gistfile1.erl

geometry(Size) ->
  Area = Size * Size,
  lists:map(
    fun(N) ->
        {N, neighbours(N, Size, Area)}
    end,
    lists:seq(1, Area)
  ).

neighbours(N, Size, Area) ->
  lists:foldl(
    fun(Direction, Acc) ->
        case neighbour(Direction, N, Size, Area) of
          undefined ->
            Acc;
          Neighbour ->
            [Neighbour|Acc]
        end
    end,
    [], [up, right, down, left]
  ).


neighbour(up, N, Size, _) when N > Size ->
  N - Size;
neighbour(right, N, Size, _) when N rem Size =/= 0 ->
  N + 1;
neighbour(down, N, Size, Area) when N =< Area - Size ->
  N + Size;
neighbour(left, N, Size, _) when N rem Size =/= 1 ->
  N - 1;
neighbour(_, _, _, _) ->
  undefined.