AdventOfCode/2020/elixir/day16/day16part2-simpler-transpos...

80 lines
2.6 KiB
Elixir

defmodule Day16Part2 do
def run do
{rules, my_ticket, tickets} = File.read!("input") |> parse_input()
tickets
|> Enum.filter(&valid?(&1, rules))
|> transpose_tickets()
|> valid_fields_for_columns(rules)
|> reduce_to_column_by_field_name()
|> Stream.filter(&match?({"departure" <> _, _idx}, &1))
|> Stream.map(fn {_name, idx} -> my_ticket[idx] end)
|> Enum.reduce(&Kernel.*/2)
|> IO.puts()
end
def valid?(ticket, rules) do
Enum.all?(ticket, fn field ->
Enum.any?(rules, fn {_name, {range1, range2}} -> field in range1 || field in range2 end)
end)
end
def transpose_tickets(tickets), do: tickets |> Enum.zip() |> Enum.map(&Tuple.to_list/1)
# Compare each column against the rules to determine the list of fields each column satisfies
def valid_fields_for_columns(transposed_tickets, rules) do
for {column, idx} <- Enum.with_index(transposed_tickets) do
{valid_fields_for(column, rules), idx}
end
end
# Reduce the list of valid fields by elimination.
# The number of valid fields for each column is unique, which doesn't seem like a coincidence.
# Start with the column that only has one possible valid field, mark the field as seen.
# Continue to the column with two valid fields, subtract the seen field, mark remaining field as seen.
# Repeat for all columns.
def reduce_to_column_by_field_name(valid_fields_by_column) do
valid_fields_by_column
|> Enum.sort_by(fn {fields, _idx} -> length(fields) end)
|> Enum.map_reduce([], fn {fields, idx}, seen ->
[field] = fields -- seen
{{field, idx}, [field | seen]}
end)
|> elem(0)
end
def valid_fields_for(column, rules) do
Enum.filter(rules, fn {_field, {range1, range2}} ->
Enum.all?(column, fn val -> val in range1 || val in range2 end)
end)
|> Enum.map(&elem(&1, 0))
end
def parse_input(input) do
[rules, [_, my_ticket | _], [_ | nearby_tickets]] =
input
|> String.split("\n\n", trim: true)
|> Enum.map(&String.split(&1, "\n", trim: true))
rules = Enum.map(rules, &parse_rule/1)
[my_ticket | nearby_tickets] =
[my_ticket | nearby_tickets]
|> Enum.map(fn ticket -> String.split(ticket, ",") |> Enum.map(&String.to_integer/1) end)
my_ticket = my_ticket |> Enum.with_index() |> Map.new(fn {val, idx} -> {idx, val} end)
{rules, my_ticket, nearby_tickets}
end
def parse_rule(rule) do
[name | ranges] =
Regex.run(~r/\A(.*): (\d+)-(\d+) or (\d+)-(\d+)\z/, rule, capture: :all_but_first)
[a, b, c, d] = Enum.map(ranges, &String.to_integer/1)
{name, {a..b, c..d}}
end
end
Day16Part2.run()