's Left-x <= previous segment's Right-x

function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)
    //console.log(A)
    //[1, 5, 2, 1, 4, 0]
    
    // convert 2D discs into 1D line segments on X axis
    let tupples = []
    let i
    let j
    
    for(i=0; i<A.length; i++){
        tupples.push( [ i-A[i], i+A[i] ] )
    }
    //console.log("source: ",tupples)
    //[ [ -1, 1 ], [ -4, 6 ], [ 0, 4 ], [ 2, 4 ], [ 0, 8 ], [ 5, 5 ] ]
    
    tupples.sort((a,b)=>a[0]-b[0])
    //console.log("sorted: ", tupples)
    //[ [ -4, 6 ], [ -1, 1 ], [ 0, 4 ], [ 0, 8 ], [ 2, 4 ], [ 5, 5 ] ]
    
    let count = 0
    for(i=0; i<A.length; i++){
        for(j=i+1; j<A.length; j++){
            if(tupples[j][0] <= tupples[i][1]){
                // next j segment start point is inside i segment
                count ++
                if(count>10000000) return -1
            }else{
                break
                // next j segments are not inside i segment
            }
        }
    }
    
    return count
}