Algorithm to calculate number of intersecting discs

Question

Given an array A of N integers we draw N discs in a 2D plane, such that i-th disc has center in (0,i) and a radius

Answer 1

Here is the PHP code that scored 100 on codility:

$sum=0;

//One way of cloning the A:
$start = array();
$end = array();

foreach ($A as $key=>$value)
{
$start[]=0;
$end[]=0;   
}  

for ($i=0; $i= count($A))   
  $end[count($A)-1]++;
  else
  $end[$i+$A[$i]]++;
}
$active=0;
for ($i=0; $i10000000) return -1;
  $active += $start[$i]-$end[$i];
}
return $sum;

However I dont understand the logic. This is just transformed C++ code from above. Folks, can you elaborate on what you were doing here, please?