That's one small step for [a] man, one giant leap for mankind. Neil Alden Armstrong (1930-2012)

In this mission the size of the hexagonal grid is 12x9 ('A1' to 'L9').

• It’s characteristics:
  • it’s flat-topped;
  • the alphabet letters represent columns and numbers represent rows;
  • the even columns are being pushed down.

You are given a set of obstacle hexes on the Moon as an input value. You have to return a set of all the candidate landing sites.

• The conditions for the landing site hex:
  • the hex isn't an obstacle hex;
  • there’re no obstacle hexes in equidistant ( 1 or more ) hexes;
  • the above radius is the longest.

• The outside of the hexagonal grid is always considered as an obstacle hex.
• If there aren’t any landing sites, return an empty set.

assert landing_site({'E5', 'E7', 'F4', 'F6', 'G4', 'G6', 'H3', 'H5'}) == {'C3', 'J7'}
assert landing_site({'A4', 'C2', 'C6', 'C9', 'D4', 'D7', 'F1', 'F5',
                     'F8', 'G4', 'H7', 'I2', 'I5', 'I9', 'K3', 'K8', 'L5'}) == {'B7', 'E3', 'J6'}

Input: The obstacles on the Moon (a set of strings).

Output: A set of all the candidate landing sites (a set of strings).

Precondition:

• all(re.fullmatch('[A-L][1-9]', i) for i in input)

How it is used: For finding the needed area.