Codility: NumberOfDiscIntersections Solution

Implementation

1function solution(A) {
2  let i = 0
3
4  // 1 means beginning, -1 mean ending
5  const points = A.map((a, i) => [
6    [i - a, 1],
7    [i + a, -1],
8  ])
9    .reduce((acc, el) => {
10      acc[i++] = el[0]
11      acc[i++] = el[1]
12      return acc
13    }, [])
14    .sort((a, b) => a[0] - b[0] || b[1] - a[1])
15
16  let intersections = (activeCircles = 0)
17  for (const [_, place] of points) {
18    if (place === 1) {
19      intersections += activeCircles
20      activeCircles += 1
21    } else {
22      activeCircles -= 1
23    }
24    if (intersections > 1e7) {
25      return -1
26    }
27  }
28
29  return intersections
30}

References

http://www.lucainvernizzi.net/blog/2014/11/21/codility-beta-challenge-number-of-disc-intersections/