Advent of Code - adventofcode.com

still not entirely sure what the :: does in grid[::d], apart from take every d'th y

The slice syntax is

grid[start:stop:stride]

and you can omit any, so start defaults to zero, stop to -1 (which means the end, since negative indices count backwards), and stride defaults to 1.

• grid[start] just gives you the element at index start (ie, the half-open interval [start,start+1) in maths language),
  
• [start:stop] gives you the sub-sequence/sub-string with default stride 1,
  
• [start::stride] would give you every stride'th element from start to the end, etc.