Second part approach:
- Get the correct mapping from segments to digit
- 1, 4, 7 and 8 are unique by the number of segments
- 6, 9, 0 all have 6 segments
- derive 9 which is the only one that fully contains 4
- derive 0 which is the only one that fully contains 7
- 6 is the remainder of the 6-segment digits
- 2, 3, 5 all have 5 segments
- derive 3 which is the only one that fully contains 7
- derive 5 which is the only one fully contained in 6
- 2 is the remainder of the 5-segment digits
def solve(stream, :second) do
stream
|> parse() # returns a tuple {patterns, output}, both being sorted charlists
|> Stream.map(&derive_numbers/1)
|> Enum.sum()
end
defp derive_numbers({patterns, output}) do
%{2 => [one], 3 => [seven], 4 => [four], 5 => len_five, 6 => len_six, 7 => [eight]} =
(output ++ patterns)
|> Enum.uniq()
|> Enum.group_by(&Kernel.length/1)
{[nine], len_six} = Enum.split_with(len_six, fn nr -> four -- nr == [] end)
{[zero], [six]} = Enum.split_with(len_six, fn nr -> seven -- nr == [] end)
{[three], len_five} = Enum.split_with(len_five, fn nr -> seven -- nr == [] end)
{[five], [two]} = Enum.split_with(len_five, fn nr -> nr -- six == [] end)
output
|> Enum.map(fn
^one -> 1
^two -> 2
^three -> 3
^four -> 4
^five -> 5
^six -> 6
^seven -> 7
^eight -> 8
^nine -> 9
^zero -> 0
end)
|> Integer.undigits()
end
