Advent Of Code - Day 16: Part 2

aoc_2020_day_16_part_two.rb

@section = File.open('input.real.txt').read.split("\n\n").compact

# Get all the rules as ranges (e.g. 0..3), with their names.
@rules = @section[0]
  .split("\n")
  .collect { |line|
    line.scan(/(.+): ([0-9\-]+)-([0-9]+) or ([0-9\-]+)-([0-9]+)/ism).flatten
  }
  .collect { |name, start_range_1, end_range_1, start_range_2, end_range_2|
    {
      name: name,
      ranges: [
        (start_range_1.to_i..end_range_1.to_i),
        (start_range_2.to_i..end_range_2.to_i),
      ],
      index: []
    }
  }

# Load my ticket numbers
@my_ticket = @section[1]
  .gsub("your ticket:\n", '')
  .split("\n")
  .compact
  .collect { |ticket| ticket.split(',').collect(&:to_i) }.first

# Load nearby ticket numbers
@nearby_tickets = @section[2]
  .gsub("nearby tickets:\n", '')
  .split("\n")
  .compact
  .collect { |ticket| ticket.split(',').collect(&:to_i) }

# Remove all the tickets that are invalid for any rules
@nearby_tickets.select! do |ticket|
  ticket.all? do |ticket_number|
    @rules.any? do |rule|
      rule[:ranges].any? { |range| range.cover?(ticket_number) }
    end
  end
end

def find_potential_indexes_for_rules(index)
  @rules.each_with_index do |rule, rule_index|
    if @nearby_tickets.all? { |ticket| rule[:ranges].any? { |range| range.cover?(ticket[index]) } }
      rule[:index] << index
    end
  end
end

# Identify which column is valid for each rules
@nearby_tickets[0].each_with_index do |_number, index|
  find_potential_indexes_for_rules(index)
end

# Keep reducing the potential indexes, until each rule only has one index left
while(@rules.collect { |rule| rule[:index] }.compact.flatten.length > @rules.length) do

  @found_indexes = @rules
    .select{ |rule| rule[:index].length == 1 }
    .collect { |rule| rule[:index] }.flatten

  @rules.each do |rule|
    next if rule[:index].length == 1

    rule[:index] -= @found_indexes
  end
end

@indexes_for_deperatures = @rules
  .select { |rule| rule[:name].start_with?("departure") }
  .collect { |rule| rule[:index] }
  .flatten

@numbers_on_my_ticket_for_deperatures = @indexes_for_deperatures.collect do |index|
  @my_ticket[index]
end

puts @numbers_on_my_ticket_for_deperatures.inspect
puts @numbers_on_my_ticket_for_deperatures.inject(&:*)